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Echohydrology as a New Tool for Sustainable Management of Estuaries and Coastal Waters (Wolanski et al, 2004)
Wetlands Ecology and Management 12: 235–276, 2004. 235
# 2004 Kluwer Academic Publishers. Printed in the Netherlands.
Ecohydrology as a new tool for sustainable management of estuaries and
coastal waters
´
E. Wolanski1,*, L.A. Boorman2, L. Chıcharo3, E. Langlois-Saliou4, R. Lara5,
A.J. Plater , R.J. Uncles and M. Zalewski8
6 7
1
Australian Institute of Marine Science, PMB No. 3, Townsville MC, Q. 4810, Australia; 2LAB Coastal,
The Maylands, Holywell, St. Ives, Cambs. PE27 4TQ, UK; 3Universidade do Algarve, CCMAR,
Campus de Gambelas, Faculdade do Mare do Ambiente, Portugal; 4Laboratoire d’Ecologie,
´
UPRES-EA 1293, Groupe de Recherche ECODIV ‘‘Biodiversite et Fonctionnement des
Ecosystmes’’, Universite de Rouen, 76821 Mont Saint Aignan, France; 5Zentrum f€r Marine
´
e u
Tropenokologie, Fahrenheitstrasse 6, 28359 Bremen, Germany; 6Department of Geography,
¨
University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT, UK; 7Plymouth Marine
Laboratory, Prospect Place, Plymouth, UK; 8Department of Applied Ecology, University
of Lodz, ul. Banacha 12/16, 902-237 Lodz, Poland; *Author for correspondence
(e-mail: e.wolanski@aims.gov.au)
Received 30 June 2003; accepted in revised form 10 December 2003
Key words: Ecohydrology, Ecology, Environmental degradation, Estuary, Hydrology, Management,
Sustainable development
Abstract
Throughout the world, estuaries and coastal waters have experienced degradation. Present proposed
remedial measures based on engineering and technological fix are not likely to restore the ecological
processes of a healthy, robust estuary and, as such, will not reinstate the full beneficial functions of the
estuary ecosystem. The successful management of estuaries and coastal waters requires an ecohydrology-
based, basin-wide approach. This necessitates changing present practices by official institutions based on
municipalities or counties as an administrative unit, or the narrowly focused approaches of managers of
specific activities (e.g., farming and fisheries, water resources, urban and economic developments, wetlands
management and nature conservationists). Without this change in thinking and management concept,
estuaries and coastal waters will continue to degrade, whatever integrated coastal management plans are
implemented. To help in this process of change there is a need to (1) develop a profound understanding of the
effects of biota and biotic processes on mediating estuary response to changing hydrology, sediment and
nutrient flux and of the biota on hydrology at the river basin scale, and (2) to develop science-based
remediation measures at the river basin scale, with elements of ecohydrology and phytotechnology at their
core, to strengthen the ability of the biota to sustain and adapt to human-induced stresses.
due to their very high biological productivity sus-
Introduction: degraded estuaries and coastal
taining a high level of food production. It is also
waters
due to the use of the rivers and estuaries as trans-
Throughout human history, the coastal plains and port routes, fundamental for economic and social
lowland river valleys have usually been the most development and improving quality of life. At
populated areas over the world. This is partially present, about 60% of the world’s population live
236
along the estuaries and the coast (Lindeboom coastal waters retain much of the nutrients and
2002). Coastal waters, including those covering other pollutants resulting from human activities.
continental shelves, supply about 90% of the global In some ecosystems, the level of extra nutrient is
fish catch. The increase of human populations in small enough that it may generate an increase of
the river basins, from natural growth and internal biological productivity without dramatic modifi-
migration within the hinterland, has resulted in a cation of biodiversity (e.g., Zalewski 2002). More
doubling of the population along many coasts over commonly, however, the load of nutrients and pol-
the last 20 years. This is degrading estuarine and lutants is so high that it degrades water quality,
coastal waters through pollution, eutrophica- ecological ‘services’, biodiversity, and productivity
tion (water quality degradation caused by excessive of coastal waters. This degradation is not restricted
nutrients), increased turbidity, overfishing and to estuaries, it is commonly observed already in
habitat destruction. The pollutant supply does not poorly flushed, semi-enclosed water bodies. In the
just include nutrients; it also includes mud from Baltic Sea, for example, the limited depth, low
eroded soil, heavy metals, radionuclides, hydro- salinity of surface water, and the supply of saline
carbons, and a number of chemicals including oxygenated water from the North Sea at the bot-
new synthetic products. tom, lead to water stagnation, eutrophication, and
The impact on estuaries is commonly still not anoxia (Gren et al. 2000). This impacts upon the
considered in environmental impact studies when commercial cod fisheries and generates increas-
dams are proposed on rivers. In this case, the ingly common toxic algal blooms (Figure 1a); in
coastal environment, coastal fisheries, and the turn, this may impact on tourism. The northwes-
local coastal people may be regarded as an expend- tern Black Sea ecosystem is similarly degraded by
able consequence of river basin development. For direct human impacts not only on the coastal eco-
instance, as we wrote this article, the dam on the system but also on the drainage basin of the
Guadiana River, Portugal, which will form the Danube River that receives the effluents from
largest man-made lake in Europe, is being com- eight European countries (Zaitsev 1992; Lancelot
pleted without a detailed audit of its impact on et al. 2002). A similar degradation is also observed
the estuary and the coastal zone, and without any in poorly flushed Seto Inland Sea, Osaka Bay, and
estuary remediation measures being planned. Tokyo Bay, Japan, where toxic algal blooms occur
Estuaries are often regarded as sites for future 100 days per year (Figure 1b; Okaichi and Yanagi
development and expansion, and have been 1997; Takahashi et al. 2000). Better flushed and
increasingly canalized and dyked for flood pro- larger systems also suffer from environmental
tection, and their wetlands infilled for residential degradation through eutrophication, as is made
areas. For example, almost all estuarine marshes apparent by some beaches of the North Sea being
have already been reclaimed in Japan and in the covered by foams of decaying algae and protozoa,
Netherlands. All these factors impact on the bio- mainly Phaeocystis and Noctiluca, and by hypoxia
diversity and productivity and, hence, the overall in the Gulf of Mexico (Richardson and Jorgensen
health and function of estuaries and coastal waters. 1996; Rabalais et al. 2002).
They increasingly lead humans away from the Until now, the solution was believed to depend
possibility of ecologically sustainable development on reducing the amount of waste and relying on
of the coastal zone. hard technology, namely the construction of
Human impact on the ecological health of sewage treatment plants and the modification of
estuarine and coastal waters is dependent on sev- farming practices and technology. While there are
eral factors. Water circulation in some estuaries exceptions, for instance, the partial restoration of
readily flushes away pollutants to the open ocean, some ecological functions in the Rhine and Thames
whereas other estuaries retain the pollutants. estuaries, in general this technological fix has not
Unfortunately many estuarine environments that restored the ecological health of estuaries in both
are especially attractive for human settlements, developed and developing nations worldwide. The
such as wetlands, lagoons, harbours, and fjords, reasons for failure are simple. First, integrated
are often poorly flushed. In these systems, through coastal zone management plans are drawn up but,
physical and biological mechanisms, estuarine and in the presence of significant river input, they are
237
environment degradation is often seen as ‘normal’
and nothing to be alarmed at. Most people do not
demand better ecosystem health and function,
possibly because in poor countries there are no
resources to restore the estuarine health (and eco-
logical issues come second to those of economic
development), and, in wealthy countries, people
have not seen their estuaries in a pristine, healthy
state and thus they may think that what they see is
normal. Further, and most commonly, solutions
are simply not applied because of socio-economic
constraints but as well because of national policies,
planning and management structures. Finally,
there is still no cost-effective technology and poli-
tical will to treat effluent from diffuse sources in
rural activities, except possibly using wetlands as
filters; a practice that is effective but rarely used
(Moore et al. 2001). The problem remains unre-
solved as to how, using only limited financial
resources, to preserve, restore and manage critical
habitats, and reduce the load of nutrients, sedi-
ments and pollutants in the face of increasing
human population and its aspirations.
In this paper, the concept of ecohydrology is
introduced as a holistic approach to the manage-
ment of estuaries and coastal zones within entire
river catchments, by adopting science-based solu-
Figure 1. (A) A photograph of a toxic algal bloom in Hel
tions to management issues that restore or enhance
Harbour, Polish Baltic Sea, in the summer of 2001. (B) An
natural processes as well as the use of technological
aerial photograph of a 3–4 km long toxic algal bloom of
solutions. Ecohydrology principles can be applied
Protoperidinium sp. at Tokyo Bay, Japan. These toxic algal
blooms are caused by eutrophication ((A) is courtesy of to control and reduce the impact of nutrients and
´
K. Skora and (B) is courtesy of the National Institute of Land and
pollutants along the rivers and the estuaries.
Infrastructure Management (NILIM), Japan (K. Furukawa)).
Ecohydrology is more than integrated river basin
management; its principles provide the guidance
bound to fail because in nearly all countries these for the development of low-cost technology for
management plans deal only with local, coastal mitigating the impact on the coastal zone of
issues, and do not consider the whole river catch- human activities throughout the river basin, using
ment as the fundamental planning unit. It is as if or enhancing the natural capacity of the water
the land, the river, the estuary, and the sea were not bodies to absorb, or process with no resulting estu-
part of the same system. When dealing with estu- ary degradation, the nutrients and pollutants. In
aries and coastal waters, in most countries land-use situations where socio-economic and political con-
managers, water-resources managers, and coastal straints do not allow the use of technology to
and fisheries managers do not cooperate effectively prevent the degradation of estuarine and coastal
due to administrative, economic and political con- waters, or to restore estuary quality and health,
straints, and the absence of a forum where their we argue that catchment-wide ecohydrology prin-
ideas and approaches are shared and discussed. A ciples should instead be applied to help rescue or
typical case is the river disposal of waste from restore these water bodies. Ecohydrology is, in
industrial swine and poultry production in North essence, a question of science-based, integrated
Carolina, USA, and the resulting pollution of estu- management of the river basin ecosystem from
aries downstream (Mallin 2000). Second, estuarine upland waters to the seas.
238
Ecohydrology: the new tool for sustainable
development of coastal zones
Ecohydrology is the science of the interplay
between hydrology and biota (Zalewski 2000,
2002). Ecohydrology is based on a set of principles.
The first principle (hydrological) defines the river Figure 2. Sketch of the robustness (R) of a river (r) and its
basin as a template of hydrological processes deter- estuary (e) and coastal waters (c) as a function of the residence
time T.
mining when and where stable and predictable
aquatic biota form and start to play a stabilizing
role on water quality. The same principle applies to enables a lower reduction in the nutrient discharge
estuaries. In the second principle, ecohydrology from rural areas, while eliminating toxic algal
considers the entire river basin as a ‘super organ- blooms without any need for an expensive techno-
ism’; this simply means that each of the inhabitants logical solution or large-scale change in land-use
of the basin is an inherent component of the entity, practices.
for example, somewhat like a cell in a body. In Extending this concept to the coastal zone, the
practice, this means that if an inhabitant discharges whole river catchment, from the headwaters down
pollutants into the system, sooner or later this pol- to the estuary and coastal waters, must be viewed
lutant will impact on another inhabitant. Such pol- as a single ecological entity (Figures 2 and 3).
lutants include eroded soil, nutrients, heavy metals, Therefore, in order to maintain healthy, produc-
and DDT and PCB residues that are now found in tive estuaries and coastal waters, it is necessary to
virtually every ecosystem of the world. Moreover, manage human impacts throughout the river
some pollutants can be bio-accumulated (‘biomag- catchment, along the river and in the coastal
nified’) in the food chain by several orders of mag- hinterland, and to enhance the natural capacity of
nitude, potentially impacting on fish and people. the river and the estuary to sequester or degrade
In the third principle, the capacity of the system pollutants, and to convert the excess nutrient into
to cope with stresses from human activities (e.g., plant biomass. Ecohydrology thus relies only on
eutrophication) can be enhanced by managing the low-cost technology; it requires innovative solu-
biota and by regulating hydrological processes tions based on an understanding of feedback
(Mitsch 1993; Jorgensen 1997; Jorgensen and de mechanisms between hydrology and biota within
Bernardi 1998; Zalewski 2000, 2002). An example the estuary and coastal waters. By relying on nat-
is the creation of wetlands to trap nutrient and ural ecological processes, it can lead to sustainable
pollutants, and convert nutrients to plant biomass; development and environmental management. In
this biomass can provide bio-energy for society and essence, these principles have been used to good
create employment. Another example is controlling effect in managing coastal dune complexes where
water levels via the biota in wetland-fringed water appropriate species of grasses and trees are used to
bodies to regulate water quality and enhance fish manipulate sediment throughput and deposition,
stocks. An additional example is the use of macro- but with careful consideration of the hydrological
phytes to avoid toxic algal blooms in rivers and demands of coastal dune communities, for
lakes. Toxic algal blooms do not appear for example, Ainsdale Dunes, UK (Wheeler et al. 1993).
P–PO4 < 30 g lÀ1 (Dunne and Leopold 1978). The paper describes a number of key processes of
Reaching such a small concentration means chan- estuaries. For convenience, these processes are
ging farming practices and, in practice, this is separated into physical, biological and human-
economically an unrealistic goal. However, it is related processes. They all impact on the estuary,
possible to avoid toxic algal blooms by reducing and successful sustainable management depends on
P–PO4 concentrations to only 120 g lÀ1, provided integrating these processes. Clearly, the prevailing
macrophytes are present. This option is attractive natural conditions and human impacts within the
because establishing macrophytes in an aquatic river catchment and its coastal hinterland, the
ecosystem may be considerably less expensive than hydrology, the biology and the buffering capacity,
reducing nutrient input from human activities; it and ‘robustness’ of each estuary (see below), vary
239
Figure 3. Sketch of the dominant pathways of water, fine sediment, nutrients, and plankton in an estuary, together with the impact on
human health.
from river catchment to river catchment and from flushing rate of water is not the only variable deter-
estuary to estuary in both time and space. Hence, mining robustness, and the following sections
there is no universally applicable formula to inte- describe the three other key determinant processes.
grate these processes in an ecohydrology model of These are: first, the rate at which fine sediment –
an estuary, but generic ecohydrologic principles soil eroded from the river catchment – is flushed;
may be employed to consider the integration of second, the efficiency with which organic matter is
these factors from one estuary to another. processed by the biology within the water column;
This paper starts with a consideration of the and third, the trapping of sediment, nutrients and
robustness of an estuary. Robustness (Figure 2) is pollutants in inter-tidal wetlands (mangroves and
controlled by the rate at which water in an estuary saltmarshes). A section describes severe estuarine
is flushed; the shorter the flushing time-scale, degradation resulting from human activities up-
the higher the robustness, that is, the lower the river to illustrate cause and effect. The next section
expected water quality problems. However, the describes aspects of human health linked with
240
estuaries; once again this is dependent on human concentration, other solutes, and SPM. To some
activities up-river, but it is also dependent on wet- degree, all these variables are controlled by the
land buffering capacity. The final section describes residence time, that is, by the rate at which
how a holistic approach is needed to understand these components are flushed out to the sea.
the health of the ecosystem. The section also Thus, the residence time of an estuary or coastal
suggests that this holistic approach can be used water body is an important parameter determining
to manage an estuary and coastal waters on an its robustness and its ability to cope with human-
ecologically-sustainable, low-cost basis. induced stresses (Figure 2). Well-flushed estuaries
are intrinsically more robust than poorly flushed
systems. The residence time is most critical in the
upper reaches of an estuary because this is where
Estuary robustness
contaminant accumulation and increased turbidity
from human influences are most likely to occur. As
The role of robustness
a result, environmental degradation is most often
apparent during periods of reduced freshwater
Estuaries and coastal waters are subject to constant
inflows, for example, during drought or when
changes in the wind, solar input, rainfall, wind,
human activities reduce the freshwater flow.
currents offshore, sea level, freshwater runoff,
Indeed, human activities in the river catchment
varying climate, and human influences. These
may cause significant reductions in dissolved
changes occur at time-scales of minutes (turbulent
oxygen, mainly through eutrophication. There is
mixing), hours and days (tides), months (hydrolo-
no other environmental variable of such ecological
gic and seasonal changes), and decades (climate).
importance to coastal marine ecosystems that has
Thus, estuaries are highly dynamic environments
changed so drastically due to human influences in
that exhibit strong temporal and spatial changes in
recent decades (Diaz and Rosenberg 1995). At
their physical, chemical and biological variables.
times this has resulted in severe deficits of dissolved
In turn, this dynamism leads to a large variability
oxygen, leading to hypoxia and anoxia. Environ-
in estuarine water quality, chemical and biological
mental degradation problems associated with the
characteristics. Further, no two estuaries are iden-
occurrence of low oxygen are increasing on a global
tical; each estuary (and coastal water body) has its
scale.
own size, shape and bathymetry, its own length of
Residence time is a key physical variable deter-
tidal influence, tidal range, wave climate, fresh-
mining if a particular estuary will or will not suffer
water inflow, turbidity and residence (or flushing)
from eutrophication. For example, the residence
times, and its own sediment properties of grain size,
time of the Tweed Estuary, UK, is generally
carbon-to-nitrogen ratio, organic carbon content,
less than 13 h and dissolved oxygen levels are high
and water-column turbidity. Thus, each estuary is
throughout the year (Uncles et al. 2000). Rapid
unique and there is no general parameter to readily
flushing ensures both low turbidity (because fine
assess the ‘health risk’ of an estuary from natural
sediments are unable to accumulate in the estuary)
and human influences.
and insufficient time for local oxygen depletion to
The ecological health of estuaries depends on the
occur, regardless of whatever biochemical oxygen
‘successful’ interaction – and the limits to the pos-
demands are present. The reverse is true in the
sible interaction that are imposed by the temporal
Humber-Ouse Estuary, UK, which has a residence
and physical variability – between organisms and
time of several weeks (Uncles et al. 1998a, b); as a
variations in salinity, currents, waves, suspended
result, this estuary experiences high turbidity and
particulate matter (SPM), bed sediments, tempera-
suffers from dissolved oxygen depletion.
ture, air exposure, hypoxia, wetland contaminants
and biodiversity.
Like the health of a living organism, the health The flushing of estuarine waters
of an estuary or a coastal water body, cannot be
measured by one single variable. A number of The simplest way to quantify the residence time is
variables are important (Balls 1994); these include through the use of a single compartment, tidally
the dissolved nutrients, the dissolved oxygen averaged box model. This method is very coarse in
241
Figure 4. Mixing diagram of a material in an estuary. Salinity is taken to be 0 in freshwater and 35 in ocean water. CR is the
concentration of the material in the river, and CS that at sea. A straight line indicates conservative mixing (no losses or gain);
concave and convex lines indicate, respectively, a loss and a gain of that material in the estuary. When the concentration of material
in the river is greater than that at sea, a negative slope exists. Conversely, a positive slope results when the concentration of material in
the river is smaller than that at sea.
basin volume with ‘new’ fresh water. The coastal
space and time, but is useful in acquiring a quali-
salinity is So (typically 33–35) and the estuarine
tative, conceptual understanding of the time-
salinity, S, is given by:
dependent solute dynamics for a range of estuarine
systems in which transients and residence times are
S=So ¼ 1 À f ð2Þ
important.
A conservative (i.e., no growth or decay) dissolved
In this method, the estuary is treated as a single
nutrient with freshwater concentration, Nf, enters
compartment of fixed volume, V. The compart-
the estuary with the freshwater and is mixed into
ment is subjected to river (and groundwater) fresh-
the coastal sea, where, for illustration, its con-
water inputs upstream, and to an equal seaward
centration is taken to be zero. The evolution of
flow at the mouth. In addition there is mixing with
estuary nutrient concentration, N, is given by:
seawater across its down-estuary face. The rate at
which the fraction of freshwater within the estuar- @t N ¼ f Nf À r N ð3Þ
ine compartment, f, changes with time (@ t f) is
Residence time and therefore r in Equations (1)
given by:
and (3) can be specified empirically in terms of tides
@t f ¼ f À r f ð1Þ and runoff. In the special case of tidally averaged,
À Á steady state conditions, during which variables do
where f ¼ Qf =V ¼ fÀ1 and r ¼ Qf þ QÃ =
not change with time (@ t 0), the freshwater
V ¼ rÀ1 ! f :
fraction is given by (from Equation (1)):
The freshwater ( f ¼ 1) flow rate into the estuary
compartment is Qf, which is equal to the flow rate f ¼ f = r ð4Þ
out of the compartment to sea, where f ¼ 0. The The estuarine nutrient concentration for this spe-
volume-mixing exchange rate with the coastal cial case is then (from Equations (2)–(4)):
waters is Q* ! 0, so that the estuarine residence
N ¼ Nf f ¼ Nf ð1 À S=So Þ ð5Þ
time of freshwater within the compartment, r, is
such that r f (from their definitions in Equation This is the classic, linear, estuarine ‘mixing-
(1)). If the estuary were completely full of fresh- diagram’ relationship between a solute and salinity
water then f would equal the estuarine (in this (Figure 4). For matter passively carried by water,
case tidal river) residence time, r, that is, the time the mixing diagram shows a linear line. An estuar-
required by the river to replace the entire estuarine ine nutrient input per unit volume of estuary and
242
per unit time, IN ! 0, can be included in the single- The residence time is, therefore, mainly affected by
compartment estuary model (see Equation (3)): TL, as would be expected for a diffusive system
(RT proportional to the square of TL) or an advec-
@t N ¼ f Nf À r N þ IN ð6Þ
tive system (i.e., proportional to TL).
Equation (8) ignores two processes that can
In the special case of a steady state estuary with a
modify the residence time in an individual estuary.
constant estuarine nutrient input, the correspond-
These are, first, the currents driven by a difference
ing steady state estuarine nutrient concentration
in density between fresh and salt water – this is
would then be (see Equation (5)):
particularly important in deep fjords, bays and
N ¼ r IN þ Nf f ¼ r IN þ Nf ð1 À S=So Þ semi-enclosed seas where the bottom waters can
! Nf ð1 À S=S0 Þ ð7Þ be nearly stagnant and where water quality can
degrade severely. Another key process is water sto-
This demonstrates that the estuarine nutrient con-
rage and buffering by intertidal wetlands, mainly
centration for this ‘input’ case lies above (i.e., con-
saltmarsh or mangrove vegetation that flank the
vex upward) the linear, ‘conservative’ line defined
main estuary and results in drag to the flow and
by the river and sea end-members (Figure 4).
temporary storage of waters. These points are dis-
A concave line suggests a loss of the matter within
cussed in more detail below in the sections on salt-
the estuary, for example, by adsorption on mud.
marshes and mangroves.
A convex line suggests the addition of matter, for
example, matter gained by degradation of organic
The effects of residence time on dissolved
material or desorption from mud. Thus, if there
substances
were in situ nutrient removal from the estuary
(i.e., IN < 0, such as might occur during an algal
An important chemical variable is dissolved oxy-
‘bloom’) then the estuarine nutrient concentration
gen, because of its necessity to the metabolism of
for this ‘removal’ case would lie below (i.e., convex
aerobic organisms, and its uptake, sometimes to
downward) the linear, ‘conservative’ line defined
hypoxic and anoxic levels, by the respiration of
by the river and sea end-members. In particular,
sewage and other waste inputs. Because of the
the influence of the local estuarine input, or
influence of varying residence times and other
removal, is directly proportional to the residence
environmental factors, such as wind speed, there
time r, and is likely to be much less important in
is usually no simple relationship between the oxy-
rapidly flushed estuaries (small r and potentially
gen demand of waste effluents and reductions in
small rIN) than in slowly flushed estuaries (large r
oxygen concentrations. The oxygen deficit depends
and potentially large rIN).
on water flow, turbidity, and oxygen supply and
demand, and this varies from estuary to estuary
Estimates of the residence time
(Owens et al. 1997).
Nutrient loads in many rivers have increased
The estuarine residence (or flushing) time of a
markedly over recent decades and this increase is
solute depends strongly on tides, freshwater run-
thought to have been partly responsible for the
off and morphological size, especially length.
changed eutrophic status of a number of estuaries
Residence times, which generally are calculated
and coastal seas (Reid et al. 1994). Once in the
from freshwater runoff and estuarine freshwater
estuary, non-conservative behaviour of nutrients
content, tend to increase with tidal length (TL)
can be pronounced. As with the dissolved oxygen
and decrease with increasing tidal range. Uncles
deficit, the fate of nutrients varies from estuary to
et al. (2002) derived the following simple, linear,
river as a function of water flow, turbidity and
multiple regression relationship for the logarithm
biota (Figure 3). In muddy estuaries, the turbidity
of the maximum residence time ( r, in days) in
varies spatially and temporally (see the section on
terms of the logarithm of the mean spring tidal
‘The fate of fine sediment in the estuary’). As a
range (MSTR, in m) and the logarithm of the
result of all these influences, phosphate usually
tidal length TL (in km),
behaves non-conservatively in turbid estuaries
r ¼ 0:23ðMSTRÞÀ0:4 ðTLÞ1:2 ð8Þ (this behaviour is particularly marked within the
243
Table 1. Comparison of the drainage areas, the sediment load
upper reaches of muddy estuaries), whereas it may
and the yield for various rivers.
be less so for silicate concentrations, while nitrate
may be essentially conservative (Morris et al. Load Yield
1981). Systems with very long residence times can (106 tonne
Area (tonne
yearÀ1) kmÀ2 yearÀ1)
(106 km2)
River
export much less of these nutrients to the coastal
zone than systems with very short residence-times. Yangtze 1.9 480 252
For example, the degree of denitrification in an Amazon 6.1 1200 190
estuary (i.e., loss of nitrogen to the atmosphere) Mississippi 3.3 210 120
Ganges/Brahmaputra 1.48 2180 1670
increases with water residence time (i.e., for N, see
Mekong 0.79 170 215
Nixon et al. 1996). There is no straightforward
Fly 0.076 116 1500
relationship between trace metal transport and Cimanuk 0.0036 15.7 6350
water residence time; this appears to be due to King Sound 0.12 6 50
5 Â 10À6 2.4 Â 10À3
non-linear metal chemistry and absorption/deso- La Sa Fua 480
rption processes of metal on mud (Morris 1988;
The effect of deforestation on estuaries is much more rapid in
Liu et al. 1998; Salomons and Forstner 1984). the tropics than in temperate zones and the difference is mainly
Once the metal is absorbed on mud, it can become due to the intense rainfall in the tropics. The Cimanuk and La Sa
buried under the substrate of the estuary or in the Fua river catchments are small and profoundly modified by
human activities in the tropical islands of Java and Guam,
fringing wetlands. This metal can still be remobi-
respectively (Wolanski et al. 2003b).
lised, and threaten the biota, by a number of pro-
cesses such as bioturbation, natural erosion and
channel migration, or through dredging (French
The fate of fine sediment in the estuary
1996).
High nutrient inputs to estuaries and associated
Riverine sediment inflow
eutrophication can lead to algal blooms. In turn,
Associated, though not exclusively, with the fresh-
this results in the consumption of dissolved oxygen
water inflow to estuaries is an input of SPM (which
by decaying algae, and in the formation of foams
includes and is usually dominated by sediment)
and toxins. The residence time, and thus the rate of
derived from erosion of the drainage basin. The
flushing of nutrients from estuaries, is therefore an
SPM concentration of river waters typically exhi-
important factor, but not the only one, in determin-
bits great variability as a function of river inflow,
ing whether excessive nutrient inputs are likely to
although it is generally larger during high river flow
lead to algal blooms and oxygen sags.
periods, especially during floods. Sediment inputs
Eutrophication is not restricted to estuaries;
into estuaries vary greatly for different rivers. For
it can also impact on coastal waters. The link
illustration, the Mekong and Fly rivers form large
between land-use and coastal water pollution can
estuaries in the wet tropics. To place these rivers in
be quantitatively determined using models. For
perspective with other major rivers in the world
instance, a 50% reduction in artificial fertilizer
(see Table 1), the Mekong, Vietnam, has a smaller
applications within the Humber drainage basin,
drainage basin than the Ganges/Brahmaputra
UK, would give a 10–15% decrease in nitrogen loads
(53%), Yangtze (41%), the Mississippi (24%) and
to the North Sea, relative to the 1994–1995 input
the Amazon (12%). The Mekong sediment yield is
(Tappin, pers. comm.). To a certain extent, residence
about twice that of the Mississippi, 85% that of the
time influences which part of the coastal zone is at
Yangtze, 12% larger than that of the Amazon, but
risk from degradation or eutrophication – whether
only about 1/7 that of the Fly and the Ganges/
coastal wetlands have sufficient time to sequester
Brahmaputra. The Fly River, Papua New Guinea,
the nutrient reservoir, or whether nutrients make
has the highest sediment yield, about 10 times
their way to the shelf without significant loss. This
higher than the Amazon and the Mississippi.
has important consequences for marine sanctuaries
Erosion increases in the catchment as a result of
and world heritage sites which flank intensively
human activities, which leads to an increase in river
used catchments, for example, Monterey Bay,
SPM. When rivers are dammed, much of the coarse
USA, and Greater St. Lucia Wetland, South
sediment (e.g., gravel, sand and coarse silt) may
Africa.
244
remain trapped in the reservoirs, which then experi- These mudflats are often flanked, in temperate
ence siltation. climates, by localised saltmarshes that are inun-
dated at HW of spring tides or, in the tropics, by
mangroves. Much of the muddy sediment is
Seaward sediment export from estuaries
derived from tributary rivers and the coastal sea,
The export of fluvial SPM varies from estuary to
and was deposited as rising sea levels of the
estuary. In practice, there are estuaries that trap
Holocene drowned former river valleys (e.g.,
essentially all the sediment, other estuaries export
Chappell and Woodroffe 1994; Roberts 1998).
seaward all the riverine sediment to the sea, and
These vegetated mudflats play a critical role in
other estuaries trap some sediment and export the
determining the robustness of the estuary, by selec-
rest. For instance, in the Mekong Estuary,
tively trapping fine sediments, influencing the
Vietnam, river floods last several months and
water residence time, sequestering nutrients and
freshwater may be found throughout the estuary,
pollutants, and converting excess nutrients within
up to the river mouth. In this case, fine sediment is
the water column into plant biomass. These points
not stored in the estuary; instead, it is discharged
are described in separate sections on saltmarshes
directly into the shallow, coastal waters (Wolanski
and mangroves, respectively.
et al. 1996). A fraction of that sediment returns in
The size, elevation and stability of non-vegetated
the estuary during the low flow period (Wolanski
mudflats depend to a considerable degree, but not
et al. 1997). In the Fly River, Papua New Guinea,
entirely, on the biology. There are relationships
river floods are rare and much of the riverine mud
between sediment erosion properties and the biolo-
remains trapped in the estuary (Wolanski et al.
gical characteristics of intertidal mudflats that
1998a). In the Fitzroy Estuary, Australia, the mud
demonstrate the importance of the surface diatom
is deposited slightly offshore during river floods;
layer (biofilm) in modifying the initial erosion of
during the rest of the year, this mud is resuspended
a surface mud layer (e.g., Sutherland et al. 1998a;
by strong tidal currents and transported back into
Widdows et al. 2000; Riethm€ller et al. 2000;
u
the estuary, so that over a time scale of a year most
Anderson 2001). Benthic diatoms produce ‘sticky’
of the riverine sediment is effectively trapped in the
extracellular polymeric substances within these
estuary (Wolanski and Spagnol 2003).
biofilms (Paterson 1994; Sutherland et al. 1998b),
which then provide a degree of binding for mudflat
and mudbank surfaces. It has been hypothesized
Formation of mudflats
that the erosion properties of cohesive, intertidal
The nature, degree and extent of sediment deposi-
mudflats depend on a balance between the physical
tion in the estuary are determined by its morphol-
and biological processes of stabilization and desta-
ogy and sediment characteristics (Figure 3). These
bilization (Widdows et al. 2001). The density of
vary widely from estuary to estuary, according to
microphytobenthos, algal mats, higher plants,
the importance of wave activity, tidal variations in
worms, mussel beds and other biological factors
water level and current speed, freshwater inflow,
can affect bio-stabilization of sediments. Bio-destabi-
shape and size of the estuary, local geology and
lization mainly results from the bioturbation
sediment availability and mineralogy. Many tidal
caused by burrowing and deposit-feeding animals,
estuaries have large areas of intertidal mudflats,
such as bivalves, polychaetes and crustaceans.
which comprise large amounts of clay and silt
mixed with varying fractions of coarser sediments.
These mudflats are exposed to the atmosphere at Estuarine turbidity maximum zone
tidal low water (LW) and border the main, deeper Once riverine-derived SPM enters the estuary, it
channels. They probably formed as turbid waters can be trapped within an estuarine turbidity
inundated the near-shore banks on a rising tide and maximum (ETM) zone (Figure 3). The ETM is
deposited sediment in the relatively slack waters frequently located in the very low salinity rea-
there, especially over high water (HW) slack, and ches of an estuary, the position of which is depen-
then failed to suspend and remove all of the depos- dent on tidal range, river flow and physical
ited sediment on the subsequent falling tide dimensions, and, therefore, shows great variabil-
(Pethick 1984). ity. Continuous exchange of primary sediment
245
particles and aggregates between water column and recycling following increased wave activity or
bed within the ETM generates a shoal of mobile tidal currents with rising sea level or increased
bed sediment, which moves down-estuary with storminess.
increased river inflow and exhibits a seasonal The affinity of organic molecules for particle
migration (Bale et al. 1985). surfaces and the incorporation of biogenic mate-
Long, strongly tidal estuaries tend to have rial within aggregates that constitute the ETM
greater SPM concentrations within their ETM make it a suitable environment for particle-
regions than either shorter estuaries (with compar- associated bacteria. The associated, localised bac-
able tidal ranges at their mouths) or weakly tidal terial respiration can cause significantly dissolved
estuaries (Uncles et al. 2002). An empirical rela- oxygen depletion within the ETM (Morris et al.
tionship between the maximum, depth-averaged 1982; Uncles et al. 1998a). Traditionally, low
SPM concentration at HW within an estuary, TL dissolved oxygen levels have been attributed to
and tidal range (TR) was derived from a systematic industrial discharges and pollution from tribu-
comparison of SPM concentrations at spring tide, tary rivers and direct effluent discharges to
low freshwater inflow conditions for 44 estuaries the estuary, especially from sewage treatment
world-wide. A simple, although crude, linear, mul- works located in the major population centres.
tiple regression relationship for the logarithm However, in the Loire Estuary, urban and indus-
of the maximum SPM concentration (mg lÀ1) in trial effluents represent a very small part of the
terms of the logarithm of the TR (in m and speci- total oxygen demand and the occurrence of
fied at the estuary mouth) and the logarithm of the anoxia is associated with the stock of organic
TL (in km) gave: material in the ETM region (Thouvenin et al.
1994).
SPM ¼ 0:0055ðTRÞ2:8 ðTLÞ1:7 ð9Þ
Longer estuaries possess faster tidal currents for a Estuaries as sieves for nutrients
given tidal range at their mouth; this in turn pro- The strongly tidal estuaries generally exhibit a tol-
duces more erosion and suspension of bed sedi- erance to eutrophication with respect to nitrogen-
ments, and therefore higher SPM concentrations containing nutrients, despite (in some cases) high
(Uncles et al. 2002). nitrogen loadings in their inflowing rivers. The
mean annual chlorophyll ‘a’ levels are significantly
Water quality aspects of SPM and deposited lower in the strongly tidal estuaries than in the
sediment weakly tidal estuaries with similar nitrogen concen-
Sediment particles and aggregates within the ETM tration (Monbet 1992). Tidal range, of course, acts
can give rise to marked changes in water quality. as a surrogate parameter to describe tidal mixing,
Fine particles can adsorb metal ions and organic current velocity, sediment suspension and turbidity
macro-molecules from solution to such an extent (Equation (9)) and therefore light penetration.
that some metals can be completely removed from Larger tides ensure that accumulated sediment is
solution within a strong ETM (Ackroyd et al. 1986; regularly suspended, leading to high turbidity and
Salomons and Forstner 1984). On longer time low light levels with less potential for bloom con-
scales, these adsorbed metals become incorporated ditions, regardless of nutrient levels.
into anoxic, reducing bed sediments. Here, heavy In non-impacted environments, the transport of
metal and radionuclide pollution may be regarded nitrogen (N) and phosphorous (P) from a drainage
as being effectively bound in an immobile sedi- basin to its watercourses is dependent on the che-
ment sink. Indeed, considerable success has been mical and mechanical weathering of soil minerals,
achieved in using down-core trends in metal and whereas in impacted environments it is thought
radionuclide concentration as a faithful record of that agriculture is the largest contributor to river
the industrial history of estuaries (e.g., McCaffrey nutrient concentrations (Tappin 2002). The fluvial
and Thomson 1980; Oldfield et al. 1993; Cundy fluxes of total N and P depend both on water and
et al. 1997; Plater et al. 1998; Fox et al. 1999). SPM fluxes and the N and P species associated with
However, these metals may become solubilised them. These species include dissolved (nitrate,
into pore water or have the potential for physical nitrite, ammonium, organic N, inorganic P,
246
organic P) and particulate (organic N, organic P, POC decreased from a range of 4.6–1.5% in
inorganic P) components. Particulate species tend the Garonne River, France, to 1.5% in the Gironde
to dominate the load, although nitrate and phos- Estuary’s ETM (Etcheber 1983). In the Delaware
phate become more important in populous regions. Estuary, USA, the POC content was 9% in the river
Once nutrients enter an estuary, non-conservative and 2.3% in the estuary’s ETM (Biggs et al. 1983).
behaviour can be pronounced as is described in In the Ems Estuary, Netherlands, POC decreased
the section on ‘The effects of residence time on from 6.1% in the non-tidal river to 2.8% in the
dissolved substances’. Key processes responsible ETM (Eisma et al. 1982). Similarly, in the Tamar
for this non-conservative behaviour include burial Estuary, UK, POC decreased from 5% in the river
in sediment reservoirs, such as tidal saltmarshes to about 2.5% within the estuarine ETM (Morris
and mangrove forests, and loss to the atmosphere, et al. 1982). Considering carbon to nitrogen ratios,
largely by bacterial denitrification of nitrate N to computed POC/PN ratios (P means particulate
gaseous N2/N2O. here) for the Humber Estuary, UK, are generally
Nitrogen compounds may exert a significant >10 and higher than many data reported elsewhere
oxygen demand in estuaries through microbially (which are typically 8–12). Similarly high values
mediated nitrogen transformations (Owens 1986; have been reported for the very low salinity reaches
Grabemann et al. 1990). Organic nitrogen is hydro- of some North Carolina estuaries, USA, where the
lyzed to ammonia and additional ammonia is input POC is characteristic of particulate terrestrial plant
to estuaries via tributary rivers and wastewater material (>15; Matson and Brinson 1990), and for
discharges. Ammonia utilizes dissolved oxygen the Vellar Estuary, India (4–140; Sivakumar et al.
during nitrification to produce nitrate, via nitrite 1983). High ratios occur in the Humber Estuary,
(e.g., Owens 1986). In conditions of low dissolved UK, when PN levels become very small, rather
oxygen, nitrate may be reduced to enable oxida- than when POC becomes very high. This might be
tion of organic carbon to proceed (e.g., Gameson a consequence of a significant coal fraction of the
1981). Other nutrients are discharged to estuaries. POC resulting from historical coal mining activity
For example, river and estuarine waters are often in the Humber’s drainage basin and estuarine-
enriched by phosphate from urban and industrial borne coal transportation.
wastewater discharges and from land runoff (e.g., Temporal variability in fluvial inputs of sus-
Grabemann et al. 1990; M€ller et al. 1991; Hager
u pended sediment also impacts on POC in estuaries.
and Schemel 1996) and they receive silicate from In the northern reaches of San Francisco Bay for
tributary river inflows via rock weathering and soil example, the POC fraction of SPM during fresh-
leaching. The river inflow, estuarine mixing and water floods was 1–2%; POC was higher (mostly
biological uptake are all important to the distribu- 2–4%) during low runoff periods (Schemel et al.
tion of dissolved silicate. 1996). Algal production probably accounted
for some of the additional POC during summer
(Tipping et al. 1997). The first major freshwater
SPM and sediments as repositories of organic
flood of winter that followed a period of relatively
carbon
low runoff could have higher POC levels, due to
Particulate organic carbon (POC) in estuaries and
mobilisation of terrestrial organic matter produced
coastal waters is derived from a number of sources
over the previous summer. Phytoplankton produc-
and, as such, its proportion in SPM varies in both
tion and fluvial loadings accounted for 85% of the
time and space. The observed POC distributions in
organic carbon sources (Jassby et al. 1993). Even if
the estuary are derived from three main sources:
only 10% of the fluvial loadings were biologically
first, fluvial plankton, both living and detrital;
available, these two sources would still account for
second, terrestrial POC detritus, mostly carried by
62% of the total.
the non-tidal, freshwater river during floods (typi-
In addition to observed seasonality, freshwater
cally 3% by dry weight of SPM; Meybeck et al.
inputs of POC are also susceptible to catchment-
1988); third, marine plankton with POC content
derived organic pollution. Bed sediment samples
up to 20%.
from the Cochin Estuary, India, had POC
Estuaries often show a POC decrease from the
contents of 0.7–3.8%, with a mean of 1.6%
non-tidal, freshwater river to the estuarine ETM.
247
(Sankaranarayanan and Panampunnayil 1979). In input is through riverflow it can also occur in the
general, values were highest during monsoon form of non-saline groundwater movements. There
periods of large freshwater runoff and they related are exceptions however, such as saltmarshes with-
this feature to the deposition of water-borne plant out any direct links to, or connections with, fresh
and animal matter from terrestrial sources. A water systems. These marshes can still be seen to be
similar study two decades later found a substantial fully functional in all respects but the potential
increase in maximum POC levels that ranged from magnitude of the fluxes that they can generate, in
0.2% to 6.2%, with a mean of 3.6%, due to organic respect of sediment, mineral and organic matter is
pollution (Seralathan et al. 1993). Mud samples inevitably reduced. They are generally much smal-
accounted for the highest POC values and muddy ler systems although their proportional contribu-
sand samples the least – the result of a strong, tion can still be very significant.
negative correlation between POC content and par- Saltmarshes (including the associated freshwater
ticle size. A similarly strong correlation between marshes) provide a living link and buffer between
POC and bed-sediment particle size was observed land and sea (Figure 3; Boorman 2000). They are
in the sewage-polluted estuary of Tolo Harbour, able to withstand much of the erosive power of the
Hong Kong, where POC reached about 2% sea during periods of storms. The erosion losses are
(Thompson and Yeung 1982). subsequently made good by increased rates of
Significantly, POC plays an important role in accretion during calmer periods (Pethick 1992).
oxygen utilisation, and hence water quality, within The possibilities for the export of organic matter,
the estuary. For example, suspended particles in the produced in the saltmarsh, to adjoining marine
Mississippi River Estuary were observed to have ecosystems have also been widely recognised
an average POC content of 1.8%, with offshore (Adam 1990; Lefeuvre and Dame 1994). The extent
particles having higher POC fractions (Trefry et al. to which export occurs appears to vary consider-
1994). There was a positive correlation between ably from marsh to marsh (Dame and Lefeuvre
SPM concentration and apparent oxygen utilisa- 1994). Saltmarshes can also act as sources or as
tion, which is due both to respiration of POC by sinks for specific components; these components
bacteria attached to the SPM, and to the existence range from organic pollutants, heavy metals and
of hypoxic waters on the Louisiana Shelf. radionuclides through the various mineral nutri-
ents to the organic and inorganic components of
´
sediments themselves (Gueune and Winett 1994).
However, future climate change, sea-level rise and
Saltmarshes
changing storm magnitude and/or frequency may
result in erosion of this sediment and remobilisa-
The importance of saltmarshes
tion of its potentially toxic pollutants. Past erosive
phases have been interpreted from morpho-sedi-
Traditionally, saltmarshes have been largely con-
mentary evidence of historical records of metal
sidered in the narrow sense of the vegetated part of
pollution in the Severn estuary (Allen 1990a). The
the tidal flat. Recent studies have been extended to
significance of future remobilisation lies in the fact
include the whole ecosystem including the vege-
that many of these saltmarsh sediment stores
tated creeks and the mudflats occurring at lower
contain heavy metals, radionuclides and organic
levels at the seaward edge of the marsh. The same
pollutants in concentrations well above currently
consideration must be given to the upper edge
accepted safety limits (Oldfield et al. 1993;
of the saltmarsh where there are gradations to
Valette-Silver 1993; Williams et al. 1994; Leggett
terrestrial communities and, more importantly,
et al. 1995).
gradations to brackish and fresh water marshes
The fate of organic material produced by the
still with a tidal influence. These intermediate habi-
saltmarsh vegetation is determined by various
tats have long been recognised in major river sys-
environmental factors, such as the local water cur-
tems such as the Mississippi, but they also occur in
rents and the maturity and the age of the saltmarsh
both a floristic and functional sense in many very
(Troccaz et al. 1994; Lefeuvre and Dame 1994;
much smaller systems that have transitions from
Boorman et al. 1994a; Lefeuvre 1996).
salt water to fresh. While usually the freshwater
248
Nutrient exchanges occurs during the processes of the decomposition
of organic matter, direct losses by the leaching of
The fluxes of mineral nutrients are dependent both nitrogen, phosphorus and also carbon, from live
on the external nutrient loading of the estuary, that plant tissues can occur as well (Turner 1993). The
is the degree of eutrophication, and on the release amounts released are high enough to account for
of nutrients by the decay of organic matter within significant increases in the activity of the estuarine
the saltmarsh. Within a saltmarsh system there are plankton community and consequently are of
considerable seasonal variations in the concentra- potential significance for many other estuarine
tions of inorganic nitrogen and these variations are communities.
even more marked when exchanges between the
different forms of nitrogen (NO3 –N, NO2 –N, and Saltmarsh sedimentation
NH4 –N) are considered. Overall, in studies in
France, England and the Netherlands, there The transport of sediment to a saltmarsh by the
appeared to be a net export of dissolved nitrogen tidal flow of sediment-rich estuarine waters into
out of the saltmarshes (Boorman et al. 1994a). The and over the marsh is a crucial part of the vertical
fate of phosphorus was less clear; phosphate may development of the surface of the marsh. Models
be exported for at least part of the year. Most of the of saltmarsh deposition consider two primary
differences in the fate of nitrogen and phosphate mechanisms of accretion: the tidal ramp and tidal
can be explained by their different sorptive beha- creeks. In the ramp model, decreasing tidal flat
viour because P is strongly adsorbed onto clay and elevation in a seaward direction increases the
N is not. The concentrations of both nitrogen and frequency and duration of tidal inundation
phosphorus in saltmarsh creeks will depend on the (¼hydroperiod). In this case, sedimentation rate is
balance between the supply from inside and outside a decreasing function of elevation within the tidal
the marsh and the rate of uptake by the growth of frame (Pethick 1981; Allen 1990b; French 1993).
saltmarsh vegetation. Saltmarsh accretion via creek processes considers
It appears that low concentrations of phos- the incoming sediment-laden waters to be funnelled
phorus can be the main limiting factor in determin- by the creek network, which then overspill once the
ing the productivity of saltmarsh vegetation within altitude of the fringing saltmarsh is exceeded.
eutrophic estuaries such as the Westerschelde, The In this example, sedimentation is focused in the
Netherlands, and the Tagus, Portugal, having immediate vicinity on the creek margins in the
enhanced productivity levels (Lefeuvre 1996). This form of levees, and vertical accretion is achieved
is in contrast to the situation regarding the primary by lateral creek migration over the long term
productivity of estuarine waters where, under sal- (Bridges and Leeder 1977). Providing that the salt-
ine conditions, phosphorus is released from terri- marsh is not limited by an insufficient supply of
genous sediments upon contact with seawater and sediment or spatially constrained by levees pre-
nitrogen becomes the limiting factor (Doering et al. venting its landward movement, a saltmarsh can
1995). There is a complex pattern of variation respond to sea-level rise via an landward retreat
in phosphorus levels with, on the one hand, the and/or enhanced sediment accretion.
release of phosphate as described above and, The magnitude and direction of sediment trans-
on the other hand, the large scale removal of port depends on the velocity of the water flow and
phosphorus by its take-up from solution during the size and density of the sediment particles. The
algal blooms. Where adequate levels of both phos- bulk flow of the water along a creek is the major
phorus and nitrogen occur, other elements, such as mode of sediment transport but there can also be a
silicon, can become limiting (Jacobsen et al. 1995). significant transport of material by near-bed move-
It is clear, however, that saltmarshes are charac- ments of hyper-concentrated suspended sediment.
terised by their large nutrient storage capacities Techniques have been developed to determine both
and that, under certain circumstances, these stores these components (Troccaz et al. 1994; Hemminga
can become ‘leaky’ with subsequent nutrient et al. 1996a). The effect of water velocity on the
releases (Turner 1993). While generally the release behaviour of the suspended sediment load is cru-
of nitrogen and phosphorus from the saltmarsh cially affected by the particle size of the sediment
249
involved. Studies at Stiffkey, Norfolk, UK, showed Sediment trapping by plants
a significant relationship between sediment load
and peak tidal velocity (r2 ¼ 0.85, p < 0.001) Distinct patterns of sedimentation and vegetation
whereas at Tollesbury, Essex, there was no signifi- dynamics start in the initial stages of colonisation
cant relationship between water velocity and by plants (Richards 1934; Gray 1992). Both
sediment load. This reflects the much higher topography and vegetation affect sedimentation
proportion of coarser sediment particles at dynamics in a feedback process. Sedimentation
Stiffkey (10% > 63 m and 40% < 2 m) compared is increased when the vegetation is taller than
with Tollesbury (2% > 63 m and 60% < 2 m). 8 cm (Boorman et al. 1998). Among the taller
The pattern of distribution of the deposited sedi- species, Spartina anglica enhances sedimentation
ment on the marsh surface is also determined by in European saltmarshes (Thompson 1990;
the sediment particle size (Pitman 1993; Boorman Gray et al. 1991; Sanchez et al. 2001). Puccinellia
1996). With the coarser sediments at Stiffkey there maritima is a perennial grass species whose popula-
was a relationship between the quantity of sedi- tions reach their maximum extent on stabilised
ment deposited in a single tide and the distance lower marshes. It also occurs in the pioneer zone,
from the nearest creek, leading to the build-up of possibly where sedimentation has started.
distinct levees. Similar results have been obtained These processes have been studied at three sites
from a Juncus roemerianus marsh in Florida of Mont St Michel Bay (Langlois et al. 2003). In
(Leonard et al. 1995). At Tollesbury, however, the this area, S. anglica is quite rare in the lower marsh
quantity of sediment deposited was independent of and P. maritima is abundant. P. maritima plays a
the distance from the creek and the deposition of role in the sediment dynamics and in the establish-
the predominantly fine sediment was largely lim- ment of micro-topography in the pioneer zones
ited to the short period at high tide when the water of lower saltmarshes. At the beginning of plant
movements were very slow. Fine grained deposi- colonisation, on mudflats, the community was
tion may, therefore, be dependent both on hydro- composed of S. anglica, Salicornia fragilis and few
period and distance to the creek (Pethick 1992; clones of P. maritima. The micro topography is
Pethick et al. 1992). absent. This plant community changes rapidly in
Rates of sediment accretion recorded at composition and in topography. We can observe
Tollesbury averaged 4.27 mm yearÀ1 over two years after 2 years a low marsh with a large cover of
of observation (Boorman 1996) but there were con- P. maritima associated with Suaeda maritima
siderable fluctuations from month to month with with a low cover and Aster tripolium. For example,
up to 6.2 mm monthÀ1 of erosion and 4.1 mm the cover of P. maritima increased significantly
monthÀ1of accretion. Calculations of the annual from 20% up to 95%. This plant community is
rate of accretion from the tidal sediment fluxes gave localised mainly on hummocks. In the presence of
a potential accretion rate of 3.9 mm yearÀ1, suggest- P. maritima, sedimentation was significantly higher
ing that the regular tidal inputs formed the major than on bare mudflats. Ninety five per cent of the
mode of sediment addition to the marsh surface. At of the P. maritima colonies were recorded on hum-
Stiffkey the rates of accretion of the marsh surface mocks and only 1% on the mudflats. By contrast,
were slightly smaller (3.08 mm yearÀ1). Here, how- all the S. anglica was encountered on mudflats, as
ever, the estimated tidal sediment fluxes were only well as all the annual S. fragilis. At the same sites,
able to account for 0.3 mm yearÀ1, a finding sug- 99% cover by S. maritima was recorded on hum-
gesting that the major sediment input comes with mocks and only 1% on the mudflats. No plants
episodic storm events. Similarly, high rates of accre- were found in the erosion zones in the low marsh.
tion in low Spartina foliosa marsh were almost The sedimentation dynamics in lower salt-
entirely limited to episodic storm-related events marshes are related to both abiotic (e.g., distur-
(Cahoon et al. 1996). On the short time scale, rates bances) and biotic factors (e.g., the vegetation type,
of saltmarsh accretion can be huge, reaching 1 m its cover and height) (Adam 1990; Pethick et al. 1992;
yearÀ1 (Li et al. 2000; Plater et al. 2002). However, Moeller et al. 1997; Dijkema 1997; Boorman et al.
when time-averaged, these rates are much smaller, 1998). Sedimentation is largest in the lowest parts of
typically a few millimetre per year. the marsh provided that vegetation is present
250
(Richards 1934; Kamps 1962; Randerson 1979; matter after plants die remains little known. Some
Stumpf 1983). In the Mont St Michel Bay, signifi- of this organic matter is moved by the tidal flow in
cant sedimentation only occurred in areas covered and out of the marsh, and the end result depends
by P. maritima (Langlois et al. 2003). Puccinellia sp. on the form of the organic matter. Three compo-
was also involved in the geomorphogenesis of the nents exist: dissolved organic matter (DOM) that is
lower marsh, as evidence by the positive linear rela- operationally defined as the fraction of organic
tion between the area of hummocks and the abun- matter not retained on 0.45 m filters; fine (sus-
dance of Puccinellia sp., leading to the formation of pended) particulate organic matter (POM); and
hummocks on which it is dominant. The sedimenta- coarse (>0.2 mm) organic matter (COM), which
tion rate reached 82 mm yearÀ1 in the pioneer zone is generally the same as the floating material. The
(Langlois et al. 2003). Similar observations were behaviour of the first two components, DOM and
reported for Danish and Dutch saltmarshes POM, is essentially the same as that of the sus-
(Jakobsen 1954; Jakobsen et al. 1955; Kamps 1962; pended sediment and is based on the flux of the
Andresen et al. 1990; Scholten and Rozema 1990; tidal water flow. The routes and destinations of
Dijkema 1997). the floating material (COM) under quiet weather
On sandy beach plain type saltmarshes, S. anglica conditions are also similar. Under strong wind
initiates the formation of small dunes that are conditions, floating material tends to be wind-driven
rapidly invaded by P. maritima (Scholten and rather than following the normal tidal routes.
Rozema 1990). Once the micro-topography has The magnitude and routes of export of COM
been established, the rates of rise of the substrate is very variable. At the Tollesbury marshes, UK,
and vegetation succession both accelerate. export of COM only amounted to 7–8% of the
Where sand dominates, the ability of Puccinellia total net above-ground primary production
sp. to stabilise the sediment is especially related to (NAPP; Lefeuvre et al. 1994). These marshes are
the density of its root system and the speed of its at a level where they are subjected to regular inun-
spatial spread by rhizome propagation, as well as its dation and, thus, there are regular opportunities
tolerance to burial (Wohlenberg 1933; Richards 1934; for the exchange of organic matter. When the
von Weihe 1979; Langlois et al. 2001). This is a marshes are at a higher level, such as those at the
perennial species that is relatively abundant in Slufter, The Netherlands, the export of COM tends
the pioneer zones, and which has a very fast rate to be dominated by the occasional storm tide when
of spreading (Langlois et al. 2003). S. anglica is the nett export can be 6 times as high as the largest
renowned for its ability to accrete large volumes import (Lefeuvre et al. 1994). Up to 1/3 of the leaf
of sediment, limiting the erosion and enhancing production of Atriplex portulacoides is exported
sediment trapping (Konig 1948; Ranwell 1972; from its point of origin (Bouchard 1996).
¨
Thompson 1990; Gray et al. 1991; Sanchez et al. A key question remains un-answered on what is
2001). However this species has a limited abund- the final destination of this material exported from
ance, typically less than 10% in sandy sediments, the saltmarsh, whether it is re-deposited elsewhere
because sand is not optimal for Spartina sp. espe- in the marsh replacing local losses, or whether it is
cially when in competition with Puccinellia (Ko deposited along the driftline at the marsh edge or
¨nig
1948; Chater and Jones 1957; Dijkema 1983; whether it is exported to coastal waters. In both
Scholten and Rozema 1990). This could be due to Europe and America, studies have indicated that
waves which inhibit the natural spread of S. anglica, usually less than 1%, but occasionally up to 10%, of
and to ergot fungus, which affects seed viability the organic matter produced is exported as COM,
and germination ability (Morley 1973; Groenendijk that is, as floating litter, to coastal waters
1984; Gray et al. 1990). (Hemminga et al. 1996b; Wolff et al. 1979; Dankers
et al. 1984; Dame and Kenny 1986). The export of
POM and DOM is better known. One major factor
Saltmarshes and fluxes of organic matter
is the relative level of the saltmarsh and the corre-
sponding frequency of inundation. This probably
Primary production and decomposition rates are
explains the apparent major differences that have
high within the saltmarsh – often equivalent to
been observed between the marshes of the east
those of tropical rainforests. The fate of the organic
251
coast of North America, where exports of over 40% (Groenendijk 1984; Boorman et al. 1994a; Lefeuvre
of NAPP are cited, and the much lower values 1996).
obtained from European saltmarsh sites (Dame Measurements show that the saltmarshes of
and Lefeuvre 1994). There can also be indirect Western Europe generally produce more than
1 kg mÀ2 yearÀ1 of above-ground dry matter and
exports of organic matter in the form of fish and
other organisms that come in from the sea to feed that this high productivity is achieved both by
within the saltmarsh and then return to the sea. pioneer and mature saltmarsh plant communities
There are also other links between the terrestrial (Boorman et al. 1994a, b; Lefeuvre 1996). Lower
and the marine ecosystems. For example, when productivities than this have been recorded in pio-
tidal water flows over a saltmarsh there is a marked neer communities, which may be due to the incom-
increase in the bacterial component of the plank- plete development of plant cover in the earlier
ton; mussel beds remove 1/3 of this enhanced stages of colonisation and, thus, do not represent
production (Newell and Krambeck 1995). The the full potential for those species or communities.
emerging picture is that, in European saltmarshes, On the other hand, the pioneer communities domi-
the primary production is not easily exported nated by S. anglica are particularly productive,
producing up to 1.5 kg mÀ2 yearÀ1 of above-
because the generally high level of marshes reduces
the frequency of tidal flooding; nevertheless at least ground dry matter. This is very much in line with
a part of primary production is integrated within the high productivities achieved by Spartina
marine trophic webs (Lefeuvre 1996). alterniflora in comparable New World marshes
(Kirby and Gosselink 1976). Many of the middle
level marsh communities are very productive and
The primary productivity of saltmarshes:
this is particularly true for the shrubby species
conversion of excess nutrients into plant biomass
Atriplex portulacoides (NAPP > 2 kg mÀ2 yearÀ1).
However, low productivity can also be observed in
The above-ground productivity of saltmarsh vege-
middle level communities where productivity is
tation is probably the most visible aspect of estuar-
limited by a combination of low nutrient levels
ine productivity and yet at the same time its
and a high degree of inter-specific and probably
determination has proved to be difficult. Much of
also intra-specific competition. Not all saltmarsh
the research has followed the classical method
systems have the full range of marsh types,
involving the monthly cutting and harvesting of
but where they occur the mature communities of
plots during the growing season and comparisons
the upper saltmarsh can also be very productive,
of the density of the living and dead material
with Elytrigia aetherica producing as much as
(Linthurst and Reimold 1978; Boorman et al.
1.5 kg mÀ2 yearÀ1 (Boorman et al. 1994a).
1994b; Lefeuvre 1996). There is an ambiguity in
Four major components of productivity can be
the method because some workers separate the
recognised in an estuarine ecosystem, namely the
dead material into standing dead and litter, using
production of the saltmarsh vegetation of higher
the standing dead in the determination of produc-
plants, the production of the algae associated with
tivity. The inclusion of litter in the dead material
the saltmarsh, the production of the microalgae on
can result in an apparent increase of net productiv-
the surface of the inter-tidal flats, and the producti-
ity by up to 10% (Bouchard 1996). Measurements
vity of the algae suspended in the water column. In
are usually carried out at monthly intervals, and do
some estuarine systems there are also submerged
not account for the loss from the dead biomass
communities of aquatic plants (sea grasses) but in
between the monthly sampling as a result of flush-
the systems under consideration their distribution
ing of the marsh by occasional, short-lived wind-
is limited and their overall contribution is small. It
driven currents. Thus, the measurement errors may
has to be noted, however, that in certain commu-
be large (Kirby and Gosselink 1976; Boorman et al.
nities, where conditions of sediment stability and
1994b). Despite these complications, this determi-
the clarity of the water favour their growth, they
nation of the primary productivity of saltmarsh
can make a significant contribution to the overall
communities, based on monthly sampling, can
productivity (Heip et al. 1995). Saltmarsh vegeta-
provide a reasonable picture of the range of produc-
tion has both above- and below-ground production.
tivity of northern temperate saltmarsh vegetation
252
In terms of the contribution of the marsh to the phenomenon is often seen in estuaries, such as
estuary as a whole it is the above-ground those in drowned river valleys, where a saltmarsh
productivity that is more important, because abuts high ground with the possibility for artesian
the products of above-ground productivity can be flows. Such flows are common also in the uplands
transported by water currents from the wetlands to (Jones 2002). Because many saltmarshes have layers
the estuary. The products of below-ground produc- of coarser sediments, the tidal movement in the main
tion are largely retained within the saltmarsh sub- creeks can be reflected by changes in the ground
system (see the section on ‘Saltmarsh impact from water levels and flows at considerable distance
the flow of ground water’). from the creeks. Seepages can frequently be seen in
The mean productivity of saltmarshes has been the banks of saltmarsh creeks and while these are
estimated as 500 g mÀ2 yearÀ1 at Tollesbury, UK, generally local saline flows, the existence of non-
and 450 g mÀ2 yearÀ1 at Stiffkey, UK (Boorman saline groundwater is likely to result in a significant
1996). These values correspond to carbon produc- freshwater contribution to the marsh system.
tion of 200 and 180 g C mÀ2 yearÀ1, respectively. The contribution of groundwater flows to salt-
The growth of algae within the saltmarsh is marshes can be significant. For example, during
included within these values; it probably accounts periods of low precipitation artesian groundwater
for 5–15 g C mÀ2 yearÀ1. The value is low because can account for essentially all of the freshwater
of the relatively small contribution of algae to the input to a marsh ecosystem in North Carolina
total biomass. However, in local areas dominated (Gramling et al. 2003). In studies in South
by macroalgae, algal production can be similar to Carolina, the ground waters in the marshes are
that of the vascular plants of the saltmarshes them- mixtures of sea water and fresh water and measur-
selves. The overall productivity of the English salt- ably mediate land–sea carbon fluxes (Cai et al.
marshes is generally similar to those in France and 2003). One of the species indicative of the presence
The Netherlands, although the marshes in the of freshwater inputs into saline environments is the
highly eutrophic Westerschelde have a markedly common reed, Phragmites australis, and the spread
higher productivity (Lefeuvre 1996). The micro- of this species has been linked to fresh groundwater
algae of the intertidal flats are important for their flows reducing salinity and the concentration of
productivity and for their contribution to the sulphide-reducing soils (Chambers et al. 2003).
stabilising of sediments through the excretion of The concentrations of the macronutrients N and
carbohydrates (Holland et al. 1974; see the section P influence the growth and development of salt-
on ‘Formation of mudflats’). marsh vegetation, and they are influenced by
The productivity of north temperate microphyto- groundwater flows (Corbett et al. 2002).
benthic communities is in the range 30–230 g C mÀ2 It is clear that saltmarsh nutrient fluxes can be
yearÀ1 (Heip et al. 1995). A typical value, and close to affected by the hydrological situation, particularly
that quoted for several English and Dutch sites, the magnitude and status of groundwater flows
would be of the order of 150 g C mÀ2 yearÀ1. This is (Anderson et al. 2001; Sutula et al. 2001). Studies
lower than the estimate of 200–300 g C mÀ2 yearÀ1 on the links between land and sea as mediated by
for the Delaware Estuary, USA, and is in line with coastal ecosystems need to take this additional
the estimate of 125 g C mÀ2 yearÀ1 for the factor into consideration. Hydrological dynamics
Westerschelde (Pennock and Sharp 1986; Nienhuis must be seen as a crucial factor for ecological pat-
1993; Jacobsen et al. 1995). terns and processes and, indeed, as the key over-
all linkage between soil, climate and vegetation
(Rodriguez-Iturbe 2003). Methods are available
Saltmarsh impact from the flow of ground water
for quantifying groundwater discharge at the
land-sea interface (Gramling et al. 2003).
The dominant factors in saltmarsh development
are the regular tidal coverage by seawater and the
varying salinity. An additional factor is the outflow Saltmarshes as fish habitats
of fresh groundwater (Figure 3). This can lead to
anomalies in the vegetation, exemplified by the The primary productivity of saltmarshes is largely
local increase of less salt-tolerant species. This assimilated though the degradation and release of
253
organic matter, particularly through the activities inundated. The total import flux of sediment at rising
tide was 12 kg mÀ2 tideÀ1, and the total export flux
of marine and terrestrial invertebrate species.
of sediment at falling tide was 2 kg mÀ2 tideÀ1. This
Associated with this are complex food chains lead-
ing to the larger vertebrates, including econo- implies for this area a net import in the mangrove
forest of fine sediment of 10 kg mÀ2 tideÀ1. Most of
mically significant species of fish. Fish form an
important component in many estuarine ecosys- this sediment settled in the mangroves within 50 m
tems. There are numerous fish species; for example, of the creek. That the mud was able to advance so
as many as 50 in the medium-sized Mira Estuary, far into the mangroves and did not deposit within a
Portugal, where, in terms of number and of bio- few m of the banks, was due to the fine-scale tur-
mass, the Common Sole (Solea vulgaris) is the most bulence of the flows through the vegetation keep-
abundant species (Costa 1988; Costa et al. 1996). ing the fine sediment in suspension. Deposition was
Pristine habitats support an extremely rich bio- observed to occur just before slack high tide and
diversity that can support commercial fisheries, lasted only about 30 min.
the success of which is closely linked to contin- Wolanski et al. (1998a) repeated such studies in
ued habitat conservation (e.g., Al-Mohanna and the mangroves of Hinchinbrook Channel, Australia,
Meakins 2003). Fish are an indicator of saltmarsh and found that the mangroves accumulated mud
at a rate of about 1000 ton kmÀ2 yearÀ1. To deter-
ecosystem health; indeed, the success of habitat
restoration can be judged by the extent to which mine how this sediment-trapping rate varies with
the fish populations have recovered (Tupper suspended sediment concentration in the rivers,
and Able 2000). A well-developed creek system is further studies were undertaken in the mangroves
important in this recovery (West and Zedler 2000). of the Ngerikiil and Ngerdorch estuaries in Palau,
Micronesia (Victor et al. 2003). These two rivers
drain adjacent, similar, mountainous areas of com-
parable area in the wet tropics. The former catch-
Mangroves
ment is being deforested, the latter is still forested.
This results in a factors of 10 differences between
There are many similarities between physical and
the sediment yield in these two catchments. The
biological processes in mangroves and saltmarshes.
mangroves comprise 3.8% of each of these two
Similarities include their role in trapping sediment,
river catchments, they flood semi-diurnally, and
converting nutrients to plant biomass, trapping
in both cases they trap about 30% of the riverine
pollutants and serving as a habitat for fish and
fine sediment. This mud trapping efficiency, while
crustaceans (Figure 3).
helpful, is however not sufficient by itself to pre-
vent degradation of coastal coral reefs from exces-
Mangrove sediment dynamics
sive sedimentation from extensive land clearing
and poor farming practices; better land care is still
During spring tides, mangroves are fully inundated
required in the river catchment.
at high tide. A deep tidal channel, fringed by inter-
Mangroves are, thus, very efficient mud traps,
tidal, mangrove forest, is usually maintained by
and this may help mangroves to keep up with the
self-scouring; this is due to a difference between
expected rise in sea level. The level of success will
peak water velocities at rising and falling tides
depend on the availability of sediment from river-
(Wolanski et al. 1980).
ine inflow and from the coastal zone (Chappell and
Within the mangrove forest, the velocities are
Woodroffe 1994; Wolanski and Chappell 1996). As
slightly larger during the rising tide than during
sea level is predicted to rise, history may be a good
the falling tide (Furukawa et al. 1997). At their
guide to predict how mangroves will fare in the
study site near Cairns, tropical Australia, the
future. Following the last glaciation, the sea level
suspended sediment concentration peaked at
rose at a rate of up to 1 cm yearÀ1. At those sites
about 150 mg LÀ1 on the rising tide and at about
30 mg LÀ1 on the falling tide, indicating import and where sediment accumulation rate kept up with the
sea level rise, the mangrove forests survived. At other
trapping of sediment. Sediment was only imported
sites mangrove migrated landwards. Elsewhere the
at spring tides; there were zero fluxes into the man-
mangroves were submerged and perished. Their
groves at neap tides because the swamp was not
254
burial sites are still recognisable by the mangrove causality (Boto and Wellington 1983, 1984; Boto
mud at the bottom of some tropical coastal seas. et al. 1989; McKee 1993). There is a pressing need
This old mangrove mud is often capped by a layer to integrate existing knowledge to understand how
of new sandy or calcareous sediment brought in a mangrove ecosystem works. The key concepts
by wave action on a sandy beach (Grindrod and include: flushing time, inundation frequency, resi-
Rhodes 1984). After sea level stabilized about 5000 dence time, degree of water-logging and stagnancy,
years ago, mangroves reestablished themselves at hyper-salinization of soils, acid sulfates, water
some, but not all, sites where they still exist. At depth and stagnancy. All of these are interrelated
other sites, the old mangrove forest has perished variables that can drastically change sediment char-
and has been buried by recent freshwater sediment acteristics. Topographic differences of a few cm can
from riverine inflow (Chappell 1993; Chappell generate major differences between sites with con-
and Woodroffe 1994). In mud-poor, reef environ- trasting sediment and vegetation characteristics.
ments, mangroves were unable to keep up with sea For example, in North Brazil, in transition areas
level rise and drowned. They recolonised the area between mangroves and saltmarshes, elevation
increases of $10 centimetre can be reflected in a
only when sea level stabilized (Fujimoto et al.
1996). In other cases such as in the Gulf of decrease of average inundation frequency from
60 to 40 days yearÀ1. This produces differences in
Carpentaria, tropical Australia, isostatic adjust-
ments lifted the mangroves out of the water, trans- sediment quality causing the existence of an ecotone
forming them into a hypersaline mudflat (Rhodes bordered by a monospecific Avicennia forest and
1980). an herbaceous plain. Mangroves react quickly and
Recent studies by Furukawa (pers. comm.) in dramatically to variations in relative sea level
mangroves fringing mud-poor reef water of (Lara et al. 2003, and references therein).
Iriomote Island, Japan, show that small wind In addition to inundation dynamics, other fac-
waves resuspend the mud, which is then exported tors are influenced by changes in relative land-sea
at ebb tide. As a result the mangroves are nutrient- level. The degree of exposure of sediments to air is
starved and stressed. They are stunted and wea- important for determining, for example, the nutri-
kened and woodborers extensively attack them, ent availability and the salinity. Tidal inundation
because stressed trees generate less tannin than produces a diffusion of oxygen from the flooding
healthy trees. water into the sediment and influences salt trans-
port (R. Lara, pers. comm.). The frequency of
inundation determines soil salinity and the fre-
Nutrients in mangrove sediments: trapping and
quency of anoxia (Boto and Wellington 1984;
mobilization
Leeuw et al. 1991). In turn, this controls nutrient
availability because a drier surface layer with lower
The capacity of mangroves to trap fine sediments is
permeability to gas exchange causes more hypoxic
essential for the ecosystem. This includes among
conditions.
others the role of sediments as a physical substrate
Mangrove sediments are commonly anaerobic,
and as a nutrient source for forest development.
organic-rich intertidal/estuarine muds. Yet, there
The role of biogeochemical transformations within
are also substantial differences in the textural
the sediment is relatively well understood (i.e.,
and physicochemical properties according to forest
microbial processes, plant/sediment relationships);
type and inundation dynamics. In contrast to
however, less is known about the role of system-
muds in temperate saltmarshes, mangrove muds
shaping agents such as inundation dynamics on
are less anaerobic, probably because of crab holes
nutrient trapping or mobilization.
and the low bioavailability of organic matter
Few researchers treat these issues in a simulta-
(Alongi and Sasekumar 1992). Crabs are a primary
neously detailed but integrative manner, building
means for incorporation of significant amounts of
causal chains between hydrology, sediment proper-
organic matter from leaf litter fall deep into man-
ties, and vegetation structure. Numerous observa-
grove sediments as well as causing an active turn-
tions have been summarized within empirical rules
over of detritus at the sediment surface (Schories
such as ‘Avicennia prefers high places’, but relatively
et al. 2003). Key characteristics of mangrove
seldom has anyone looked beyond correlations into
255
sediments such as redox potential or sulphide levels (Walbridge and Struthers 1993). Particularly
can be also modified by the root systems of adult redox potential has a significant effect on the P
trees (McKee 1993). availability in mangrove sediments. Increasing
Other typical features of mangrove sediments are inundation frequency elevates redox potential in
relatively low concentrations of dissolved inorganic the sediment, that is, it creates more oxidative con-
nutrients, for example, nitrate, ammonium and ditions by diffusive aeration, and elevates the con-
phosphate in porewater, and the presence of tan- centration of available P. At lower elevation sites,
nins derived from leaching and decomposing roots the higher flooding frequency accounts for a higher
and litter (Alongi 1987; Boto et al. 1989). The total P content. Higher P availability is reflected
reasons for low nutrient concentrations in pore- in vegetation biomass (Boto and Wellington 1984)
water are rapid turnover, such as high uptake by and P content in leaves (Lara, unpubl. data).
trees and bacteria, as well as redox conditions and Freshwater flood plain ecosystems are controlled
the recalcitrant character of organic matter. by a combination of topographic, hydrographic and
Ammonium is the main form of inorganic N in chemical processes (Townsend and Walsh 1998); an
mangrove soils because nitrification is prevented equivalent understanding of mangrove ecosystems
due to the lack of oxygen to oxidize it into nitrite/ is largely missing (Cohen and Lara 2003).
nitrate. Concentrations of ammonium are higher
in muddy than sandy soils and are influenced by
the degree of tidal wetting, rainfall, and biological Estuarine food webs
uptake and production. Concentrations of dis-
solved organic N can be higher than dissolved inor- When summarising key biological processes within
ganic N. The high sodium level in most mangrove the water column, it is particularly difficult to keep
soils displaces ammonium, which is thus mostly a balance between a simplistic description that
found in the interstitial waters where it can be highlights the interplay between components of
leached by rainfall or tidal inundation or drained the system, and a scientifically thorough approach
(Alongi et al. 1992). Tidal inundation generates a that does justice to the ecological complexity,
nutrient exchange between sediment and estuarine where the general picture can be lost. One can
waters (Dittmar and Lara 2001). find in the scientific literature dozens of figures
Interactions between inundation dynamics, sedi- describing estuarine food webs for different estu-
ment characteristics, and vegetation structure in aries. The description of estuarine food webs often
the basin can be particularly well illustrated with gets bogged down in local details. Some of these
reference to phosphorus mobility. For example, figures show simplistic food webs, others show
changes in porewater phosphate concentrations in extremely complex food webs with numerous feed-
sediments beneath mangrove communities in the backs; other figures focus on biomass and highlight
Indian River, USA, seemed to be linked to varia- phytoplankton dominance, while yet others
tions in tidal inundation frequency (Carlson et al. emphasise the key role of detritus.
1983). Due to its dependence on tidal height, P can
become limiting in elevated mangrove forest areas Generic description of estuarine food webs
(Boto and Wellington 1983). Concentration of
dissolved inorganic P in mangroves is generally A generic approach is used to describe food webs in
low (Alongi et al. 1992; Lara and Dittmar 1999). estuaries (Figure 3). Estuaries are converters of
Bacteria and microalgae in mangrove muds are living phytoplankton to detrital particles; they are
also probably P-limited. A close microbe-nutrient- also conveyors of both allochthonous and auto-
plant connection may serve as a path to conserve chthonous detrital particles to the sea. Herbivorous
scarce nutrients necessary for the existence of these fishes transfer energy and matter from estuarine
forests (Alongi et al. 1993). plants to upper trophic levels and to the coastal
Retention of dissolved phosphate and long-term zone. However, direct grazing by herbivores con-
P storage in wetlands is probably controlled by sumes only a small proportion of the macrophyte
adsorption by Al and Fe oxyhydroxides and by and macroalgal production. The great bulk of
precipitation of Al, Fe and Ca phosphates the organic matter produced (sometimes 90%) is
256
processed through the detrital system. Zooplank- production (Rhee 1972). This situation may occur
ton, planktivorous fish, interstitial micro and meio- in estuaries with large fringing saltmarshes and
fauna, surface deposit-feeding molluscs, fishes and with little external input of inorganic N. More
polychaeta, and filter-feeding invertebrates con- commonly though, estuarine waters are often
sume a much greater proportion of the primary enriched with N and P from rural, urban and
production of the phytoplankton and benthic industrial wastewater discharges (Uncles et al.
microalgae. Annual plant growth and decay pro- 1998a, b).
vides continuing large quantities of organic detri- The fate of nutrients depends on a number of
tus. In addition, there is often a considerable input physical, biological and chemical processes.Physical
of detritus from river inflow. Detrital particles and processes include mixing, flushing and sedimenta-
their associated microorganisms provide the basic tion. Chemical processes include absorption and
food source for primary consumers such as zoo- desorption. Biological processes include stripping
plankton, most benthic invertebrates and some of dissolved and particulate nutrients, primarily by
fishes. Many of the estuarine consumers are selec- bacteria and phytoplankton (Alongi 1998). Biologi-
tive and indiscriminate feeders on particles in sus- cal and chemical processes increase in importance
pension in the water column, or in the sediment with increasing values of the residence time because,
that they ingest. Thus, most of the biota that inha- when the residence time is large, there is more time
bits estuaries can be considered as particle produ- for these processes to act (see the section on ‘The
cers (microalgae and detritus derived from plant flushing of estuarine waters’).
growth) and particle consumers. The first trophic For example, organic N entering the estuary is
level in the estuarine ecosystem is therefore best degraded to ammonia and further nitrified by aero-
described as a mixed trophic level of detritus con- bic bacteria; this increases nitrate concentration,
sumers, which in varying degrees are herbivores, resulting in a convex curve of nitrate versus salinity
omnivores or primary carnivores (Knox 1986). (see the section on ‘Estuaries as sieves for nutrients’
and Figure 4). These biological processes consume
oxygen and lead to depletion of oxygen content if
Environmental factors affecting estuarine and
the water column is not well aerated. Phytoplank-
coastal biota
ton consumes the produced nitrate rapidly, in
which case a removal of nitrate may be observed.
Estuaries are highly productive ecosystems because
High loads of N and P may lead to depletion of
they are often nutrient rich and have multiple
silicate (see the section on ‘Estuaries as sieves for
sources of organic carbon to sustain the population
nutrients’) during phytoplankton blooms, condi-
of heterotrophs. The nutrient sources include riv-
tioning the successive phytoplankton assemblages
erine and waste inputs and autochthonous primary
during the remaining productive period. Sub-
production by vascular plants, macroalgae, phyto-
sequent low river discharges may provide an
plankton and benthic microalgae (Cloern 1987).
environment with low Si : N and N : P relative avail-
These nutrients sustain a trophic structure that
ability, and this may favour dominance of cyano-
varies from estuary to estuary (Degan et al. 1994).
bacteria and even toxic algal blooms (Rocha et al.
Inorganic nutrient loading stimulates primary pro-
2002). Pulses of freshwater discharges (and their
duction and generates a shift to larger-sized phyto-
nutrients) stimulate the development of a more
plankton (Boynton et al. 1982). Organic matter
diversified phytoplankton assemblage that supports
loading stimulates bacterial production, in differ-
zooplankton (Rey et al. 1991), that is, top-down
ent ways depending on the form of dissolved
control, and this hinders harmful algae blooms.
organic matter. Microbes, through the microbial
Geochemical and biological transformations are
loop (Figure 3) serve as N-remineralizers when
concentrated in the ETM (see the section on ‘SPM
metabolizing N-rich organic compounds because
and sediments as repositories of organic carbon’ and
they take up inorganic nutrients when organic
Simenstad et al. 1994). The long retention time of
inputs are deficient in N or P (Kirchman et al.
SPM in the ETM – and the nutrients it contains –
1989). Increased riverine imports of organic matter
enables bacteria to consume particulate nutrients
in an estuary may result in decreased nutrients
and to incorporate them into primary levels of the
availability for algae that may limit phytoplankton
257
estuarine food web by increasing the importance of stocks of prey species attracts secondary and tertiary
heterotrophic processes (Reed and Donovan 1994; consumers in this region (Simenstad et al. 1990).
Thayer 1974). However, this may also result in dep- Salinity changes in the estuary induce a physio-
leting the dissolved oxygen (see the section on ‘SPM logical stress in organisms that must then spend
and sediments as repositories of organic carbon’). extra energy in physiological adaptation changes.
Light penetration is a limiting factor in the ETM, This may reduce growth and body size in organ-
inhibiting phytoplankton production (see the sec- isms. In fact, because of the extra energy spent in
tion on ‘Estuaries as sieves for nutrients’). This physiological adaptation, brackish water organ-
decrease of primary production by phytoplankton isms are usually smaller than their marine relatives
results in minimal excretion of photosynthetic pro- (Harris et al. 2000). Production and consumption
ducts that are a food source to bacteria (Goosen are determined by the relative availability of food
et al. 1999). When SPM exceeds 50 mg LÀ1, phyto- (Simenstad et al. 1990). The critical process deter-
plankton photosynthesis is severely limited mining the impact of hydrology on estuarine and
(Cloern, 1987). In estuaries where primary produc- coastal food webs is related to organism size. Size is
tion rates are less than 160 g C mÀ2 yearÀ1, phyto- a determinant feature in prey–predator interac-
plankton are light-limited and these systems can be tions. Thus different food webs for the same species
considered heterotrophic (Heip et al. 1995). The can exist in estuaries and coastal areas. Similarly,
phytoplankton biomass can also be regulated by natural and human-induced changes in river flows
vertical stratification in salinity (Cloern 1987). lead to different food webs.
Microphytobenthos comprises the microscopic,
photosynthetic eukaryotic algae and cyanobacteria Feedback between biological and physical
that grow on, or close to, the sediment–water inter- processes
face in illuminated inshore areas. Microphyto-
benthos, anaerobic and chemosynthetic bacteria There are numerous feedbacks between physical
may also contribute to microphytobenthic produc- and biological processes in considering food webs;
tion. Because they are found at the sediment inter- a few dominant ones are summarized here. Larval
face, the microphytobenthos also modulates the and juvenile species exist in estuaries, both in the
exchange of nutrients between the sediment and water column and the bottom. Coastal species,
water column (Ser^dio and Catarino 1999).
o such as the anchovy Engraulis encrasicolus or the
Freshwater phytoplankton and bacterioplank- crustacean Crangon crangon, use the estuary as a
ton in the river are subject to salt stress when fresh- ´
nursery ground (Chıcharo et al. 2002). In turn, these
water mixes with saltwater (Flameling and groups affect the uptake and release of organic and
Kromkamp 1994). The freshwater microbial popu- inorganic materials (from excretion products) and
lations die in this zone (Goosen et al. 1995). influence fluxes between water and sediment.
Salinity changes also affect estuarine plankton On the mud and sand flats, detritivorous macro-
and some invertebrate species (e.g., bivalves). Salt- benthic invertebrates benefit from the deposition of
hardy invasive species are increasingly invading particulate organic matter that they use as a food
this ecological niche. For instance, salinity changes source (Snelgrove and Butman 1994). These
in the Guadiana Estuary, Portugal, favour the benthic organisms change the physical and chemi-
introduced, salt-resistant, Asian clam (Corbicula cal properties of sediments (see the section on
fluminea) that is increasingly out-competing the ‘Formation of mudflats’), modifying, in turn, the
salt-intolerant indigenous clam (Anodonta cygnea) bottom shear stress, currents and flushing of the
´
(L. Chıcharo, pers. comm.). estuary (Jumars and Nowell 1984; Brown et al.
Estuarine fishes suffer less impact from salinity 1998). Through bioturbation, these organisms can
changes due to their ability to utilize osmotic reg- also cause sediment mobilization or, conversely,
ulation (Whitfield and Wooldridge 1994; Thayer increase stability as in the case of dense mats of
et al. 1999). In the ETM, primary consumers are worm tubes, or by enhancing the deposition of fine
generalist and omnivorous feeders, capable of ex- particles through biodeposition (Schaffner 1990;
ploiting both autotrophic and heterotrophic food Luckenbach et al. 1988; Widdows et al. 1998;
web pathways. The presence of higher standing Noji and Noji 1991; Ayukai and Wolanski 1997).
258
The amphipod Corophium volutator (Pallas) may et al. 2002). Strong outwelling or river plumes are
reduce sediment binding by grazing benthic dia- important for these species because they require tur-
toms that contribute to sediment aggregation bidity or salinity signals for orientation to find ade-
(Gerdol and Hughes 1994). Birds, by feeding on quate spawning areas (Kingsford and Suthers 1994).
this amphipod, decrease the erosion of the sedi-
ment (Brown et al. 1998).
Enhanced erosion and resuspension of sediment Human health and aquatic ecosystems
through bioturbation may result in greater sedi-
ment accretion on adjacent saltmarshes (Brown Ecological integrity is central to human health
et al. 1998). Bivalves also play an important role (Epstein 1999). Threats to human health arising
in reducing phytoplankton densities and control- from human interaction with aquatic ecosystems
ling algal blooms (Takeda and Kurihara 1994). can originate from multiple factors, which might
Attrition of particles in the gut, together with selec- be broadly grouped into three main categories:
tive feeding of fine particle sizes by suspension and * derived from the effects of water pollution (che-
deposit-feeders and incorporation into faecal mical, microbiological, radioactive, thermal) on
pellets, results in deposition of finer mineral and humans and on the physiology of individual
organic particles in the surrounding sediment, with organisms;
an increase of absorbed and adsorbed nutrient and * those resulting from management of aquatic
trace elements concentrations (Brown 1986). The resources (e.g., wetland drainage, aquaculture,
resulting availability of these nutrients enables the dam construction);
growth of macrophytes and macroalgae, which are * effect of global changes affecting climate and
contributors to the primary production in estuaries the hydrological cycle (e.g., habitat degradation,
(Simenstad et al. 1990). warming, increased rainfall, storms).
These categories are interlinked.
The transition between estuaries and coastal
zones Examples of health threats derived from water
pollution
Compared with the extensive production and
transport of detrital carbon, the consumption by A wide range of activities on land contributes to the
detritivores constitutes only a minor reduction release of contaminants (typically organic chemi-
of the organic matter exported to the nearshore cals, heavy metals, microorganisms, sewage, nutri-
coastal waters. Detritus can be removed via grazing ents) into the sea by rivers, run-off, groundwater
by zooplankton, epibenthos and benthic fauna. and the atmosphere. The bulk of these contami-
The exchange of zooplankton between estuary nants remains in coastal waters, particularly in
and adjacent coastal waters results in the export poorly flushed areas. Water related diseases cause
of organic carbon (Simenstad et al. 1990). The millions of preventable deaths every year, especially
export of nutrients from the estuary also contri- among children (UNDP 1998). Greater incidence of
butes to primary production in coastal zones. illness due to consumption of contaminated fish and
The coastal fish community in the Guadiana shellfish is an increasing concern. Harmful algal
Estuary, Portugal, changed from planktivorous blooms in many coastal regions in the world cause
during high river inflow periods, to demersal- a number of diseases, including poisoning, neurolo-
carnivorous during low river discharge periods gical disorders and gastroenteritis (HEED 1999;
´
(Figure 3; L. Chıcharo, pers. comm.). During per- UNEP 1999). Increased flooding as a result of
iods of high river inflow, the increase in primary changes in precipitation may cause contamination
productivity and consequent availability of food for of water supplies, which leads to greater incidence
planktivorous pelagic fish species, like anchovy of faecal-oral contamination (WHO 1996). Typhoid
(Engraulis encrasicolus) or sardine (Sardina pilchardus), and paratyphoid fevers, produced by Salmonella
may explain higher fishery catches in the typhi and S. paratyphi A, B, C can occur in areas
´ ´ ´
coastal zone (Soberon-Chavez and Yanez-Aracibia with poor sanitation and inadequate sewerage sys-
´
1985; Ray 1996; Sklar and Browder 1998; Chıcharo tems. They are transmitted by contaminated water
259
and food. Crop irrigation with untreated sewage In some of the tropical regions of the developing
can cause higher Salmonella infection rates among world, the incidence of malaria has increased in
children living in the wastewater-spreading area recent years as the mosquito and the malaria para-
(Melloul and Hassani 1999). site it spreads have evolved more resistance to
Also, eutrophication is depriving lakeside resi- DDT alternatives and to the medications used
dents of good water quality in many densely popu- to prevent or treat the disease (Thurow 2001).
lated areas of the world (UNEP 1999). Malaria now strikes more than 300 million people
In the developed countries, Cryptosporidium and per year and kills about a million of those affected.
Giardia are two of the most common agents of In some regions (e.g., South Africa) DDT is again
surface water-associated infections and disease increasingly used to control mosquitoes. In gen-
outbreaks. These parasitic infections, primarily eral, land claim for agriculture, deforestation and
causing diarrhoea, are difficult to control due to changes in land use most probably bear the greatest
the high resistance of the oocysts and cysts to blame for the climatic and habitat changes produ-
environmental stress and disinfectants. The level cing this development. Yet, it was primarily the
of contamination is particularly dependent on drainage of wetlands for agricultural purposes
the land use in the water-catchment area, on waste- that contributed most significantly to the eradica-
water discharge, and on weather conditions tion of malaria in Europe (Reiter 2000).
(Exner et al. 2001). It is today often erroneously thought that larvae
For decades, a great deal of research has concen- of malaria-transmitting mosquitoes can only
trated on the direct influence of contaminants on develop in freshwater. Furthermore, malaria is
aquatic biota, their bioaccumulation in trophic not restricted to the tropics. Only in 1975 did
webs, and their detrimental effect on humans. the World Health Organization declare that
Examples of threats to human health arising from Europe was free of malaria. About 200 years ago
the degradation ofmarine, freshwater and watershed malaria was a leading cause of death in many
ecosystems are summarised in Table 2. Informa- marshlands communities along the coast of south-
tion about these topics is available in the literature ern England (Reiter 2000). There, extensive salt-
and will not be further related here. marshes provided high-quality grazing for sheep
and cattle, but they were also a favoured habitat
for a highly effective malaria vector, Anopheles
atroparvus, which prefers to breed in brackish
Examples of health threats derived from
water along river estuaries and in the presence of
management of aquatic resources
abundant algae. Until the 19th century, malaria
was a major mortality factor in The Netherlands.
Wetlands
By the end of that century transmission had
These ecosystems filter floodwaters and support
dropped precipitously in the more prosperous
the genetic, species and functional group diversity
countries of North Europe. A major factor contri-
that ensures resilience to stress and resistance to
buting to this decline was that the mosquito habitat
pests and pathogens (Epstein 1999). Mangroves,
was eliminated by improved drainage and extensive
marshes and floodplains have been devastated in
land claim. However, major epidemics occurred in
the last centuries and decades. In the 20th century
Russia and Poland in the 1920s, with high death
alone, some 10 million square kilometres of wet-
rates reaching regions near the Arctic Circle.
lands have been drained across the globe, an area
Today, malaria is again common in many parts
about the size of Canada (Chivian 2002). This has
of Central America, northern South America, tro-
brought losses of habitat and retention capacity of
pical and subtropical Asia, some Mediterranean
water and sediments, as well as the filtering func-
countries and many of the republics of the former
tion for waterborne nutrients. Interrelated issues
USSR. Thus, policies on wetland creation or
affecting aquatic ecosystems such as loss of biolo-
restoration must carefully consider, besides the
gical diversity, endangered physical coastal stabi-
benefits of the re-establishment of lost ecological
lity, floods, and eutrophication make it a priority
services, the potential consequences of increased
to adequately restore these environments and the
areas of slow-flowing or stagnant waters on disease
‘ecological services’ they offer.
260
Table 2. Examples of threats to human health arising from the degradation of marine, freshwater and watershed ecosystems. Modified
from UNEP (1999).
Driving forces Changing ecological pattern Influence on human health/example
Marine
Pollution from oil, industry, naval Deterioration of marine ecosystems from Decrease in life expectancy, skin and eye
operations and sewage discharge imbalances due to dense ship traffic diseases (Black/Azov seas, Caspian Sea)
Biological/bacterial contamination due Effects on fish and algae Typhoid, malaria, diphtheria (Central
to hydrological changes Asia)
Rising sea levels Ecosystem destruction and pasture See main text in the frame of climate
degradation; changes in depth and change
amplitude of water fluctuations lead to
reduction of wildlife habitats
Biological contamination of surface Gastroenteritis, eye and skin infections
water with waste water (UK, France, S. Africa)
Biological and chemical contamination: Shellfish poisoning, wildlife mortalities, Poisonings, diarrhoea, dehydration,
harmful (toxic and nontoxic) algal sunlight penetration prevention, oxygen headaches, confusion, dizziness, memory
blooms from the rapid reproduction shortages, reservoirs for bacteria loss, weakness, gastroenteritis, bacterial
and localized dominance of infections, swimming related illnesses,
phytoplankton neurological diseases, deaths (Florida,
Gulf States, S. America)
Contamination: cholera contaminated Coastal shellfish and fish contamination Cholera (Peru and other 16 countries)
sea plankton due to contaminated
ships’ hulls
Freshwater
Dam construction Inundating of lands Schistosomiasis epidemics
(Senegal River: Manantali and
Diama dams)
Changes in hydrogeological cycle Habitat alteration, new breeding sites for Intestinal nematode infections
mosquitoes (Yangtze River)
Changes in water flow: High levels of suspended soils and faecal Rift valley fever (Senegal)
scarcity and degradation of water bacterial content Faecal infections, intestinal diseases
resources contaminated river systems (S. Africa)
Pollution
Pesticides, herbicides, fertilizers and Water quality deterioration Health problems (general), infant
defoliants. Airborne toxic salts mortality and morbidity (Central Asia)
Water contamination with heavy metals Accumulation in biochains Progressive and irreversible kidney
(Cadmium) damage (Japan)
Faecal water pollution Diarrhoea, Hepatitis (Bangladesh, India)
Declines in ground water levels and Declines have been implicated in the Poisoning (Bangladesh, India)
groundwater contamination widespread arsenic contamination of well
water
Degradation, e.g., fallen water tables, Severe increase in salinity Cholera outbreak (1995), infectious
chemical contamination diseases, intestinal parasites (Gazan
aquifer, Palestine, Israel)
Catchment
Deforestation Erosion, changes in hydrological cycles Malaria (S. Africa)
Firewood shortage, land degradation Trauma, allergic reactions, infections,
‘‘slash and burn’’ use, increased wood respiratory diseases, cancer (Gambia,
transport and processing infrastructure Kenya, South Africa, Zimbabwe)
New breeding grounds for insects Yellow fever (Ghana, Nigeria, Zaire,
Sudan, Liberia, Cote d’Ivoire)
Agroecosystem development: e.g., land New breeding grounds for insects Japanese encephalitis (Japan, China,
inundation for rice growing India)
Intensified agriculture, irrigation Insecticide resistance Malaria (Central America)
schemes damaging ecosystems
261
vector proliferation, particularly under a scenario water of ships. As many as 500,000 people were
of increasing temperatures, which could allow dis- afflicted; there were 5000 deaths.
ease causing organisms to significantly extend their
range.
Habitat degradation
The degradation of ecosystems, such as forests,
Aquaculture wetlands and coastal waters, is the single most
Threats to human health from aquaculture pro- important factor behind the current mass extinction
ducts can arise from the misuse of therapeutic of species. Compared with the knowledge of the
drugs (e.g., chloramphenicol), chemicals (e.g., tri- impact of human pressure on aquatic ecosystems,
butyltin), fertilizers (e.g., raw chicken manure con- almost nothing is known about the effects of
taining salmonella), and viruses from shrimp farms diversity loss in these systems on human health. A
spreading to wild stocks (US EPA 1998). The aban- new category of extremely serious threats to eco-
donment of small aquaculture ponds in tropical system and human health arises from the so-called
countries leads to an extension of mosquito habi- ‘emerging infectious diseases’ (EIDs), which in sev-
tats and concomitant increases in malaria. Escape eral cases arise from human-induced habitat
of exotic species and animal pathogens into the destruction or invasion. EIDs link health and the
environment can have a tragic impact on native fate of humans and wildlife; EIDs are ‘one of
aquatic species. The FAO and the WHO recom- the most significant, yet underestimated, anthro-
mend that the Hazard Analysis and Critical pogenic threats to biodiversity conservation’
Control Points (HACCP) concept be applied to (Daszak et al. 2000). In turn, disturbances in bio-
fresh water aquaculture programs to control food- diversity are probably a driving force in major
borne digenetic nematode infections in humans emerging disease threats to humans. These include
(Garret et al. 1997; and references therein). Hantavirus, influenza, AIDS, Lyme’s disease,
Ebola virus – they are all zoonotic or have resulted
from pathogens switching hosts from wild animal
Examples of health threats derived from global
reservoirs to humans.
changes
If knowledge of the above field is just beginning
for terrestrial ecosystems, much less is known
Global warming
about human-relevant EIDs in aquatic ecosystems.
The effect of global warming on aquatic ecosys-
Mass mortalities due to disease outbreak have
tems, mainly concerning deforestation of water-
recently affected major marine taxa. Emerging dis-
sheds and plankton blooms, is considered one of
eases are associated with algal toxins, and bacteria
the driving forces behind re-emerging diseases such
and viruses are affecting fish, shorebirds, and
as cholera and malaria. Strong evidence links the
mammals. Of great concern also are diseases that
outbreak of cholera in recent years to warmer seas
attack coral and sea grasses, essential habitats that
caused by El Ni~o. There is a biological basis for a
n
sustain mobile species (Epstein 1999). An increase
link between sea surface temperature (SST), marine
in the severity of coral bleaching in 1997–1998 is
ecology, and human cholera (Colwell 1996).
coincident with high El Ni~o temperatures. Such
n
Zooplankton, which feed on algae, accumulate
climate-mediated physiological stresses may com-
Vibrio cholerae and other enteric pathogens. This
promise host resistance and increase frequency of
may explain why cholera follows seasonal warming
opportunistic diseases. New diseases typically have
of SST that can enhance plankton blooms. Vibrio
emerged through host or range-shifts of known
spp. in general are influenced by temperature and
pathogens. Both climate and human activities
salinity (Lipp and Rose 1997). Many others factors
may have also accelerated global transport of spe-
promote algal growth, the reservoir of cholera,
cies, bringing together pathogens and previously
including pollution, eutrophication, the loss of
unexposed host populations. There is hitherto no
mangrove filtration systems, and declining fisheries
evidence of switch in the host-pathogen interac-
in offshore waters. The result was an epidemic
tions at the level of aquatic fauna and humans,
between 1991 and 1993, which started in Asia and
yet this may be ‘absence of evidence, rather than
may have spread to South America in the bilge
262
evidence of absence of a (climate) effect’ (Kovats Human impacts in estuaries from upland
et al. 2001). activities
Direct human impacts on estuaries arise from sev-
Changes in hydrological cycles
eral sources. They range from the consequences
Hydrological change is a main threat to human
of engineering works (e.g., damming of rivers,
health (WHO 2000). Through the joint WHO/
abstraction of water for irrigation, dredging, land
UNICEF programme ‘Health Map’, specific GIS
claim of wetlands and harbour constructions) on
software was developed for this purpose, combin-
the estuarine water residence time and erosion and
ing a standardized geographic database, a data
sedimentation patterns, to the effects of wastewater
manager and a mapping interface. However, human
discharges on the health of biota. Anthropogenic
health is not usually included in interdisciplinary
discharges include an array of chemical conta-
research projects dealing with coastal wetlands or
minants, such as metals, organometals, and petro-
river basin management programs. Increasingly,
leum hydrocarbons, organic compounds from
human health needs to become a major issue to
pesticides, industrial waste products and nutrients.
be considered in management strategies.
Dams built in uplands for water storage or power
There are many pathways through which hydro-
generation can result in lower river flows and raised
logically relevant events can affect health; notably
estuarine salinities. Indirect human impacts in estu-
when a river or stream inundates the flood plains
aries can be just as severe (see below) as a result of
(see the arrow labelled H in Figure 3). This pro-
poor land-use management in the river catchment
duces changes in mosquito abundance (malaria,
and of damming rivers.
dengue fever), or contamination of surface water
with human or animal waste, for example, rodent
urine (leptospirosis). Conversely, droughts can
Impacts from land clearing, soil erosion and
produce changes in vector abundance, for example,
damming
a vector breeding in stagnant river ponds. Flooding
may become more intense with climate change and
Land clearing, overgrazing and other poor farming
can affect health through the spread of disease
practices considerably increase soil erosion
(Noji 1997; Menne et al. 1999). Thus, the elabora-
(Wolanski and Spagnol 2000). Most of the eroded
tion of high-resolution topographic models (digital
soil is fine, cohesive sediment. Much of this mud is
elevation models) of basins is necessary for a better
not retained in reservoirs and flows right through to
prediction of the dispersal of inundation waters
the estuary. When the excess sediment is deposited
and the location of stagnant waters. Precisely in
in the coastal waters, changes in coastal properties
tropical coastal areas, where the impact of climate
may be very rapid, occurring within a few decades.
change on vector-transmitted diseases is of high
Examples of this are found in some bays along the
concern, there is frequently a lack of topographic
Great Barrier Reef, which have become perma-
information with an adequate resolution for
nently muddy in only a few decades following land
low-lying sectors. In vulnerable regions, the combi-
clearing and accelerated soil erosion (Wolanski and
nation of risks to both food and water can exacer-
Duke 2002). The coral reefs are also being degraded
bate the health impact of even minor weather
by this excess fine sediment (Wolanski et al. 2003a).
extremes such as floods and droughts (Webb and
An example is the La Sa Fua River, in the wet,
Iskandarani 1998). The only way to reduce vulner-
tropical island of Guam, Mariana Islands (Table 1).
ability is to ensure the infrastructure for removal of
This river drains a steep sloping drainage basin,
solid waste and wastewater and to supply potable
1.5% that of the Cimanuk River. Due to poor
water. No sanitation technology is ‘safe’ when cov-
land-uses practices in this catchment of highly erod-
ered by floodwaters, as faecal matter mixes with
ible volcanic soils, the sediment yield is twice that of
floodwaters and is spread wherever the floodwaters
the Mekong and Yangtze. The river has no delta
run. Similarly, groundwater quality deterioration
and discharges directly to a shallow, coral-fringed
by saline intrusion due to climate change and
bay where coral reefs are smothered by mud
sea-level rise must be considered (Sherif and Singh
(Wolanski et al. 2003b).
1999).
263
Much of the eroded soil remains trapped in estu- per year before construction of the Aswan High
aries, resulting in increased siltation and flooding, Dam (Khafagy et al. 1992; Fanos 1995; Stanley
increased turbidity, decreased primary productiv- and Warne 1998). About 90% of this sediment is
ity, and a loss of aesthetics resulting in an economic now trapped in the reservoirs. As a result, coastal
loss for the tourism industry. As a result, the estu- erosion is intense – the Rosetta and Damietta pro-
montories are eroding at the rates of 106 m yearÀ1
ary can be severely degraded. An example is the
and 10 m yearÀ1, respectively. Similarly, the
Cimanuk River, Indonesia (Table 1). Compared to
the Mekong (Table 1), the Cimanuk River is very Ribarroja-Mequinenza dam on the Ebro River,
small. However, due to human activities, its sedi- Spain, traps about 96% of the riverine sediment.
ment yield is thirty times larger. The catchment is This has led to coastal recession at the river mouth
yielding sediment at a rate exceeding 6000 ton mud area – stopping the previous seaward progradation
kmÀ2 yearÀ1, possibly the highest recorded erosion ´ ´
of the delta (Jimenez and Sanchez-Arcilla 1993;
rates in the world (Milliman and Meade 1983; Guillen and Palanques 1997). The story is repeated
Wolanski and Spagnol 2000). The river is now for the Colorado River, USA and Mexico, which
once supplied 150 Â 106 ton of sediment per year to
profoundly degraded and little more than a drain
for eroded soil and human waste. As a result of the Gulf of California before damming (Meckel
siltation, flooding is common. To protect the farms 1975). Damming has resulted in coastal recession.
from flooding, levees have been constructed; these The Mississippi River suspended sediment load has
need to be raised at a rate of 0.05–0.1 m yearÀ1 and decreased by about 40% between 1963 and 1989,
reach 4 m in height. Other costs involve hard engi- and this may be the major cause for the recession of
neering structures along the river to combat ero- the Mississippi deltaic coast (Coleman et al. 1998;
sion of the levees and maintenance-dredging to Streever 2001). It is also likely that the Three
combat siltation. Gorges Dam in China, under construction, will
The Fly Estuary, Papua New Guinea, is rapidly generate coastal erosion and recession, thus rever-
silting due to increased sediment input from activ- sing the present trend of progradation of the river
ities in the river catchment, as well as from sedi- delta (Chen et al. 1985; Xiqing 1998; Yang et al.
ment imported from coastal waters by tidal and 2001). Water diversion from China’s Luanhe River
wind-driven currents (Wolanski et al. 1998b). The has decreased the riverine sediment load by 95%
Jiaojiang River estuary, China, is infilling at a rate and resulted in its delta’s recession at a rate of
of about 0.13 m yearÀ1, requiring continuous dred- about 17.4 m yearÀ1 (Qian 1994). These problems
ging to maintain navigability (Guan et al. 1998). occur for both small and large rivers. Indeed, similar
In the Mekong Estuary, Vietnam, some of the coastal problems due to water regulation in the
sediment that is flushed to the coastal zone during catchment of small rivers are degrading coastal
freshwater floods may return to the estuary during embayments (Poulos et al. 1994; Jay and Simenstad
the low-flow season. Over time-scales of years, the 1996). The on-going, rapid shoreline retreat in sev-
natural system may be at quasi-equilibrium in the eral segments of Portugal is mainly caused by human
sense that the size, shape and depth of the system activity and not natural fluctuations (Granja and
may change only very slowly, evolving at time- Soares-de-Carvalho 1999; Dias et al. 2000).
scales of decades. This is important because it In semi-arid regions, river floods are short-lived
gives humans the time to adapt to changing estu- and can be suppressed by large dams. The balance
aries and coasts. However, there are plans for 100 between scouring of the estuary during occasional
hydroelectric dams and water diversion structures river floods, and the regular import into the estuary
in the Mekong River and tributaries in riparian of coastal zone sediment by tidal pumping, is then
countries upstream of Vietnam, and this will dis- disturbed by dams. The estuary may then silt, such
turb this quasi-equilibrium (Wolanski et al. 1996). as did indeed happen in the Ord Estuary, Australia,
which has silted by 30 Â 106 m3 over the last 30 years
Many estuaries are dammed, and these dams
trap much of the non-cohesive sediment (e.g., since the river was dammed (Wolanski et al. 2001).
sand). As a result, many temperate estuaries are Mangrove creeks are self-scouring; this effect is
generally suffering from sediment starvation. The proportional to the mangrove area. If the latter
Nile River carried about 110 Â 106 ton of sediment decreases because of mangrove land claim, then
264
tidal creeks will silt in a few years (Wattayakorn recover from an earlier collapse due to overfishing
et al. 1990). (Jackson et al. 2001).
Human populations and economic activities in
Changes to estuarine biology and variability the coastal zone are increasing in most countries.
At the same time, more rivers are dammed world-
Changes in river flows due to irrigation, damming wide. Deforestation is continuing and in many
and water diversion change the entire food web – countries, accelerating, while urban and rural
even up to the level of fisheries – with significant activities are growing. Thus, even though sewage
negative socio-economic consequences (see above). discharge from cities may increasingly be treated,
Longer-term climatic variability is also an diffuse sources remain unchecked, overall the river
important factor because it affects the density of flow is increasingly modified, and the load of sedi-
important species and influences predator–prey ment and nutrients is increasing. No solution has
relationships. The ultimate survival of many yet been offered of how, using only limited finan-
estuarine species depends both on the environ- cial resources, to preserve, restore and manage cri-
mental state of the individual estuaries where they tical estuarine habitats, and reduce the load of
occur and critically on the continued existence of a nutrients and pollutants in waters in the face of
chains of habitats (estuaries) to enable them to increasing human population and its aspirations.
respond to long-term climatic changes. A solution that managers advocate the most at
Global warming, sea level rise and changes in present is to reduce the amount of waste and to rely
precipitation and freshwater runoff to estuaries on a technological fix, namely to construct sewage
that result from greenhouse-gas emissions will treatment plants and to modify farming practices
alter not only the physical system but also its biol- and technology. This technological approach is
ogy. At this stage it is difficult to predict these logical; nevertheless it often fails for reasons out-
changes, partly because the predictions of eco- lined in the section on ‘Introduction: degraded
nomic and social development within the catch- estuaries and coastal waters’.
ment and of climate change remain uncertain. An ecohydrology-based solution is needed to
restore the health of estuaries. Ecologically, the estu-
ary is part of the river basin. It is the end-user of the
river where incremental small-scale impacts become
Ecohydrology as a solution for estuarine and
cumulative, and where the summation of minor
coastal management
environmental stresses becomes a critical issue. In
practice, people have commonly not considered this
Estuary and coastal water degradation
integrative, end-member aspect of estuaries. Criti-
cally, in most countries, land-use managers dealing
Estuary and coastal water degradation occurs
with development within the river catchment, water-
worldwide and is made apparent by the fact that
resource managers dealing with hydrology and
coastal fisheries are collapsing worldwide. Large
water quality, city councils dealing with growth
indicator species, such as sea turtles, sea lions,
and waste disposal in residential areas, and coastal
dugongs and manatees, are disappearing and
and fisheries managers do not have the forum or
critical habitats such as saltmarshes, mangroves,
mechanism to cooperate as a result of their various
coral reefs and seagrass beds are also declining
socio-economic and political imperatives.
(Pauly et al. 1998; Lindeboom 2002; Hughes et al.
2003; Diop 2003). In many cases, particularly in
less developed countries, it is not that traditional
Ecohydrology as a solution
management has failed but that management is
piecemeal or that there is no management at all;
In comparison with purely technological solutions,
overexploitation is the rule. In countries where
ecohydrology works from within ecosystems.
management policies and strategies are now in place,
Implementation of ecohydrologic solutions requires
but did not exist in the past, the destruction of
a profound knowledge of the interdependences
habitats and decreased water quality may be an
between water and biota. By manipulating these
important reason for the failure of fisheries to
265
countries it has been, degraded by human activities
that alter the hydrologic regime, increase the nutri-
ent load from the catchment, and destroy wetlands.
Some of the aquatic ecosystem functions can be
restored by a combination of engineering, ecohy-
drology, and phytotechnology (i.e., using plants)
solutions. As described in the previous sections
(see a summary in Figure 3), the estuary ecosystem
is more robust than the river ecosystem, especially
if the latter is impounded (Figure 2). However,
while the system is more robust, it is more difficult
to restore the system after it has been degraded,
Figure 5. Sketch of the relationship between productivity (and
because the range of options is more limited. Under
also biodiversity) and the nutrient inflow in an estuary.
human influences, the level of degradation of the
Extra nutrients in small quantities may generate an increase
estuarine ecosystem will depend on two key factors,
of biological productivity without dramatic modification of
biodiversity. More commonly the load of nutrients and namely (1) the ratio between the nutrient flux and
pollutants is so high that it degrades the water quality, the
the flushing time, and (2) the robustness of the
ecological services, biodiversity, and productivity of coastal
system. The robustness of the estuary is controlled
waters, leading in extreme cases to toxic algae blooms.
both by a number of parameters, including the
residence time, the estuarine food webs within
the water column, and the buffer effect and the
interdependences, it is possible to control ecosys- habitats provided by the fringing wetlands (princi-
tem characteristics and increase system robustness. pally mudflats, saltmarshes and mangroves). In the
Estuarine food webs show a strong dependence estuary it is generally not possible to use engineer-
on river flow, the ecosystem shifting as a result of ing solutions to solve environmental degradation
changing river inflow and salinity regimes. problems, though in some cases it is possible to
Therefore, it is necessary for estuary managers to diminish the residence time by dredging, opening
take into account in their decision-making the new river mouths, and, for dammed rivers, creating
dependence of food webs on natural (hydrologic artificial river floods. Generally estuarine environ-
and oceanic) factors, as well as on human-induced mental restoration can only be carried out by
changes of river inflows of water, nutrients, pollu- enhancing the biotic integrity (Figure 3); restoring
tants and eroded soils. Key processes are sketched intertidal wetlands is one such key option. For non-
in Figures 3 and 5. Robust systems are less likely to robust estuaries (Figure 5) none of these solutions
become degraded than non-robust systems from will be adequate and the health of the estuary can
disturbances such as fertilisation and changes in only be restored by adopting a basin-wide ecohy-
hydrology. Changes in estuarine and coastal zones drology solution. This solution requires (1) regulat-
biota will affect humans by degrading water quality ing basin-wide human activities that impact on the
and reducing its uses, and also by impacting on river, and (2) manipulating the river ecosystem to
fisheries, with consequent socio-economic impacts. decrease its impact on the estuary (Figure 6). The
The ecologically sustainable solution to estuary coastal zone is the most robust (Figure 2) and the
management is to adopt ecohydrology as the only option available to restore environmental
underpinning principle to guide the management health of the coastal zone is to adopt a basin-wide
of the entire river basin from the headwaters down ecohydrology solution (Figure 6).
to the coastal zone (Figure 6). Environmental A holistic, basin-scale approach is necessary
degradation can only be remedied by restoring because the whole river basin is a functioning,
some of the working of the ecosystem and helping inter-related ecosystem (Zalewski 2002). UNEP
the partially restored system improve itself natu- recently advocated an integrated approach to the
rally. In the freshwater part of the river and espe- hydrological environment, emphasising links
cially in impoundments, the aquatic ecosystem is between river and coastal systems (Coccossis et al.
the least robust (Figure 2). It can be, and in most 1999; Burt 2003). The Mersey Basin Campaign,
266
Figure 6. Sketch of the ecohydrology-based management of a river basin that is necessary to enable sustainable development of an
estuary.
UK, is one of a number of recent examples in quality by converting excess nutrients into plant
Europe, North America and Australasia where biomass, and by sequestering pollutants.
this basin-wide approach has been taken in envir- The interplay between hydrology and biota, and
onmental management, but even here coastal associated fluxes and exchanges of sediment, nutri-
waters are not considered explicitly in the cam- ents and pollutants, can be used in the development
paign’s main objectives. Similarly, ecohydrology of sustainable ecological solutions for water man-
principles may be adopted in the restoration of agement. A number of practical steps are possible,
specific tracts, watercourses and habitats, but they the first of which is to reduce the discharge to the
do not guide the over-arching ethos of the manage- river of excess nutrients, sediment and pollutants
ment strategy. Until this is changed, estuaries and from rural and urban areas through environmen-
coastal waters will continue to degrade. tally sensitive land-use and lifestyles in addition
Significantly, ecohydrology is a low-cost, uni- to high cost technology. More importantly,
formly applicable principle. Ecohydrology is a ecohydrology-based management relies on the use
crucial element of future river basin and coastal of feedback mechanisms between hydrology and
management where growing human populations biota as a management tool by using and manip-
are putting increasing pressure on resources and ulating plants and trees within the rivers, lakes,
the environment, particularly in the developing wetlands and flood plains, and, indeed, the coastal
world, and where, in addition, the coast is also zone, to address specific problems of water, sedi-
threatened by climate change and sea-level rise. ment, nutrients and pollutants. For example, wet-
Ecohydrology relies on using our understanding lands can be used as a remediation tool to sequester
of biota and process-interaction to moderate all excess nutrients into biomass, that is, production of
elements of catchment and estuary robustness bio-energy or timber, to limit their delivery to the
(e.g., flow rates, residence times and productivity) estuary. Similarly, aquatic biota may be manipu-
to enhance the ability of the river and coastal lated to control toxic algae. Woodlands and wet-
waters to recover from impacts. At the more spe- lands may even be used to regulate water flow,
cific level, phytotechnology is an element of ecohy- water quality and sediment delivery to the coast.
drology that relies on using plants to improve water The level at which these activities are implemented
267
will be dependent upon the robustness of the estu- annually from wetlands to remove nutrients. Such
ary. Riverine woodlands are another example of wetland manipulations go hand-in-hand with basing
how biota may be used to regulate catchment- remediation measures on biological function and
dependent fluxes; much of which has been lost in productivity. The practical steps that can be taken
lowland Europe and North America to intensive vary from estuary to estuary. Thus, the use of eco-
agriculture and urban development and these need hydrology and phytotechnology in solving practical
to be restored where possible (Peterken and issues in an estuary, depends on the wise choice of
Hughes 1995). Floodplain woodlands have draw- appropriate management practices. These must
backs, such as limited control of floodwaters and reconcile the different spatial and temporal scales
increased evapotranspiration. Nevertheless, they controlling nutrient and pollutant supply, the ro-
offer improved sediment and pollutant retention bustness of the estuary, the hydrologic regime, the
and nutrient sequestration and, indeed, reduced tidal patterns and wave climate, and the biota; these
diffuse discharge as a result of displacing flood- are all factors that can be managed and balanced
plain farmland. They help reduce peak flood flow to enhance the self-purification ability and capa-
and maintain low flow; they increase habitat diver- city of estuaries and coastal zones (Figure 6).
sity and landscape quality, and timber production Also, it is absolutely necessary to consider the
(Kerr and Nisbet 1996). They need restoration to environmental and socio-economic factors of the
improve the health of estuaries downstream. whole river catchment, including the estuary and
In estuaries and coastal waters, ecohydrology- the coastal zone, when proposing developments
based solutions call for the restoration and creation such as land clearance, urbanisation, intensive agri-
of wetlands, including mudflats, mangroves and culture and/or river damming. The environmental
saltmarshes, because of their ability to trap sedi- impacts of such developments must be fully evalu-
ment and pollutants, to convert excess water-born ated and quantified before being included in the
nutrients into plant biomass, to provide habitats final cost-benefit analysis.
for demersal and pelagic species, and, in some The issue of human health is critical when creat-
cases, to protect the coast from increased erosion ing and managing wetlands, to seek equilibrium
following sea-level rise and sediment starvation between the necessary water residence time for effi-
from damming. This is an important element of cient nutrient sequestration and the minimisation
estuarine ecohydrology where the ‘knock-on’ con- of diseases (e.g., bilharzia and malaria) and vector
sequences of catchment land-clearance may be breeding grounds. In intertidal wetlands, this may
exceeding the capacity of intertidal wetlands to be less of an issue than in freshwater wetlands,
‘absorb’ influxes of sediment, nutrients and pollu- depending on the flushing rate – which can, of
tants, which then by-pass the estuary and enter course, be manipulated using ecohydrologic
coastal waters. Wetland creation of this kind neces- principles.
sitates a dynamic coast, using the balance between Thus, the successful management of estuaries
prevailing tidal levels and accommodation space and coastal waters requires an ecohydrology-
to determine the pace at which intertidal wetlands based, basin-wide management, which considers
may be manipulated to address degradation pro- the river basin as the fundamental unit of territorial
blems from the mild to the acute. These wetlands management (Zalewski 2002). This necessitates
must be managed to reduce human health pro- changing present practices by official institutions
blems. In coastal waters, further applications of based on municipalities or counties as an adminis-
ecohydrology are the use of macrophytes to trative unit, or based on managers of specific activ-
enhance the internal consumption rate, and benthic ities (e.g., farming, water resources, fisheries, urban
suspension feeders, such as bivalve molluscs, developments). Generally, at present, the limits of
sponges, tunicates, and polychaetes, to filter and these administrative or usage units do not
pelletize excess nutrients and plankton. There are, coincide with basin boundaries. The ecohydrology
thus, several steps that may be taken in parallel. approach also necessitates a high level of colla-
The ecohydrologic approaches considered above boration and opportunity to share approaches
have an important element of seasonality. Where and experience on a range of forums to develop
possible, plant biomass may need to be harvested best practice. Without these changes, estuaries and
268
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# 2004 Kluwer Academic Publishers. Printed in the Netherlands.
Ecohydrology as a new tool for sustainable management of estuaries and
coastal waters
´
E. Wolanski1,*, L.A. Boorman2, L. Chıcharo3, E. Langlois-Saliou4, R. Lara5,
A.J. Plater , R.J. Uncles and M. Zalewski8
6 7
1
Australian Institute of Marine Science, PMB No. 3, Townsville MC, Q. 4810, Australia; 2LAB Coastal,
The Maylands, Holywell, St. Ives, Cambs. PE27 4TQ, UK; 3Universidade do Algarve, CCMAR,
Campus de Gambelas, Faculdade do Mare do Ambiente, Portugal; 4Laboratoire d’Ecologie,
´
UPRES-EA 1293, Groupe de Recherche ECODIV ‘‘Biodiversite et Fonctionnement des
Ecosystmes’’, Universite de Rouen, 76821 Mont Saint Aignan, France; 5Zentrum f€r Marine
´
e u
Tropenokologie, Fahrenheitstrasse 6, 28359 Bremen, Germany; 6Department of Geography,
¨
University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT, UK; 7Plymouth Marine
Laboratory, Prospect Place, Plymouth, UK; 8Department of Applied Ecology, University
of Lodz, ul. Banacha 12/16, 902-237 Lodz, Poland; *Author for correspondence
(e-mail: e.wolanski@aims.gov.au)
Received 30 June 2003; accepted in revised form 10 December 2003
Key words: Ecohydrology, Ecology, Environmental degradation, Estuary, Hydrology, Management,
Sustainable development
Abstract
Throughout the world, estuaries and coastal waters have experienced degradation. Present proposed
remedial measures based on engineering and technological fix are not likely to restore the ecological
processes of a healthy, robust estuary and, as such, will not reinstate the full beneficial functions of the
estuary ecosystem. The successful management of estuaries and coastal waters requires an ecohydrology-
based, basin-wide approach. This necessitates changing present practices by official institutions based on
municipalities or counties as an administrative unit, or the narrowly focused approaches of managers of
specific activities (e.g., farming and fisheries, water resources, urban and economic developments, wetlands
management and nature conservationists). Without this change in thinking and management concept,
estuaries and coastal waters will continue to degrade, whatever integrated coastal management plans are
implemented. To help in this process of change there is a need to (1) develop a profound understanding of the
effects of biota and biotic processes on mediating estuary response to changing hydrology, sediment and
nutrient flux and of the biota on hydrology at the river basin scale, and (2) to develop science-based
remediation measures at the river basin scale, with elements of ecohydrology and phytotechnology at their
core, to strengthen the ability of the biota to sustain and adapt to human-induced stresses.
due to their very high biological productivity sus-
Introduction: degraded estuaries and coastal
taining a high level of food production. It is also
waters
due to the use of the rivers and estuaries as trans-
Throughout human history, the coastal plains and port routes, fundamental for economic and social
lowland river valleys have usually been the most development and improving quality of life. At
populated areas over the world. This is partially present, about 60% of the world’s population live
236
along the estuaries and the coast (Lindeboom coastal waters retain much of the nutrients and
2002). Coastal waters, including those covering other pollutants resulting from human activities.
continental shelves, supply about 90% of the global In some ecosystems, the level of extra nutrient is
fish catch. The increase of human populations in small enough that it may generate an increase of
the river basins, from natural growth and internal biological productivity without dramatic modifi-
migration within the hinterland, has resulted in a cation of biodiversity (e.g., Zalewski 2002). More
doubling of the population along many coasts over commonly, however, the load of nutrients and pol-
the last 20 years. This is degrading estuarine and lutants is so high that it degrades water quality,
coastal waters through pollution, eutrophica- ecological ‘services’, biodiversity, and productivity
tion (water quality degradation caused by excessive of coastal waters. This degradation is not restricted
nutrients), increased turbidity, overfishing and to estuaries, it is commonly observed already in
habitat destruction. The pollutant supply does not poorly flushed, semi-enclosed water bodies. In the
just include nutrients; it also includes mud from Baltic Sea, for example, the limited depth, low
eroded soil, heavy metals, radionuclides, hydro- salinity of surface water, and the supply of saline
carbons, and a number of chemicals including oxygenated water from the North Sea at the bot-
new synthetic products. tom, lead to water stagnation, eutrophication, and
The impact on estuaries is commonly still not anoxia (Gren et al. 2000). This impacts upon the
considered in environmental impact studies when commercial cod fisheries and generates increas-
dams are proposed on rivers. In this case, the ingly common toxic algal blooms (Figure 1a); in
coastal environment, coastal fisheries, and the turn, this may impact on tourism. The northwes-
local coastal people may be regarded as an expend- tern Black Sea ecosystem is similarly degraded by
able consequence of river basin development. For direct human impacts not only on the coastal eco-
instance, as we wrote this article, the dam on the system but also on the drainage basin of the
Guadiana River, Portugal, which will form the Danube River that receives the effluents from
largest man-made lake in Europe, is being com- eight European countries (Zaitsev 1992; Lancelot
pleted without a detailed audit of its impact on et al. 2002). A similar degradation is also observed
the estuary and the coastal zone, and without any in poorly flushed Seto Inland Sea, Osaka Bay, and
estuary remediation measures being planned. Tokyo Bay, Japan, where toxic algal blooms occur
Estuaries are often regarded as sites for future 100 days per year (Figure 1b; Okaichi and Yanagi
development and expansion, and have been 1997; Takahashi et al. 2000). Better flushed and
increasingly canalized and dyked for flood pro- larger systems also suffer from environmental
tection, and their wetlands infilled for residential degradation through eutrophication, as is made
areas. For example, almost all estuarine marshes apparent by some beaches of the North Sea being
have already been reclaimed in Japan and in the covered by foams of decaying algae and protozoa,
Netherlands. All these factors impact on the bio- mainly Phaeocystis and Noctiluca, and by hypoxia
diversity and productivity and, hence, the overall in the Gulf of Mexico (Richardson and Jorgensen
health and function of estuaries and coastal waters. 1996; Rabalais et al. 2002).
They increasingly lead humans away from the Until now, the solution was believed to depend
possibility of ecologically sustainable development on reducing the amount of waste and relying on
of the coastal zone. hard technology, namely the construction of
Human impact on the ecological health of sewage treatment plants and the modification of
estuarine and coastal waters is dependent on sev- farming practices and technology. While there are
eral factors. Water circulation in some estuaries exceptions, for instance, the partial restoration of
readily flushes away pollutants to the open ocean, some ecological functions in the Rhine and Thames
whereas other estuaries retain the pollutants. estuaries, in general this technological fix has not
Unfortunately many estuarine environments that restored the ecological health of estuaries in both
are especially attractive for human settlements, developed and developing nations worldwide. The
such as wetlands, lagoons, harbours, and fjords, reasons for failure are simple. First, integrated
are often poorly flushed. In these systems, through coastal zone management plans are drawn up but,
physical and biological mechanisms, estuarine and in the presence of significant river input, they are
237
environment degradation is often seen as ‘normal’
and nothing to be alarmed at. Most people do not
demand better ecosystem health and function,
possibly because in poor countries there are no
resources to restore the estuarine health (and eco-
logical issues come second to those of economic
development), and, in wealthy countries, people
have not seen their estuaries in a pristine, healthy
state and thus they may think that what they see is
normal. Further, and most commonly, solutions
are simply not applied because of socio-economic
constraints but as well because of national policies,
planning and management structures. Finally,
there is still no cost-effective technology and poli-
tical will to treat effluent from diffuse sources in
rural activities, except possibly using wetlands as
filters; a practice that is effective but rarely used
(Moore et al. 2001). The problem remains unre-
solved as to how, using only limited financial
resources, to preserve, restore and manage critical
habitats, and reduce the load of nutrients, sedi-
ments and pollutants in the face of increasing
human population and its aspirations.
In this paper, the concept of ecohydrology is
introduced as a holistic approach to the manage-
ment of estuaries and coastal zones within entire
river catchments, by adopting science-based solu-
Figure 1. (A) A photograph of a toxic algal bloom in Hel
tions to management issues that restore or enhance
Harbour, Polish Baltic Sea, in the summer of 2001. (B) An
natural processes as well as the use of technological
aerial photograph of a 3–4 km long toxic algal bloom of
solutions. Ecohydrology principles can be applied
Protoperidinium sp. at Tokyo Bay, Japan. These toxic algal
blooms are caused by eutrophication ((A) is courtesy of to control and reduce the impact of nutrients and
´
K. Skora and (B) is courtesy of the National Institute of Land and
pollutants along the rivers and the estuaries.
Infrastructure Management (NILIM), Japan (K. Furukawa)).
Ecohydrology is more than integrated river basin
management; its principles provide the guidance
bound to fail because in nearly all countries these for the development of low-cost technology for
management plans deal only with local, coastal mitigating the impact on the coastal zone of
issues, and do not consider the whole river catch- human activities throughout the river basin, using
ment as the fundamental planning unit. It is as if or enhancing the natural capacity of the water
the land, the river, the estuary, and the sea were not bodies to absorb, or process with no resulting estu-
part of the same system. When dealing with estu- ary degradation, the nutrients and pollutants. In
aries and coastal waters, in most countries land-use situations where socio-economic and political con-
managers, water-resources managers, and coastal straints do not allow the use of technology to
and fisheries managers do not cooperate effectively prevent the degradation of estuarine and coastal
due to administrative, economic and political con- waters, or to restore estuary quality and health,
straints, and the absence of a forum where their we argue that catchment-wide ecohydrology prin-
ideas and approaches are shared and discussed. A ciples should instead be applied to help rescue or
typical case is the river disposal of waste from restore these water bodies. Ecohydrology is, in
industrial swine and poultry production in North essence, a question of science-based, integrated
Carolina, USA, and the resulting pollution of estu- management of the river basin ecosystem from
aries downstream (Mallin 2000). Second, estuarine upland waters to the seas.
238
Ecohydrology: the new tool for sustainable
development of coastal zones
Ecohydrology is the science of the interplay
between hydrology and biota (Zalewski 2000,
2002). Ecohydrology is based on a set of principles.
The first principle (hydrological) defines the river Figure 2. Sketch of the robustness (R) of a river (r) and its
basin as a template of hydrological processes deter- estuary (e) and coastal waters (c) as a function of the residence
time T.
mining when and where stable and predictable
aquatic biota form and start to play a stabilizing
role on water quality. The same principle applies to enables a lower reduction in the nutrient discharge
estuaries. In the second principle, ecohydrology from rural areas, while eliminating toxic algal
considers the entire river basin as a ‘super organ- blooms without any need for an expensive techno-
ism’; this simply means that each of the inhabitants logical solution or large-scale change in land-use
of the basin is an inherent component of the entity, practices.
for example, somewhat like a cell in a body. In Extending this concept to the coastal zone, the
practice, this means that if an inhabitant discharges whole river catchment, from the headwaters down
pollutants into the system, sooner or later this pol- to the estuary and coastal waters, must be viewed
lutant will impact on another inhabitant. Such pol- as a single ecological entity (Figures 2 and 3).
lutants include eroded soil, nutrients, heavy metals, Therefore, in order to maintain healthy, produc-
and DDT and PCB residues that are now found in tive estuaries and coastal waters, it is necessary to
virtually every ecosystem of the world. Moreover, manage human impacts throughout the river
some pollutants can be bio-accumulated (‘biomag- catchment, along the river and in the coastal
nified’) in the food chain by several orders of mag- hinterland, and to enhance the natural capacity of
nitude, potentially impacting on fish and people. the river and the estuary to sequester or degrade
In the third principle, the capacity of the system pollutants, and to convert the excess nutrient into
to cope with stresses from human activities (e.g., plant biomass. Ecohydrology thus relies only on
eutrophication) can be enhanced by managing the low-cost technology; it requires innovative solu-
biota and by regulating hydrological processes tions based on an understanding of feedback
(Mitsch 1993; Jorgensen 1997; Jorgensen and de mechanisms between hydrology and biota within
Bernardi 1998; Zalewski 2000, 2002). An example the estuary and coastal waters. By relying on nat-
is the creation of wetlands to trap nutrient and ural ecological processes, it can lead to sustainable
pollutants, and convert nutrients to plant biomass; development and environmental management. In
this biomass can provide bio-energy for society and essence, these principles have been used to good
create employment. Another example is controlling effect in managing coastal dune complexes where
water levels via the biota in wetland-fringed water appropriate species of grasses and trees are used to
bodies to regulate water quality and enhance fish manipulate sediment throughput and deposition,
stocks. An additional example is the use of macro- but with careful consideration of the hydrological
phytes to avoid toxic algal blooms in rivers and demands of coastal dune communities, for
lakes. Toxic algal blooms do not appear for example, Ainsdale Dunes, UK (Wheeler et al. 1993).
P–PO4 < 30 g lÀ1 (Dunne and Leopold 1978). The paper describes a number of key processes of
Reaching such a small concentration means chan- estuaries. For convenience, these processes are
ging farming practices and, in practice, this is separated into physical, biological and human-
economically an unrealistic goal. However, it is related processes. They all impact on the estuary,
possible to avoid toxic algal blooms by reducing and successful sustainable management depends on
P–PO4 concentrations to only 120 g lÀ1, provided integrating these processes. Clearly, the prevailing
macrophytes are present. This option is attractive natural conditions and human impacts within the
because establishing macrophytes in an aquatic river catchment and its coastal hinterland, the
ecosystem may be considerably less expensive than hydrology, the biology and the buffering capacity,
reducing nutrient input from human activities; it and ‘robustness’ of each estuary (see below), vary
239
Figure 3. Sketch of the dominant pathways of water, fine sediment, nutrients, and plankton in an estuary, together with the impact on
human health.
from river catchment to river catchment and from flushing rate of water is not the only variable deter-
estuary to estuary in both time and space. Hence, mining robustness, and the following sections
there is no universally applicable formula to inte- describe the three other key determinant processes.
grate these processes in an ecohydrology model of These are: first, the rate at which fine sediment –
an estuary, but generic ecohydrologic principles soil eroded from the river catchment – is flushed;
may be employed to consider the integration of second, the efficiency with which organic matter is
these factors from one estuary to another. processed by the biology within the water column;
This paper starts with a consideration of the and third, the trapping of sediment, nutrients and
robustness of an estuary. Robustness (Figure 2) is pollutants in inter-tidal wetlands (mangroves and
controlled by the rate at which water in an estuary saltmarshes). A section describes severe estuarine
is flushed; the shorter the flushing time-scale, degradation resulting from human activities up-
the higher the robustness, that is, the lower the river to illustrate cause and effect. The next section
expected water quality problems. However, the describes aspects of human health linked with
240
estuaries; once again this is dependent on human concentration, other solutes, and SPM. To some
activities up-river, but it is also dependent on wet- degree, all these variables are controlled by the
land buffering capacity. The final section describes residence time, that is, by the rate at which
how a holistic approach is needed to understand these components are flushed out to the sea.
the health of the ecosystem. The section also Thus, the residence time of an estuary or coastal
suggests that this holistic approach can be used water body is an important parameter determining
to manage an estuary and coastal waters on an its robustness and its ability to cope with human-
ecologically-sustainable, low-cost basis. induced stresses (Figure 2). Well-flushed estuaries
are intrinsically more robust than poorly flushed
systems. The residence time is most critical in the
upper reaches of an estuary because this is where
Estuary robustness
contaminant accumulation and increased turbidity
from human influences are most likely to occur. As
The role of robustness
a result, environmental degradation is most often
apparent during periods of reduced freshwater
Estuaries and coastal waters are subject to constant
inflows, for example, during drought or when
changes in the wind, solar input, rainfall, wind,
human activities reduce the freshwater flow.
currents offshore, sea level, freshwater runoff,
Indeed, human activities in the river catchment
varying climate, and human influences. These
may cause significant reductions in dissolved
changes occur at time-scales of minutes (turbulent
oxygen, mainly through eutrophication. There is
mixing), hours and days (tides), months (hydrolo-
no other environmental variable of such ecological
gic and seasonal changes), and decades (climate).
importance to coastal marine ecosystems that has
Thus, estuaries are highly dynamic environments
changed so drastically due to human influences in
that exhibit strong temporal and spatial changes in
recent decades (Diaz and Rosenberg 1995). At
their physical, chemical and biological variables.
times this has resulted in severe deficits of dissolved
In turn, this dynamism leads to a large variability
oxygen, leading to hypoxia and anoxia. Environ-
in estuarine water quality, chemical and biological
mental degradation problems associated with the
characteristics. Further, no two estuaries are iden-
occurrence of low oxygen are increasing on a global
tical; each estuary (and coastal water body) has its
scale.
own size, shape and bathymetry, its own length of
Residence time is a key physical variable deter-
tidal influence, tidal range, wave climate, fresh-
mining if a particular estuary will or will not suffer
water inflow, turbidity and residence (or flushing)
from eutrophication. For example, the residence
times, and its own sediment properties of grain size,
time of the Tweed Estuary, UK, is generally
carbon-to-nitrogen ratio, organic carbon content,
less than 13 h and dissolved oxygen levels are high
and water-column turbidity. Thus, each estuary is
throughout the year (Uncles et al. 2000). Rapid
unique and there is no general parameter to readily
flushing ensures both low turbidity (because fine
assess the ‘health risk’ of an estuary from natural
sediments are unable to accumulate in the estuary)
and human influences.
and insufficient time for local oxygen depletion to
The ecological health of estuaries depends on the
occur, regardless of whatever biochemical oxygen
‘successful’ interaction – and the limits to the pos-
demands are present. The reverse is true in the
sible interaction that are imposed by the temporal
Humber-Ouse Estuary, UK, which has a residence
and physical variability – between organisms and
time of several weeks (Uncles et al. 1998a, b); as a
variations in salinity, currents, waves, suspended
result, this estuary experiences high turbidity and
particulate matter (SPM), bed sediments, tempera-
suffers from dissolved oxygen depletion.
ture, air exposure, hypoxia, wetland contaminants
and biodiversity.
Like the health of a living organism, the health The flushing of estuarine waters
of an estuary or a coastal water body, cannot be
measured by one single variable. A number of The simplest way to quantify the residence time is
variables are important (Balls 1994); these include through the use of a single compartment, tidally
the dissolved nutrients, the dissolved oxygen averaged box model. This method is very coarse in
241
Figure 4. Mixing diagram of a material in an estuary. Salinity is taken to be 0 in freshwater and 35 in ocean water. CR is the
concentration of the material in the river, and CS that at sea. A straight line indicates conservative mixing (no losses or gain);
concave and convex lines indicate, respectively, a loss and a gain of that material in the estuary. When the concentration of material
in the river is greater than that at sea, a negative slope exists. Conversely, a positive slope results when the concentration of material in
the river is smaller than that at sea.
basin volume with ‘new’ fresh water. The coastal
space and time, but is useful in acquiring a quali-
salinity is So (typically 33–35) and the estuarine
tative, conceptual understanding of the time-
salinity, S, is given by:
dependent solute dynamics for a range of estuarine
systems in which transients and residence times are
S=So ¼ 1 À f ð2Þ
important.
A conservative (i.e., no growth or decay) dissolved
In this method, the estuary is treated as a single
nutrient with freshwater concentration, Nf, enters
compartment of fixed volume, V. The compart-
the estuary with the freshwater and is mixed into
ment is subjected to river (and groundwater) fresh-
the coastal sea, where, for illustration, its con-
water inputs upstream, and to an equal seaward
centration is taken to be zero. The evolution of
flow at the mouth. In addition there is mixing with
estuary nutrient concentration, N, is given by:
seawater across its down-estuary face. The rate at
which the fraction of freshwater within the estuar- @t N ¼ f Nf À r N ð3Þ
ine compartment, f, changes with time (@ t f) is
Residence time and therefore r in Equations (1)
given by:
and (3) can be specified empirically in terms of tides
@t f ¼ f À r f ð1Þ and runoff. In the special case of tidally averaged,
À Á steady state conditions, during which variables do
where f ¼ Qf =V ¼ fÀ1 and r ¼ Qf þ QÃ =
not change with time (@ t 0), the freshwater
V ¼ rÀ1 ! f :
fraction is given by (from Equation (1)):
The freshwater ( f ¼ 1) flow rate into the estuary
compartment is Qf, which is equal to the flow rate f ¼ f = r ð4Þ
out of the compartment to sea, where f ¼ 0. The The estuarine nutrient concentration for this spe-
volume-mixing exchange rate with the coastal cial case is then (from Equations (2)–(4)):
waters is Q* ! 0, so that the estuarine residence
N ¼ Nf f ¼ Nf ð1 À S=So Þ ð5Þ
time of freshwater within the compartment, r, is
such that r f (from their definitions in Equation This is the classic, linear, estuarine ‘mixing-
(1)). If the estuary were completely full of fresh- diagram’ relationship between a solute and salinity
water then f would equal the estuarine (in this (Figure 4). For matter passively carried by water,
case tidal river) residence time, r, that is, the time the mixing diagram shows a linear line. An estuar-
required by the river to replace the entire estuarine ine nutrient input per unit volume of estuary and
242
per unit time, IN ! 0, can be included in the single- The residence time is, therefore, mainly affected by
compartment estuary model (see Equation (3)): TL, as would be expected for a diffusive system
(RT proportional to the square of TL) or an advec-
@t N ¼ f Nf À r N þ IN ð6Þ
tive system (i.e., proportional to TL).
Equation (8) ignores two processes that can
In the special case of a steady state estuary with a
modify the residence time in an individual estuary.
constant estuarine nutrient input, the correspond-
These are, first, the currents driven by a difference
ing steady state estuarine nutrient concentration
in density between fresh and salt water – this is
would then be (see Equation (5)):
particularly important in deep fjords, bays and
N ¼ r IN þ Nf f ¼ r IN þ Nf ð1 À S=So Þ semi-enclosed seas where the bottom waters can
! Nf ð1 À S=S0 Þ ð7Þ be nearly stagnant and where water quality can
degrade severely. Another key process is water sto-
This demonstrates that the estuarine nutrient con-
rage and buffering by intertidal wetlands, mainly
centration for this ‘input’ case lies above (i.e., con-
saltmarsh or mangrove vegetation that flank the
vex upward) the linear, ‘conservative’ line defined
main estuary and results in drag to the flow and
by the river and sea end-members (Figure 4).
temporary storage of waters. These points are dis-
A concave line suggests a loss of the matter within
cussed in more detail below in the sections on salt-
the estuary, for example, by adsorption on mud.
marshes and mangroves.
A convex line suggests the addition of matter, for
example, matter gained by degradation of organic
The effects of residence time on dissolved
material or desorption from mud. Thus, if there
substances
were in situ nutrient removal from the estuary
(i.e., IN < 0, such as might occur during an algal
An important chemical variable is dissolved oxy-
‘bloom’) then the estuarine nutrient concentration
gen, because of its necessity to the metabolism of
for this ‘removal’ case would lie below (i.e., convex
aerobic organisms, and its uptake, sometimes to
downward) the linear, ‘conservative’ line defined
hypoxic and anoxic levels, by the respiration of
by the river and sea end-members. In particular,
sewage and other waste inputs. Because of the
the influence of the local estuarine input, or
influence of varying residence times and other
removal, is directly proportional to the residence
environmental factors, such as wind speed, there
time r, and is likely to be much less important in
is usually no simple relationship between the oxy-
rapidly flushed estuaries (small r and potentially
gen demand of waste effluents and reductions in
small rIN) than in slowly flushed estuaries (large r
oxygen concentrations. The oxygen deficit depends
and potentially large rIN).
on water flow, turbidity, and oxygen supply and
demand, and this varies from estuary to estuary
Estimates of the residence time
(Owens et al. 1997).
Nutrient loads in many rivers have increased
The estuarine residence (or flushing) time of a
markedly over recent decades and this increase is
solute depends strongly on tides, freshwater run-
thought to have been partly responsible for the
off and morphological size, especially length.
changed eutrophic status of a number of estuaries
Residence times, which generally are calculated
and coastal seas (Reid et al. 1994). Once in the
from freshwater runoff and estuarine freshwater
estuary, non-conservative behaviour of nutrients
content, tend to increase with tidal length (TL)
can be pronounced. As with the dissolved oxygen
and decrease with increasing tidal range. Uncles
deficit, the fate of nutrients varies from estuary to
et al. (2002) derived the following simple, linear,
river as a function of water flow, turbidity and
multiple regression relationship for the logarithm
biota (Figure 3). In muddy estuaries, the turbidity
of the maximum residence time ( r, in days) in
varies spatially and temporally (see the section on
terms of the logarithm of the mean spring tidal
‘The fate of fine sediment in the estuary’). As a
range (MSTR, in m) and the logarithm of the
result of all these influences, phosphate usually
tidal length TL (in km),
behaves non-conservatively in turbid estuaries
r ¼ 0:23ðMSTRÞÀ0:4 ðTLÞ1:2 ð8Þ (this behaviour is particularly marked within the
243
Table 1. Comparison of the drainage areas, the sediment load
upper reaches of muddy estuaries), whereas it may
and the yield for various rivers.
be less so for silicate concentrations, while nitrate
may be essentially conservative (Morris et al. Load Yield
1981). Systems with very long residence times can (106 tonne
Area (tonne
yearÀ1) kmÀ2 yearÀ1)
(106 km2)
River
export much less of these nutrients to the coastal
zone than systems with very short residence-times. Yangtze 1.9 480 252
For example, the degree of denitrification in an Amazon 6.1 1200 190
estuary (i.e., loss of nitrogen to the atmosphere) Mississippi 3.3 210 120
Ganges/Brahmaputra 1.48 2180 1670
increases with water residence time (i.e., for N, see
Mekong 0.79 170 215
Nixon et al. 1996). There is no straightforward
Fly 0.076 116 1500
relationship between trace metal transport and Cimanuk 0.0036 15.7 6350
water residence time; this appears to be due to King Sound 0.12 6 50
5 Â 10À6 2.4 Â 10À3
non-linear metal chemistry and absorption/deso- La Sa Fua 480
rption processes of metal on mud (Morris 1988;
The effect of deforestation on estuaries is much more rapid in
Liu et al. 1998; Salomons and Forstner 1984). the tropics than in temperate zones and the difference is mainly
Once the metal is absorbed on mud, it can become due to the intense rainfall in the tropics. The Cimanuk and La Sa
buried under the substrate of the estuary or in the Fua river catchments are small and profoundly modified by
human activities in the tropical islands of Java and Guam,
fringing wetlands. This metal can still be remobi-
respectively (Wolanski et al. 2003b).
lised, and threaten the biota, by a number of pro-
cesses such as bioturbation, natural erosion and
channel migration, or through dredging (French
The fate of fine sediment in the estuary
1996).
High nutrient inputs to estuaries and associated
Riverine sediment inflow
eutrophication can lead to algal blooms. In turn,
Associated, though not exclusively, with the fresh-
this results in the consumption of dissolved oxygen
water inflow to estuaries is an input of SPM (which
by decaying algae, and in the formation of foams
includes and is usually dominated by sediment)
and toxins. The residence time, and thus the rate of
derived from erosion of the drainage basin. The
flushing of nutrients from estuaries, is therefore an
SPM concentration of river waters typically exhi-
important factor, but not the only one, in determin-
bits great variability as a function of river inflow,
ing whether excessive nutrient inputs are likely to
although it is generally larger during high river flow
lead to algal blooms and oxygen sags.
periods, especially during floods. Sediment inputs
Eutrophication is not restricted to estuaries;
into estuaries vary greatly for different rivers. For
it can also impact on coastal waters. The link
illustration, the Mekong and Fly rivers form large
between land-use and coastal water pollution can
estuaries in the wet tropics. To place these rivers in
be quantitatively determined using models. For
perspective with other major rivers in the world
instance, a 50% reduction in artificial fertilizer
(see Table 1), the Mekong, Vietnam, has a smaller
applications within the Humber drainage basin,
drainage basin than the Ganges/Brahmaputra
UK, would give a 10–15% decrease in nitrogen loads
(53%), Yangtze (41%), the Mississippi (24%) and
to the North Sea, relative to the 1994–1995 input
the Amazon (12%). The Mekong sediment yield is
(Tappin, pers. comm.). To a certain extent, residence
about twice that of the Mississippi, 85% that of the
time influences which part of the coastal zone is at
Yangtze, 12% larger than that of the Amazon, but
risk from degradation or eutrophication – whether
only about 1/7 that of the Fly and the Ganges/
coastal wetlands have sufficient time to sequester
Brahmaputra. The Fly River, Papua New Guinea,
the nutrient reservoir, or whether nutrients make
has the highest sediment yield, about 10 times
their way to the shelf without significant loss. This
higher than the Amazon and the Mississippi.
has important consequences for marine sanctuaries
Erosion increases in the catchment as a result of
and world heritage sites which flank intensively
human activities, which leads to an increase in river
used catchments, for example, Monterey Bay,
SPM. When rivers are dammed, much of the coarse
USA, and Greater St. Lucia Wetland, South
sediment (e.g., gravel, sand and coarse silt) may
Africa.
244
remain trapped in the reservoirs, which then experi- These mudflats are often flanked, in temperate
ence siltation. climates, by localised saltmarshes that are inun-
dated at HW of spring tides or, in the tropics, by
mangroves. Much of the muddy sediment is
Seaward sediment export from estuaries
derived from tributary rivers and the coastal sea,
The export of fluvial SPM varies from estuary to
and was deposited as rising sea levels of the
estuary. In practice, there are estuaries that trap
Holocene drowned former river valleys (e.g.,
essentially all the sediment, other estuaries export
Chappell and Woodroffe 1994; Roberts 1998).
seaward all the riverine sediment to the sea, and
These vegetated mudflats play a critical role in
other estuaries trap some sediment and export the
determining the robustness of the estuary, by selec-
rest. For instance, in the Mekong Estuary,
tively trapping fine sediments, influencing the
Vietnam, river floods last several months and
water residence time, sequestering nutrients and
freshwater may be found throughout the estuary,
pollutants, and converting excess nutrients within
up to the river mouth. In this case, fine sediment is
the water column into plant biomass. These points
not stored in the estuary; instead, it is discharged
are described in separate sections on saltmarshes
directly into the shallow, coastal waters (Wolanski
and mangroves, respectively.
et al. 1996). A fraction of that sediment returns in
The size, elevation and stability of non-vegetated
the estuary during the low flow period (Wolanski
mudflats depend to a considerable degree, but not
et al. 1997). In the Fly River, Papua New Guinea,
entirely, on the biology. There are relationships
river floods are rare and much of the riverine mud
between sediment erosion properties and the biolo-
remains trapped in the estuary (Wolanski et al.
gical characteristics of intertidal mudflats that
1998a). In the Fitzroy Estuary, Australia, the mud
demonstrate the importance of the surface diatom
is deposited slightly offshore during river floods;
layer (biofilm) in modifying the initial erosion of
during the rest of the year, this mud is resuspended
a surface mud layer (e.g., Sutherland et al. 1998a;
by strong tidal currents and transported back into
Widdows et al. 2000; Riethm€ller et al. 2000;
u
the estuary, so that over a time scale of a year most
Anderson 2001). Benthic diatoms produce ‘sticky’
of the riverine sediment is effectively trapped in the
extracellular polymeric substances within these
estuary (Wolanski and Spagnol 2003).
biofilms (Paterson 1994; Sutherland et al. 1998b),
which then provide a degree of binding for mudflat
and mudbank surfaces. It has been hypothesized
Formation of mudflats
that the erosion properties of cohesive, intertidal
The nature, degree and extent of sediment deposi-
mudflats depend on a balance between the physical
tion in the estuary are determined by its morphol-
and biological processes of stabilization and desta-
ogy and sediment characteristics (Figure 3). These
bilization (Widdows et al. 2001). The density of
vary widely from estuary to estuary, according to
microphytobenthos, algal mats, higher plants,
the importance of wave activity, tidal variations in
worms, mussel beds and other biological factors
water level and current speed, freshwater inflow,
can affect bio-stabilization of sediments. Bio-destabi-
shape and size of the estuary, local geology and
lization mainly results from the bioturbation
sediment availability and mineralogy. Many tidal
caused by burrowing and deposit-feeding animals,
estuaries have large areas of intertidal mudflats,
such as bivalves, polychaetes and crustaceans.
which comprise large amounts of clay and silt
mixed with varying fractions of coarser sediments.
These mudflats are exposed to the atmosphere at Estuarine turbidity maximum zone
tidal low water (LW) and border the main, deeper Once riverine-derived SPM enters the estuary, it
channels. They probably formed as turbid waters can be trapped within an estuarine turbidity
inundated the near-shore banks on a rising tide and maximum (ETM) zone (Figure 3). The ETM is
deposited sediment in the relatively slack waters frequently located in the very low salinity rea-
there, especially over high water (HW) slack, and ches of an estuary, the position of which is depen-
then failed to suspend and remove all of the depos- dent on tidal range, river flow and physical
ited sediment on the subsequent falling tide dimensions, and, therefore, shows great variabil-
(Pethick 1984). ity. Continuous exchange of primary sediment
245
particles and aggregates between water column and recycling following increased wave activity or
bed within the ETM generates a shoal of mobile tidal currents with rising sea level or increased
bed sediment, which moves down-estuary with storminess.
increased river inflow and exhibits a seasonal The affinity of organic molecules for particle
migration (Bale et al. 1985). surfaces and the incorporation of biogenic mate-
Long, strongly tidal estuaries tend to have rial within aggregates that constitute the ETM
greater SPM concentrations within their ETM make it a suitable environment for particle-
regions than either shorter estuaries (with compar- associated bacteria. The associated, localised bac-
able tidal ranges at their mouths) or weakly tidal terial respiration can cause significantly dissolved
estuaries (Uncles et al. 2002). An empirical rela- oxygen depletion within the ETM (Morris et al.
tionship between the maximum, depth-averaged 1982; Uncles et al. 1998a). Traditionally, low
SPM concentration at HW within an estuary, TL dissolved oxygen levels have been attributed to
and tidal range (TR) was derived from a systematic industrial discharges and pollution from tribu-
comparison of SPM concentrations at spring tide, tary rivers and direct effluent discharges to
low freshwater inflow conditions for 44 estuaries the estuary, especially from sewage treatment
world-wide. A simple, although crude, linear, mul- works located in the major population centres.
tiple regression relationship for the logarithm However, in the Loire Estuary, urban and indus-
of the maximum SPM concentration (mg lÀ1) in trial effluents represent a very small part of the
terms of the logarithm of the TR (in m and speci- total oxygen demand and the occurrence of
fied at the estuary mouth) and the logarithm of the anoxia is associated with the stock of organic
TL (in km) gave: material in the ETM region (Thouvenin et al.
1994).
SPM ¼ 0:0055ðTRÞ2:8 ðTLÞ1:7 ð9Þ
Longer estuaries possess faster tidal currents for a Estuaries as sieves for nutrients
given tidal range at their mouth; this in turn pro- The strongly tidal estuaries generally exhibit a tol-
duces more erosion and suspension of bed sedi- erance to eutrophication with respect to nitrogen-
ments, and therefore higher SPM concentrations containing nutrients, despite (in some cases) high
(Uncles et al. 2002). nitrogen loadings in their inflowing rivers. The
mean annual chlorophyll ‘a’ levels are significantly
Water quality aspects of SPM and deposited lower in the strongly tidal estuaries than in the
sediment weakly tidal estuaries with similar nitrogen concen-
Sediment particles and aggregates within the ETM tration (Monbet 1992). Tidal range, of course, acts
can give rise to marked changes in water quality. as a surrogate parameter to describe tidal mixing,
Fine particles can adsorb metal ions and organic current velocity, sediment suspension and turbidity
macro-molecules from solution to such an extent (Equation (9)) and therefore light penetration.
that some metals can be completely removed from Larger tides ensure that accumulated sediment is
solution within a strong ETM (Ackroyd et al. 1986; regularly suspended, leading to high turbidity and
Salomons and Forstner 1984). On longer time low light levels with less potential for bloom con-
scales, these adsorbed metals become incorporated ditions, regardless of nutrient levels.
into anoxic, reducing bed sediments. Here, heavy In non-impacted environments, the transport of
metal and radionuclide pollution may be regarded nitrogen (N) and phosphorous (P) from a drainage
as being effectively bound in an immobile sedi- basin to its watercourses is dependent on the che-
ment sink. Indeed, considerable success has been mical and mechanical weathering of soil minerals,
achieved in using down-core trends in metal and whereas in impacted environments it is thought
radionuclide concentration as a faithful record of that agriculture is the largest contributor to river
the industrial history of estuaries (e.g., McCaffrey nutrient concentrations (Tappin 2002). The fluvial
and Thomson 1980; Oldfield et al. 1993; Cundy fluxes of total N and P depend both on water and
et al. 1997; Plater et al. 1998; Fox et al. 1999). SPM fluxes and the N and P species associated with
However, these metals may become solubilised them. These species include dissolved (nitrate,
into pore water or have the potential for physical nitrite, ammonium, organic N, inorganic P,
246
organic P) and particulate (organic N, organic P, POC decreased from a range of 4.6–1.5% in
inorganic P) components. Particulate species tend the Garonne River, France, to 1.5% in the Gironde
to dominate the load, although nitrate and phos- Estuary’s ETM (Etcheber 1983). In the Delaware
phate become more important in populous regions. Estuary, USA, the POC content was 9% in the river
Once nutrients enter an estuary, non-conservative and 2.3% in the estuary’s ETM (Biggs et al. 1983).
behaviour can be pronounced as is described in In the Ems Estuary, Netherlands, POC decreased
the section on ‘The effects of residence time on from 6.1% in the non-tidal river to 2.8% in the
dissolved substances’. Key processes responsible ETM (Eisma et al. 1982). Similarly, in the Tamar
for this non-conservative behaviour include burial Estuary, UK, POC decreased from 5% in the river
in sediment reservoirs, such as tidal saltmarshes to about 2.5% within the estuarine ETM (Morris
and mangrove forests, and loss to the atmosphere, et al. 1982). Considering carbon to nitrogen ratios,
largely by bacterial denitrification of nitrate N to computed POC/PN ratios (P means particulate
gaseous N2/N2O. here) for the Humber Estuary, UK, are generally
Nitrogen compounds may exert a significant >10 and higher than many data reported elsewhere
oxygen demand in estuaries through microbially (which are typically 8–12). Similarly high values
mediated nitrogen transformations (Owens 1986; have been reported for the very low salinity reaches
Grabemann et al. 1990). Organic nitrogen is hydro- of some North Carolina estuaries, USA, where the
lyzed to ammonia and additional ammonia is input POC is characteristic of particulate terrestrial plant
to estuaries via tributary rivers and wastewater material (>15; Matson and Brinson 1990), and for
discharges. Ammonia utilizes dissolved oxygen the Vellar Estuary, India (4–140; Sivakumar et al.
during nitrification to produce nitrate, via nitrite 1983). High ratios occur in the Humber Estuary,
(e.g., Owens 1986). In conditions of low dissolved UK, when PN levels become very small, rather
oxygen, nitrate may be reduced to enable oxida- than when POC becomes very high. This might be
tion of organic carbon to proceed (e.g., Gameson a consequence of a significant coal fraction of the
1981). Other nutrients are discharged to estuaries. POC resulting from historical coal mining activity
For example, river and estuarine waters are often in the Humber’s drainage basin and estuarine-
enriched by phosphate from urban and industrial borne coal transportation.
wastewater discharges and from land runoff (e.g., Temporal variability in fluvial inputs of sus-
Grabemann et al. 1990; M€ller et al. 1991; Hager
u pended sediment also impacts on POC in estuaries.
and Schemel 1996) and they receive silicate from In the northern reaches of San Francisco Bay for
tributary river inflows via rock weathering and soil example, the POC fraction of SPM during fresh-
leaching. The river inflow, estuarine mixing and water floods was 1–2%; POC was higher (mostly
biological uptake are all important to the distribu- 2–4%) during low runoff periods (Schemel et al.
tion of dissolved silicate. 1996). Algal production probably accounted
for some of the additional POC during summer
(Tipping et al. 1997). The first major freshwater
SPM and sediments as repositories of organic
flood of winter that followed a period of relatively
carbon
low runoff could have higher POC levels, due to
Particulate organic carbon (POC) in estuaries and
mobilisation of terrestrial organic matter produced
coastal waters is derived from a number of sources
over the previous summer. Phytoplankton produc-
and, as such, its proportion in SPM varies in both
tion and fluvial loadings accounted for 85% of the
time and space. The observed POC distributions in
organic carbon sources (Jassby et al. 1993). Even if
the estuary are derived from three main sources:
only 10% of the fluvial loadings were biologically
first, fluvial plankton, both living and detrital;
available, these two sources would still account for
second, terrestrial POC detritus, mostly carried by
62% of the total.
the non-tidal, freshwater river during floods (typi-
In addition to observed seasonality, freshwater
cally 3% by dry weight of SPM; Meybeck et al.
inputs of POC are also susceptible to catchment-
1988); third, marine plankton with POC content
derived organic pollution. Bed sediment samples
up to 20%.
from the Cochin Estuary, India, had POC
Estuaries often show a POC decrease from the
contents of 0.7–3.8%, with a mean of 1.6%
non-tidal, freshwater river to the estuarine ETM.
247
(Sankaranarayanan and Panampunnayil 1979). In input is through riverflow it can also occur in the
general, values were highest during monsoon form of non-saline groundwater movements. There
periods of large freshwater runoff and they related are exceptions however, such as saltmarshes with-
this feature to the deposition of water-borne plant out any direct links to, or connections with, fresh
and animal matter from terrestrial sources. A water systems. These marshes can still be seen to be
similar study two decades later found a substantial fully functional in all respects but the potential
increase in maximum POC levels that ranged from magnitude of the fluxes that they can generate, in
0.2% to 6.2%, with a mean of 3.6%, due to organic respect of sediment, mineral and organic matter is
pollution (Seralathan et al. 1993). Mud samples inevitably reduced. They are generally much smal-
accounted for the highest POC values and muddy ler systems although their proportional contribu-
sand samples the least – the result of a strong, tion can still be very significant.
negative correlation between POC content and par- Saltmarshes (including the associated freshwater
ticle size. A similarly strong correlation between marshes) provide a living link and buffer between
POC and bed-sediment particle size was observed land and sea (Figure 3; Boorman 2000). They are
in the sewage-polluted estuary of Tolo Harbour, able to withstand much of the erosive power of the
Hong Kong, where POC reached about 2% sea during periods of storms. The erosion losses are
(Thompson and Yeung 1982). subsequently made good by increased rates of
Significantly, POC plays an important role in accretion during calmer periods (Pethick 1992).
oxygen utilisation, and hence water quality, within The possibilities for the export of organic matter,
the estuary. For example, suspended particles in the produced in the saltmarsh, to adjoining marine
Mississippi River Estuary were observed to have ecosystems have also been widely recognised
an average POC content of 1.8%, with offshore (Adam 1990; Lefeuvre and Dame 1994). The extent
particles having higher POC fractions (Trefry et al. to which export occurs appears to vary consider-
1994). There was a positive correlation between ably from marsh to marsh (Dame and Lefeuvre
SPM concentration and apparent oxygen utilisa- 1994). Saltmarshes can also act as sources or as
tion, which is due both to respiration of POC by sinks for specific components; these components
bacteria attached to the SPM, and to the existence range from organic pollutants, heavy metals and
of hypoxic waters on the Louisiana Shelf. radionuclides through the various mineral nutri-
ents to the organic and inorganic components of
´
sediments themselves (Gueune and Winett 1994).
However, future climate change, sea-level rise and
Saltmarshes
changing storm magnitude and/or frequency may
result in erosion of this sediment and remobilisa-
The importance of saltmarshes
tion of its potentially toxic pollutants. Past erosive
phases have been interpreted from morpho-sedi-
Traditionally, saltmarshes have been largely con-
mentary evidence of historical records of metal
sidered in the narrow sense of the vegetated part of
pollution in the Severn estuary (Allen 1990a). The
the tidal flat. Recent studies have been extended to
significance of future remobilisation lies in the fact
include the whole ecosystem including the vege-
that many of these saltmarsh sediment stores
tated creeks and the mudflats occurring at lower
contain heavy metals, radionuclides and organic
levels at the seaward edge of the marsh. The same
pollutants in concentrations well above currently
consideration must be given to the upper edge
accepted safety limits (Oldfield et al. 1993;
of the saltmarsh where there are gradations to
Valette-Silver 1993; Williams et al. 1994; Leggett
terrestrial communities and, more importantly,
et al. 1995).
gradations to brackish and fresh water marshes
The fate of organic material produced by the
still with a tidal influence. These intermediate habi-
saltmarsh vegetation is determined by various
tats have long been recognised in major river sys-
environmental factors, such as the local water cur-
tems such as the Mississippi, but they also occur in
rents and the maturity and the age of the saltmarsh
both a floristic and functional sense in many very
(Troccaz et al. 1994; Lefeuvre and Dame 1994;
much smaller systems that have transitions from
Boorman et al. 1994a; Lefeuvre 1996).
salt water to fresh. While usually the freshwater
248
Nutrient exchanges occurs during the processes of the decomposition
of organic matter, direct losses by the leaching of
The fluxes of mineral nutrients are dependent both nitrogen, phosphorus and also carbon, from live
on the external nutrient loading of the estuary, that plant tissues can occur as well (Turner 1993). The
is the degree of eutrophication, and on the release amounts released are high enough to account for
of nutrients by the decay of organic matter within significant increases in the activity of the estuarine
the saltmarsh. Within a saltmarsh system there are plankton community and consequently are of
considerable seasonal variations in the concentra- potential significance for many other estuarine
tions of inorganic nitrogen and these variations are communities.
even more marked when exchanges between the
different forms of nitrogen (NO3 –N, NO2 –N, and Saltmarsh sedimentation
NH4 –N) are considered. Overall, in studies in
France, England and the Netherlands, there The transport of sediment to a saltmarsh by the
appeared to be a net export of dissolved nitrogen tidal flow of sediment-rich estuarine waters into
out of the saltmarshes (Boorman et al. 1994a). The and over the marsh is a crucial part of the vertical
fate of phosphorus was less clear; phosphate may development of the surface of the marsh. Models
be exported for at least part of the year. Most of the of saltmarsh deposition consider two primary
differences in the fate of nitrogen and phosphate mechanisms of accretion: the tidal ramp and tidal
can be explained by their different sorptive beha- creeks. In the ramp model, decreasing tidal flat
viour because P is strongly adsorbed onto clay and elevation in a seaward direction increases the
N is not. The concentrations of both nitrogen and frequency and duration of tidal inundation
phosphorus in saltmarsh creeks will depend on the (¼hydroperiod). In this case, sedimentation rate is
balance between the supply from inside and outside a decreasing function of elevation within the tidal
the marsh and the rate of uptake by the growth of frame (Pethick 1981; Allen 1990b; French 1993).
saltmarsh vegetation. Saltmarsh accretion via creek processes considers
It appears that low concentrations of phos- the incoming sediment-laden waters to be funnelled
phorus can be the main limiting factor in determin- by the creek network, which then overspill once the
ing the productivity of saltmarsh vegetation within altitude of the fringing saltmarsh is exceeded.
eutrophic estuaries such as the Westerschelde, The In this example, sedimentation is focused in the
Netherlands, and the Tagus, Portugal, having immediate vicinity on the creek margins in the
enhanced productivity levels (Lefeuvre 1996). This form of levees, and vertical accretion is achieved
is in contrast to the situation regarding the primary by lateral creek migration over the long term
productivity of estuarine waters where, under sal- (Bridges and Leeder 1977). Providing that the salt-
ine conditions, phosphorus is released from terri- marsh is not limited by an insufficient supply of
genous sediments upon contact with seawater and sediment or spatially constrained by levees pre-
nitrogen becomes the limiting factor (Doering et al. venting its landward movement, a saltmarsh can
1995). There is a complex pattern of variation respond to sea-level rise via an landward retreat
in phosphorus levels with, on the one hand, the and/or enhanced sediment accretion.
release of phosphate as described above and, The magnitude and direction of sediment trans-
on the other hand, the large scale removal of port depends on the velocity of the water flow and
phosphorus by its take-up from solution during the size and density of the sediment particles. The
algal blooms. Where adequate levels of both phos- bulk flow of the water along a creek is the major
phorus and nitrogen occur, other elements, such as mode of sediment transport but there can also be a
silicon, can become limiting (Jacobsen et al. 1995). significant transport of material by near-bed move-
It is clear, however, that saltmarshes are charac- ments of hyper-concentrated suspended sediment.
terised by their large nutrient storage capacities Techniques have been developed to determine both
and that, under certain circumstances, these stores these components (Troccaz et al. 1994; Hemminga
can become ‘leaky’ with subsequent nutrient et al. 1996a). The effect of water velocity on the
releases (Turner 1993). While generally the release behaviour of the suspended sediment load is cru-
of nitrogen and phosphorus from the saltmarsh cially affected by the particle size of the sediment
249
involved. Studies at Stiffkey, Norfolk, UK, showed Sediment trapping by plants
a significant relationship between sediment load
and peak tidal velocity (r2 ¼ 0.85, p < 0.001) Distinct patterns of sedimentation and vegetation
whereas at Tollesbury, Essex, there was no signifi- dynamics start in the initial stages of colonisation
cant relationship between water velocity and by plants (Richards 1934; Gray 1992). Both
sediment load. This reflects the much higher topography and vegetation affect sedimentation
proportion of coarser sediment particles at dynamics in a feedback process. Sedimentation
Stiffkey (10% > 63 m and 40% < 2 m) compared is increased when the vegetation is taller than
with Tollesbury (2% > 63 m and 60% < 2 m). 8 cm (Boorman et al. 1998). Among the taller
The pattern of distribution of the deposited sedi- species, Spartina anglica enhances sedimentation
ment on the marsh surface is also determined by in European saltmarshes (Thompson 1990;
the sediment particle size (Pitman 1993; Boorman Gray et al. 1991; Sanchez et al. 2001). Puccinellia
1996). With the coarser sediments at Stiffkey there maritima is a perennial grass species whose popula-
was a relationship between the quantity of sedi- tions reach their maximum extent on stabilised
ment deposited in a single tide and the distance lower marshes. It also occurs in the pioneer zone,
from the nearest creek, leading to the build-up of possibly where sedimentation has started.
distinct levees. Similar results have been obtained These processes have been studied at three sites
from a Juncus roemerianus marsh in Florida of Mont St Michel Bay (Langlois et al. 2003). In
(Leonard et al. 1995). At Tollesbury, however, the this area, S. anglica is quite rare in the lower marsh
quantity of sediment deposited was independent of and P. maritima is abundant. P. maritima plays a
the distance from the creek and the deposition of role in the sediment dynamics and in the establish-
the predominantly fine sediment was largely lim- ment of micro-topography in the pioneer zones
ited to the short period at high tide when the water of lower saltmarshes. At the beginning of plant
movements were very slow. Fine grained deposi- colonisation, on mudflats, the community was
tion may, therefore, be dependent both on hydro- composed of S. anglica, Salicornia fragilis and few
period and distance to the creek (Pethick 1992; clones of P. maritima. The micro topography is
Pethick et al. 1992). absent. This plant community changes rapidly in
Rates of sediment accretion recorded at composition and in topography. We can observe
Tollesbury averaged 4.27 mm yearÀ1 over two years after 2 years a low marsh with a large cover of
of observation (Boorman 1996) but there were con- P. maritima associated with Suaeda maritima
siderable fluctuations from month to month with with a low cover and Aster tripolium. For example,
up to 6.2 mm monthÀ1 of erosion and 4.1 mm the cover of P. maritima increased significantly
monthÀ1of accretion. Calculations of the annual from 20% up to 95%. This plant community is
rate of accretion from the tidal sediment fluxes gave localised mainly on hummocks. In the presence of
a potential accretion rate of 3.9 mm yearÀ1, suggest- P. maritima, sedimentation was significantly higher
ing that the regular tidal inputs formed the major than on bare mudflats. Ninety five per cent of the
mode of sediment addition to the marsh surface. At of the P. maritima colonies were recorded on hum-
Stiffkey the rates of accretion of the marsh surface mocks and only 1% on the mudflats. By contrast,
were slightly smaller (3.08 mm yearÀ1). Here, how- all the S. anglica was encountered on mudflats, as
ever, the estimated tidal sediment fluxes were only well as all the annual S. fragilis. At the same sites,
able to account for 0.3 mm yearÀ1, a finding sug- 99% cover by S. maritima was recorded on hum-
gesting that the major sediment input comes with mocks and only 1% on the mudflats. No plants
episodic storm events. Similarly, high rates of accre- were found in the erosion zones in the low marsh.
tion in low Spartina foliosa marsh were almost The sedimentation dynamics in lower salt-
entirely limited to episodic storm-related events marshes are related to both abiotic (e.g., distur-
(Cahoon et al. 1996). On the short time scale, rates bances) and biotic factors (e.g., the vegetation type,
of saltmarsh accretion can be huge, reaching 1 m its cover and height) (Adam 1990; Pethick et al. 1992;
yearÀ1 (Li et al. 2000; Plater et al. 2002). However, Moeller et al. 1997; Dijkema 1997; Boorman et al.
when time-averaged, these rates are much smaller, 1998). Sedimentation is largest in the lowest parts of
typically a few millimetre per year. the marsh provided that vegetation is present
250
(Richards 1934; Kamps 1962; Randerson 1979; matter after plants die remains little known. Some
Stumpf 1983). In the Mont St Michel Bay, signifi- of this organic matter is moved by the tidal flow in
cant sedimentation only occurred in areas covered and out of the marsh, and the end result depends
by P. maritima (Langlois et al. 2003). Puccinellia sp. on the form of the organic matter. Three compo-
was also involved in the geomorphogenesis of the nents exist: dissolved organic matter (DOM) that is
lower marsh, as evidence by the positive linear rela- operationally defined as the fraction of organic
tion between the area of hummocks and the abun- matter not retained on 0.45 m filters; fine (sus-
dance of Puccinellia sp., leading to the formation of pended) particulate organic matter (POM); and
hummocks on which it is dominant. The sedimenta- coarse (>0.2 mm) organic matter (COM), which
tion rate reached 82 mm yearÀ1 in the pioneer zone is generally the same as the floating material. The
(Langlois et al. 2003). Similar observations were behaviour of the first two components, DOM and
reported for Danish and Dutch saltmarshes POM, is essentially the same as that of the sus-
(Jakobsen 1954; Jakobsen et al. 1955; Kamps 1962; pended sediment and is based on the flux of the
Andresen et al. 1990; Scholten and Rozema 1990; tidal water flow. The routes and destinations of
Dijkema 1997). the floating material (COM) under quiet weather
On sandy beach plain type saltmarshes, S. anglica conditions are also similar. Under strong wind
initiates the formation of small dunes that are conditions, floating material tends to be wind-driven
rapidly invaded by P. maritima (Scholten and rather than following the normal tidal routes.
Rozema 1990). Once the micro-topography has The magnitude and routes of export of COM
been established, the rates of rise of the substrate is very variable. At the Tollesbury marshes, UK,
and vegetation succession both accelerate. export of COM only amounted to 7–8% of the
Where sand dominates, the ability of Puccinellia total net above-ground primary production
sp. to stabilise the sediment is especially related to (NAPP; Lefeuvre et al. 1994). These marshes are
the density of its root system and the speed of its at a level where they are subjected to regular inun-
spatial spread by rhizome propagation, as well as its dation and, thus, there are regular opportunities
tolerance to burial (Wohlenberg 1933; Richards 1934; for the exchange of organic matter. When the
von Weihe 1979; Langlois et al. 2001). This is a marshes are at a higher level, such as those at the
perennial species that is relatively abundant in Slufter, The Netherlands, the export of COM tends
the pioneer zones, and which has a very fast rate to be dominated by the occasional storm tide when
of spreading (Langlois et al. 2003). S. anglica is the nett export can be 6 times as high as the largest
renowned for its ability to accrete large volumes import (Lefeuvre et al. 1994). Up to 1/3 of the leaf
of sediment, limiting the erosion and enhancing production of Atriplex portulacoides is exported
sediment trapping (Konig 1948; Ranwell 1972; from its point of origin (Bouchard 1996).
¨
Thompson 1990; Gray et al. 1991; Sanchez et al. A key question remains un-answered on what is
2001). However this species has a limited abund- the final destination of this material exported from
ance, typically less than 10% in sandy sediments, the saltmarsh, whether it is re-deposited elsewhere
because sand is not optimal for Spartina sp. espe- in the marsh replacing local losses, or whether it is
cially when in competition with Puccinellia (Ko deposited along the driftline at the marsh edge or
¨nig
1948; Chater and Jones 1957; Dijkema 1983; whether it is exported to coastal waters. In both
Scholten and Rozema 1990). This could be due to Europe and America, studies have indicated that
waves which inhibit the natural spread of S. anglica, usually less than 1%, but occasionally up to 10%, of
and to ergot fungus, which affects seed viability the organic matter produced is exported as COM,
and germination ability (Morley 1973; Groenendijk that is, as floating litter, to coastal waters
1984; Gray et al. 1990). (Hemminga et al. 1996b; Wolff et al. 1979; Dankers
et al. 1984; Dame and Kenny 1986). The export of
POM and DOM is better known. One major factor
Saltmarshes and fluxes of organic matter
is the relative level of the saltmarsh and the corre-
sponding frequency of inundation. This probably
Primary production and decomposition rates are
explains the apparent major differences that have
high within the saltmarsh – often equivalent to
been observed between the marshes of the east
those of tropical rainforests. The fate of the organic
251
coast of North America, where exports of over 40% (Groenendijk 1984; Boorman et al. 1994a; Lefeuvre
of NAPP are cited, and the much lower values 1996).
obtained from European saltmarsh sites (Dame Measurements show that the saltmarshes of
and Lefeuvre 1994). There can also be indirect Western Europe generally produce more than
1 kg mÀ2 yearÀ1 of above-ground dry matter and
exports of organic matter in the form of fish and
other organisms that come in from the sea to feed that this high productivity is achieved both by
within the saltmarsh and then return to the sea. pioneer and mature saltmarsh plant communities
There are also other links between the terrestrial (Boorman et al. 1994a, b; Lefeuvre 1996). Lower
and the marine ecosystems. For example, when productivities than this have been recorded in pio-
tidal water flows over a saltmarsh there is a marked neer communities, which may be due to the incom-
increase in the bacterial component of the plank- plete development of plant cover in the earlier
ton; mussel beds remove 1/3 of this enhanced stages of colonisation and, thus, do not represent
production (Newell and Krambeck 1995). The the full potential for those species or communities.
emerging picture is that, in European saltmarshes, On the other hand, the pioneer communities domi-
the primary production is not easily exported nated by S. anglica are particularly productive,
producing up to 1.5 kg mÀ2 yearÀ1 of above-
because the generally high level of marshes reduces
the frequency of tidal flooding; nevertheless at least ground dry matter. This is very much in line with
a part of primary production is integrated within the high productivities achieved by Spartina
marine trophic webs (Lefeuvre 1996). alterniflora in comparable New World marshes
(Kirby and Gosselink 1976). Many of the middle
level marsh communities are very productive and
The primary productivity of saltmarshes:
this is particularly true for the shrubby species
conversion of excess nutrients into plant biomass
Atriplex portulacoides (NAPP > 2 kg mÀ2 yearÀ1).
However, low productivity can also be observed in
The above-ground productivity of saltmarsh vege-
middle level communities where productivity is
tation is probably the most visible aspect of estuar-
limited by a combination of low nutrient levels
ine productivity and yet at the same time its
and a high degree of inter-specific and probably
determination has proved to be difficult. Much of
also intra-specific competition. Not all saltmarsh
the research has followed the classical method
systems have the full range of marsh types,
involving the monthly cutting and harvesting of
but where they occur the mature communities of
plots during the growing season and comparisons
the upper saltmarsh can also be very productive,
of the density of the living and dead material
with Elytrigia aetherica producing as much as
(Linthurst and Reimold 1978; Boorman et al.
1.5 kg mÀ2 yearÀ1 (Boorman et al. 1994a).
1994b; Lefeuvre 1996). There is an ambiguity in
Four major components of productivity can be
the method because some workers separate the
recognised in an estuarine ecosystem, namely the
dead material into standing dead and litter, using
production of the saltmarsh vegetation of higher
the standing dead in the determination of produc-
plants, the production of the algae associated with
tivity. The inclusion of litter in the dead material
the saltmarsh, the production of the microalgae on
can result in an apparent increase of net productiv-
the surface of the inter-tidal flats, and the producti-
ity by up to 10% (Bouchard 1996). Measurements
vity of the algae suspended in the water column. In
are usually carried out at monthly intervals, and do
some estuarine systems there are also submerged
not account for the loss from the dead biomass
communities of aquatic plants (sea grasses) but in
between the monthly sampling as a result of flush-
the systems under consideration their distribution
ing of the marsh by occasional, short-lived wind-
is limited and their overall contribution is small. It
driven currents. Thus, the measurement errors may
has to be noted, however, that in certain commu-
be large (Kirby and Gosselink 1976; Boorman et al.
nities, where conditions of sediment stability and
1994b). Despite these complications, this determi-
the clarity of the water favour their growth, they
nation of the primary productivity of saltmarsh
can make a significant contribution to the overall
communities, based on monthly sampling, can
productivity (Heip et al. 1995). Saltmarsh vegeta-
provide a reasonable picture of the range of produc-
tion has both above- and below-ground production.
tivity of northern temperate saltmarsh vegetation
252
In terms of the contribution of the marsh to the phenomenon is often seen in estuaries, such as
estuary as a whole it is the above-ground those in drowned river valleys, where a saltmarsh
productivity that is more important, because abuts high ground with the possibility for artesian
the products of above-ground productivity can be flows. Such flows are common also in the uplands
transported by water currents from the wetlands to (Jones 2002). Because many saltmarshes have layers
the estuary. The products of below-ground produc- of coarser sediments, the tidal movement in the main
tion are largely retained within the saltmarsh sub- creeks can be reflected by changes in the ground
system (see the section on ‘Saltmarsh impact from water levels and flows at considerable distance
the flow of ground water’). from the creeks. Seepages can frequently be seen in
The mean productivity of saltmarshes has been the banks of saltmarsh creeks and while these are
estimated as 500 g mÀ2 yearÀ1 at Tollesbury, UK, generally local saline flows, the existence of non-
and 450 g mÀ2 yearÀ1 at Stiffkey, UK (Boorman saline groundwater is likely to result in a significant
1996). These values correspond to carbon produc- freshwater contribution to the marsh system.
tion of 200 and 180 g C mÀ2 yearÀ1, respectively. The contribution of groundwater flows to salt-
The growth of algae within the saltmarsh is marshes can be significant. For example, during
included within these values; it probably accounts periods of low precipitation artesian groundwater
for 5–15 g C mÀ2 yearÀ1. The value is low because can account for essentially all of the freshwater
of the relatively small contribution of algae to the input to a marsh ecosystem in North Carolina
total biomass. However, in local areas dominated (Gramling et al. 2003). In studies in South
by macroalgae, algal production can be similar to Carolina, the ground waters in the marshes are
that of the vascular plants of the saltmarshes them- mixtures of sea water and fresh water and measur-
selves. The overall productivity of the English salt- ably mediate land–sea carbon fluxes (Cai et al.
marshes is generally similar to those in France and 2003). One of the species indicative of the presence
The Netherlands, although the marshes in the of freshwater inputs into saline environments is the
highly eutrophic Westerschelde have a markedly common reed, Phragmites australis, and the spread
higher productivity (Lefeuvre 1996). The micro- of this species has been linked to fresh groundwater
algae of the intertidal flats are important for their flows reducing salinity and the concentration of
productivity and for their contribution to the sulphide-reducing soils (Chambers et al. 2003).
stabilising of sediments through the excretion of The concentrations of the macronutrients N and
carbohydrates (Holland et al. 1974; see the section P influence the growth and development of salt-
on ‘Formation of mudflats’). marsh vegetation, and they are influenced by
The productivity of north temperate microphyto- groundwater flows (Corbett et al. 2002).
benthic communities is in the range 30–230 g C mÀ2 It is clear that saltmarsh nutrient fluxes can be
yearÀ1 (Heip et al. 1995). A typical value, and close to affected by the hydrological situation, particularly
that quoted for several English and Dutch sites, the magnitude and status of groundwater flows
would be of the order of 150 g C mÀ2 yearÀ1. This is (Anderson et al. 2001; Sutula et al. 2001). Studies
lower than the estimate of 200–300 g C mÀ2 yearÀ1 on the links between land and sea as mediated by
for the Delaware Estuary, USA, and is in line with coastal ecosystems need to take this additional
the estimate of 125 g C mÀ2 yearÀ1 for the factor into consideration. Hydrological dynamics
Westerschelde (Pennock and Sharp 1986; Nienhuis must be seen as a crucial factor for ecological pat-
1993; Jacobsen et al. 1995). terns and processes and, indeed, as the key over-
all linkage between soil, climate and vegetation
(Rodriguez-Iturbe 2003). Methods are available
Saltmarsh impact from the flow of ground water
for quantifying groundwater discharge at the
land-sea interface (Gramling et al. 2003).
The dominant factors in saltmarsh development
are the regular tidal coverage by seawater and the
varying salinity. An additional factor is the outflow Saltmarshes as fish habitats
of fresh groundwater (Figure 3). This can lead to
anomalies in the vegetation, exemplified by the The primary productivity of saltmarshes is largely
local increase of less salt-tolerant species. This assimilated though the degradation and release of
253
organic matter, particularly through the activities inundated. The total import flux of sediment at rising
tide was 12 kg mÀ2 tideÀ1, and the total export flux
of marine and terrestrial invertebrate species.
of sediment at falling tide was 2 kg mÀ2 tideÀ1. This
Associated with this are complex food chains lead-
ing to the larger vertebrates, including econo- implies for this area a net import in the mangrove
forest of fine sediment of 10 kg mÀ2 tideÀ1. Most of
mically significant species of fish. Fish form an
important component in many estuarine ecosys- this sediment settled in the mangroves within 50 m
tems. There are numerous fish species; for example, of the creek. That the mud was able to advance so
as many as 50 in the medium-sized Mira Estuary, far into the mangroves and did not deposit within a
Portugal, where, in terms of number and of bio- few m of the banks, was due to the fine-scale tur-
mass, the Common Sole (Solea vulgaris) is the most bulence of the flows through the vegetation keep-
abundant species (Costa 1988; Costa et al. 1996). ing the fine sediment in suspension. Deposition was
Pristine habitats support an extremely rich bio- observed to occur just before slack high tide and
diversity that can support commercial fisheries, lasted only about 30 min.
the success of which is closely linked to contin- Wolanski et al. (1998a) repeated such studies in
ued habitat conservation (e.g., Al-Mohanna and the mangroves of Hinchinbrook Channel, Australia,
Meakins 2003). Fish are an indicator of saltmarsh and found that the mangroves accumulated mud
at a rate of about 1000 ton kmÀ2 yearÀ1. To deter-
ecosystem health; indeed, the success of habitat
restoration can be judged by the extent to which mine how this sediment-trapping rate varies with
the fish populations have recovered (Tupper suspended sediment concentration in the rivers,
and Able 2000). A well-developed creek system is further studies were undertaken in the mangroves
important in this recovery (West and Zedler 2000). of the Ngerikiil and Ngerdorch estuaries in Palau,
Micronesia (Victor et al. 2003). These two rivers
drain adjacent, similar, mountainous areas of com-
parable area in the wet tropics. The former catch-
Mangroves
ment is being deforested, the latter is still forested.
This results in a factors of 10 differences between
There are many similarities between physical and
the sediment yield in these two catchments. The
biological processes in mangroves and saltmarshes.
mangroves comprise 3.8% of each of these two
Similarities include their role in trapping sediment,
river catchments, they flood semi-diurnally, and
converting nutrients to plant biomass, trapping
in both cases they trap about 30% of the riverine
pollutants and serving as a habitat for fish and
fine sediment. This mud trapping efficiency, while
crustaceans (Figure 3).
helpful, is however not sufficient by itself to pre-
vent degradation of coastal coral reefs from exces-
Mangrove sediment dynamics
sive sedimentation from extensive land clearing
and poor farming practices; better land care is still
During spring tides, mangroves are fully inundated
required in the river catchment.
at high tide. A deep tidal channel, fringed by inter-
Mangroves are, thus, very efficient mud traps,
tidal, mangrove forest, is usually maintained by
and this may help mangroves to keep up with the
self-scouring; this is due to a difference between
expected rise in sea level. The level of success will
peak water velocities at rising and falling tides
depend on the availability of sediment from river-
(Wolanski et al. 1980).
ine inflow and from the coastal zone (Chappell and
Within the mangrove forest, the velocities are
Woodroffe 1994; Wolanski and Chappell 1996). As
slightly larger during the rising tide than during
sea level is predicted to rise, history may be a good
the falling tide (Furukawa et al. 1997). At their
guide to predict how mangroves will fare in the
study site near Cairns, tropical Australia, the
future. Following the last glaciation, the sea level
suspended sediment concentration peaked at
rose at a rate of up to 1 cm yearÀ1. At those sites
about 150 mg LÀ1 on the rising tide and at about
30 mg LÀ1 on the falling tide, indicating import and where sediment accumulation rate kept up with the
sea level rise, the mangrove forests survived. At other
trapping of sediment. Sediment was only imported
sites mangrove migrated landwards. Elsewhere the
at spring tides; there were zero fluxes into the man-
mangroves were submerged and perished. Their
groves at neap tides because the swamp was not
254
burial sites are still recognisable by the mangrove causality (Boto and Wellington 1983, 1984; Boto
mud at the bottom of some tropical coastal seas. et al. 1989; McKee 1993). There is a pressing need
This old mangrove mud is often capped by a layer to integrate existing knowledge to understand how
of new sandy or calcareous sediment brought in a mangrove ecosystem works. The key concepts
by wave action on a sandy beach (Grindrod and include: flushing time, inundation frequency, resi-
Rhodes 1984). After sea level stabilized about 5000 dence time, degree of water-logging and stagnancy,
years ago, mangroves reestablished themselves at hyper-salinization of soils, acid sulfates, water
some, but not all, sites where they still exist. At depth and stagnancy. All of these are interrelated
other sites, the old mangrove forest has perished variables that can drastically change sediment char-
and has been buried by recent freshwater sediment acteristics. Topographic differences of a few cm can
from riverine inflow (Chappell 1993; Chappell generate major differences between sites with con-
and Woodroffe 1994). In mud-poor, reef environ- trasting sediment and vegetation characteristics.
ments, mangroves were unable to keep up with sea For example, in North Brazil, in transition areas
level rise and drowned. They recolonised the area between mangroves and saltmarshes, elevation
increases of $10 centimetre can be reflected in a
only when sea level stabilized (Fujimoto et al.
1996). In other cases such as in the Gulf of decrease of average inundation frequency from
60 to 40 days yearÀ1. This produces differences in
Carpentaria, tropical Australia, isostatic adjust-
ments lifted the mangroves out of the water, trans- sediment quality causing the existence of an ecotone
forming them into a hypersaline mudflat (Rhodes bordered by a monospecific Avicennia forest and
1980). an herbaceous plain. Mangroves react quickly and
Recent studies by Furukawa (pers. comm.) in dramatically to variations in relative sea level
mangroves fringing mud-poor reef water of (Lara et al. 2003, and references therein).
Iriomote Island, Japan, show that small wind In addition to inundation dynamics, other fac-
waves resuspend the mud, which is then exported tors are influenced by changes in relative land-sea
at ebb tide. As a result the mangroves are nutrient- level. The degree of exposure of sediments to air is
starved and stressed. They are stunted and wea- important for determining, for example, the nutri-
kened and woodborers extensively attack them, ent availability and the salinity. Tidal inundation
because stressed trees generate less tannin than produces a diffusion of oxygen from the flooding
healthy trees. water into the sediment and influences salt trans-
port (R. Lara, pers. comm.). The frequency of
inundation determines soil salinity and the fre-
Nutrients in mangrove sediments: trapping and
quency of anoxia (Boto and Wellington 1984;
mobilization
Leeuw et al. 1991). In turn, this controls nutrient
availability because a drier surface layer with lower
The capacity of mangroves to trap fine sediments is
permeability to gas exchange causes more hypoxic
essential for the ecosystem. This includes among
conditions.
others the role of sediments as a physical substrate
Mangrove sediments are commonly anaerobic,
and as a nutrient source for forest development.
organic-rich intertidal/estuarine muds. Yet, there
The role of biogeochemical transformations within
are also substantial differences in the textural
the sediment is relatively well understood (i.e.,
and physicochemical properties according to forest
microbial processes, plant/sediment relationships);
type and inundation dynamics. In contrast to
however, less is known about the role of system-
muds in temperate saltmarshes, mangrove muds
shaping agents such as inundation dynamics on
are less anaerobic, probably because of crab holes
nutrient trapping or mobilization.
and the low bioavailability of organic matter
Few researchers treat these issues in a simulta-
(Alongi and Sasekumar 1992). Crabs are a primary
neously detailed but integrative manner, building
means for incorporation of significant amounts of
causal chains between hydrology, sediment proper-
organic matter from leaf litter fall deep into man-
ties, and vegetation structure. Numerous observa-
grove sediments as well as causing an active turn-
tions have been summarized within empirical rules
over of detritus at the sediment surface (Schories
such as ‘Avicennia prefers high places’, but relatively
et al. 2003). Key characteristics of mangrove
seldom has anyone looked beyond correlations into
255
sediments such as redox potential or sulphide levels (Walbridge and Struthers 1993). Particularly
can be also modified by the root systems of adult redox potential has a significant effect on the P
trees (McKee 1993). availability in mangrove sediments. Increasing
Other typical features of mangrove sediments are inundation frequency elevates redox potential in
relatively low concentrations of dissolved inorganic the sediment, that is, it creates more oxidative con-
nutrients, for example, nitrate, ammonium and ditions by diffusive aeration, and elevates the con-
phosphate in porewater, and the presence of tan- centration of available P. At lower elevation sites,
nins derived from leaching and decomposing roots the higher flooding frequency accounts for a higher
and litter (Alongi 1987; Boto et al. 1989). The total P content. Higher P availability is reflected
reasons for low nutrient concentrations in pore- in vegetation biomass (Boto and Wellington 1984)
water are rapid turnover, such as high uptake by and P content in leaves (Lara, unpubl. data).
trees and bacteria, as well as redox conditions and Freshwater flood plain ecosystems are controlled
the recalcitrant character of organic matter. by a combination of topographic, hydrographic and
Ammonium is the main form of inorganic N in chemical processes (Townsend and Walsh 1998); an
mangrove soils because nitrification is prevented equivalent understanding of mangrove ecosystems
due to the lack of oxygen to oxidize it into nitrite/ is largely missing (Cohen and Lara 2003).
nitrate. Concentrations of ammonium are higher
in muddy than sandy soils and are influenced by
the degree of tidal wetting, rainfall, and biological Estuarine food webs
uptake and production. Concentrations of dis-
solved organic N can be higher than dissolved inor- When summarising key biological processes within
ganic N. The high sodium level in most mangrove the water column, it is particularly difficult to keep
soils displaces ammonium, which is thus mostly a balance between a simplistic description that
found in the interstitial waters where it can be highlights the interplay between components of
leached by rainfall or tidal inundation or drained the system, and a scientifically thorough approach
(Alongi et al. 1992). Tidal inundation generates a that does justice to the ecological complexity,
nutrient exchange between sediment and estuarine where the general picture can be lost. One can
waters (Dittmar and Lara 2001). find in the scientific literature dozens of figures
Interactions between inundation dynamics, sedi- describing estuarine food webs for different estu-
ment characteristics, and vegetation structure in aries. The description of estuarine food webs often
the basin can be particularly well illustrated with gets bogged down in local details. Some of these
reference to phosphorus mobility. For example, figures show simplistic food webs, others show
changes in porewater phosphate concentrations in extremely complex food webs with numerous feed-
sediments beneath mangrove communities in the backs; other figures focus on biomass and highlight
Indian River, USA, seemed to be linked to varia- phytoplankton dominance, while yet others
tions in tidal inundation frequency (Carlson et al. emphasise the key role of detritus.
1983). Due to its dependence on tidal height, P can
become limiting in elevated mangrove forest areas Generic description of estuarine food webs
(Boto and Wellington 1983). Concentration of
dissolved inorganic P in mangroves is generally A generic approach is used to describe food webs in
low (Alongi et al. 1992; Lara and Dittmar 1999). estuaries (Figure 3). Estuaries are converters of
Bacteria and microalgae in mangrove muds are living phytoplankton to detrital particles; they are
also probably P-limited. A close microbe-nutrient- also conveyors of both allochthonous and auto-
plant connection may serve as a path to conserve chthonous detrital particles to the sea. Herbivorous
scarce nutrients necessary for the existence of these fishes transfer energy and matter from estuarine
forests (Alongi et al. 1993). plants to upper trophic levels and to the coastal
Retention of dissolved phosphate and long-term zone. However, direct grazing by herbivores con-
P storage in wetlands is probably controlled by sumes only a small proportion of the macrophyte
adsorption by Al and Fe oxyhydroxides and by and macroalgal production. The great bulk of
precipitation of Al, Fe and Ca phosphates the organic matter produced (sometimes 90%) is
256
processed through the detrital system. Zooplank- production (Rhee 1972). This situation may occur
ton, planktivorous fish, interstitial micro and meio- in estuaries with large fringing saltmarshes and
fauna, surface deposit-feeding molluscs, fishes and with little external input of inorganic N. More
polychaeta, and filter-feeding invertebrates con- commonly though, estuarine waters are often
sume a much greater proportion of the primary enriched with N and P from rural, urban and
production of the phytoplankton and benthic industrial wastewater discharges (Uncles et al.
microalgae. Annual plant growth and decay pro- 1998a, b).
vides continuing large quantities of organic detri- The fate of nutrients depends on a number of
tus. In addition, there is often a considerable input physical, biological and chemical processes.Physical
of detritus from river inflow. Detrital particles and processes include mixing, flushing and sedimenta-
their associated microorganisms provide the basic tion. Chemical processes include absorption and
food source for primary consumers such as zoo- desorption. Biological processes include stripping
plankton, most benthic invertebrates and some of dissolved and particulate nutrients, primarily by
fishes. Many of the estuarine consumers are selec- bacteria and phytoplankton (Alongi 1998). Biologi-
tive and indiscriminate feeders on particles in sus- cal and chemical processes increase in importance
pension in the water column, or in the sediment with increasing values of the residence time because,
that they ingest. Thus, most of the biota that inha- when the residence time is large, there is more time
bits estuaries can be considered as particle produ- for these processes to act (see the section on ‘The
cers (microalgae and detritus derived from plant flushing of estuarine waters’).
growth) and particle consumers. The first trophic For example, organic N entering the estuary is
level in the estuarine ecosystem is therefore best degraded to ammonia and further nitrified by aero-
described as a mixed trophic level of detritus con- bic bacteria; this increases nitrate concentration,
sumers, which in varying degrees are herbivores, resulting in a convex curve of nitrate versus salinity
omnivores or primary carnivores (Knox 1986). (see the section on ‘Estuaries as sieves for nutrients’
and Figure 4). These biological processes consume
oxygen and lead to depletion of oxygen content if
Environmental factors affecting estuarine and
the water column is not well aerated. Phytoplank-
coastal biota
ton consumes the produced nitrate rapidly, in
which case a removal of nitrate may be observed.
Estuaries are highly productive ecosystems because
High loads of N and P may lead to depletion of
they are often nutrient rich and have multiple
silicate (see the section on ‘Estuaries as sieves for
sources of organic carbon to sustain the population
nutrients’) during phytoplankton blooms, condi-
of heterotrophs. The nutrient sources include riv-
tioning the successive phytoplankton assemblages
erine and waste inputs and autochthonous primary
during the remaining productive period. Sub-
production by vascular plants, macroalgae, phyto-
sequent low river discharges may provide an
plankton and benthic microalgae (Cloern 1987).
environment with low Si : N and N : P relative avail-
These nutrients sustain a trophic structure that
ability, and this may favour dominance of cyano-
varies from estuary to estuary (Degan et al. 1994).
bacteria and even toxic algal blooms (Rocha et al.
Inorganic nutrient loading stimulates primary pro-
2002). Pulses of freshwater discharges (and their
duction and generates a shift to larger-sized phyto-
nutrients) stimulate the development of a more
plankton (Boynton et al. 1982). Organic matter
diversified phytoplankton assemblage that supports
loading stimulates bacterial production, in differ-
zooplankton (Rey et al. 1991), that is, top-down
ent ways depending on the form of dissolved
control, and this hinders harmful algae blooms.
organic matter. Microbes, through the microbial
Geochemical and biological transformations are
loop (Figure 3) serve as N-remineralizers when
concentrated in the ETM (see the section on ‘SPM
metabolizing N-rich organic compounds because
and sediments as repositories of organic carbon’ and
they take up inorganic nutrients when organic
Simenstad et al. 1994). The long retention time of
inputs are deficient in N or P (Kirchman et al.
SPM in the ETM – and the nutrients it contains –
1989). Increased riverine imports of organic matter
enables bacteria to consume particulate nutrients
in an estuary may result in decreased nutrients
and to incorporate them into primary levels of the
availability for algae that may limit phytoplankton
257
estuarine food web by increasing the importance of stocks of prey species attracts secondary and tertiary
heterotrophic processes (Reed and Donovan 1994; consumers in this region (Simenstad et al. 1990).
Thayer 1974). However, this may also result in dep- Salinity changes in the estuary induce a physio-
leting the dissolved oxygen (see the section on ‘SPM logical stress in organisms that must then spend
and sediments as repositories of organic carbon’). extra energy in physiological adaptation changes.
Light penetration is a limiting factor in the ETM, This may reduce growth and body size in organ-
inhibiting phytoplankton production (see the sec- isms. In fact, because of the extra energy spent in
tion on ‘Estuaries as sieves for nutrients’). This physiological adaptation, brackish water organ-
decrease of primary production by phytoplankton isms are usually smaller than their marine relatives
results in minimal excretion of photosynthetic pro- (Harris et al. 2000). Production and consumption
ducts that are a food source to bacteria (Goosen are determined by the relative availability of food
et al. 1999). When SPM exceeds 50 mg LÀ1, phyto- (Simenstad et al. 1990). The critical process deter-
plankton photosynthesis is severely limited mining the impact of hydrology on estuarine and
(Cloern, 1987). In estuaries where primary produc- coastal food webs is related to organism size. Size is
tion rates are less than 160 g C mÀ2 yearÀ1, phyto- a determinant feature in prey–predator interac-
plankton are light-limited and these systems can be tions. Thus different food webs for the same species
considered heterotrophic (Heip et al. 1995). The can exist in estuaries and coastal areas. Similarly,
phytoplankton biomass can also be regulated by natural and human-induced changes in river flows
vertical stratification in salinity (Cloern 1987). lead to different food webs.
Microphytobenthos comprises the microscopic,
photosynthetic eukaryotic algae and cyanobacteria Feedback between biological and physical
that grow on, or close to, the sediment–water inter- processes
face in illuminated inshore areas. Microphyto-
benthos, anaerobic and chemosynthetic bacteria There are numerous feedbacks between physical
may also contribute to microphytobenthic produc- and biological processes in considering food webs;
tion. Because they are found at the sediment inter- a few dominant ones are summarized here. Larval
face, the microphytobenthos also modulates the and juvenile species exist in estuaries, both in the
exchange of nutrients between the sediment and water column and the bottom. Coastal species,
water column (Ser^dio and Catarino 1999).
o such as the anchovy Engraulis encrasicolus or the
Freshwater phytoplankton and bacterioplank- crustacean Crangon crangon, use the estuary as a
ton in the river are subject to salt stress when fresh- ´
nursery ground (Chıcharo et al. 2002). In turn, these
water mixes with saltwater (Flameling and groups affect the uptake and release of organic and
Kromkamp 1994). The freshwater microbial popu- inorganic materials (from excretion products) and
lations die in this zone (Goosen et al. 1995). influence fluxes between water and sediment.
Salinity changes also affect estuarine plankton On the mud and sand flats, detritivorous macro-
and some invertebrate species (e.g., bivalves). Salt- benthic invertebrates benefit from the deposition of
hardy invasive species are increasingly invading particulate organic matter that they use as a food
this ecological niche. For instance, salinity changes source (Snelgrove and Butman 1994). These
in the Guadiana Estuary, Portugal, favour the benthic organisms change the physical and chemi-
introduced, salt-resistant, Asian clam (Corbicula cal properties of sediments (see the section on
fluminea) that is increasingly out-competing the ‘Formation of mudflats’), modifying, in turn, the
salt-intolerant indigenous clam (Anodonta cygnea) bottom shear stress, currents and flushing of the
´
(L. Chıcharo, pers. comm.). estuary (Jumars and Nowell 1984; Brown et al.
Estuarine fishes suffer less impact from salinity 1998). Through bioturbation, these organisms can
changes due to their ability to utilize osmotic reg- also cause sediment mobilization or, conversely,
ulation (Whitfield and Wooldridge 1994; Thayer increase stability as in the case of dense mats of
et al. 1999). In the ETM, primary consumers are worm tubes, or by enhancing the deposition of fine
generalist and omnivorous feeders, capable of ex- particles through biodeposition (Schaffner 1990;
ploiting both autotrophic and heterotrophic food Luckenbach et al. 1988; Widdows et al. 1998;
web pathways. The presence of higher standing Noji and Noji 1991; Ayukai and Wolanski 1997).
258
The amphipod Corophium volutator (Pallas) may et al. 2002). Strong outwelling or river plumes are
reduce sediment binding by grazing benthic dia- important for these species because they require tur-
toms that contribute to sediment aggregation bidity or salinity signals for orientation to find ade-
(Gerdol and Hughes 1994). Birds, by feeding on quate spawning areas (Kingsford and Suthers 1994).
this amphipod, decrease the erosion of the sedi-
ment (Brown et al. 1998).
Enhanced erosion and resuspension of sediment Human health and aquatic ecosystems
through bioturbation may result in greater sedi-
ment accretion on adjacent saltmarshes (Brown Ecological integrity is central to human health
et al. 1998). Bivalves also play an important role (Epstein 1999). Threats to human health arising
in reducing phytoplankton densities and control- from human interaction with aquatic ecosystems
ling algal blooms (Takeda and Kurihara 1994). can originate from multiple factors, which might
Attrition of particles in the gut, together with selec- be broadly grouped into three main categories:
tive feeding of fine particle sizes by suspension and * derived from the effects of water pollution (che-
deposit-feeders and incorporation into faecal mical, microbiological, radioactive, thermal) on
pellets, results in deposition of finer mineral and humans and on the physiology of individual
organic particles in the surrounding sediment, with organisms;
an increase of absorbed and adsorbed nutrient and * those resulting from management of aquatic
trace elements concentrations (Brown 1986). The resources (e.g., wetland drainage, aquaculture,
resulting availability of these nutrients enables the dam construction);
growth of macrophytes and macroalgae, which are * effect of global changes affecting climate and
contributors to the primary production in estuaries the hydrological cycle (e.g., habitat degradation,
(Simenstad et al. 1990). warming, increased rainfall, storms).
These categories are interlinked.
The transition between estuaries and coastal
zones Examples of health threats derived from water
pollution
Compared with the extensive production and
transport of detrital carbon, the consumption by A wide range of activities on land contributes to the
detritivores constitutes only a minor reduction release of contaminants (typically organic chemi-
of the organic matter exported to the nearshore cals, heavy metals, microorganisms, sewage, nutri-
coastal waters. Detritus can be removed via grazing ents) into the sea by rivers, run-off, groundwater
by zooplankton, epibenthos and benthic fauna. and the atmosphere. The bulk of these contami-
The exchange of zooplankton between estuary nants remains in coastal waters, particularly in
and adjacent coastal waters results in the export poorly flushed areas. Water related diseases cause
of organic carbon (Simenstad et al. 1990). The millions of preventable deaths every year, especially
export of nutrients from the estuary also contri- among children (UNDP 1998). Greater incidence of
butes to primary production in coastal zones. illness due to consumption of contaminated fish and
The coastal fish community in the Guadiana shellfish is an increasing concern. Harmful algal
Estuary, Portugal, changed from planktivorous blooms in many coastal regions in the world cause
during high river inflow periods, to demersal- a number of diseases, including poisoning, neurolo-
carnivorous during low river discharge periods gical disorders and gastroenteritis (HEED 1999;
´
(Figure 3; L. Chıcharo, pers. comm.). During per- UNEP 1999). Increased flooding as a result of
iods of high river inflow, the increase in primary changes in precipitation may cause contamination
productivity and consequent availability of food for of water supplies, which leads to greater incidence
planktivorous pelagic fish species, like anchovy of faecal-oral contamination (WHO 1996). Typhoid
(Engraulis encrasicolus) or sardine (Sardina pilchardus), and paratyphoid fevers, produced by Salmonella
may explain higher fishery catches in the typhi and S. paratyphi A, B, C can occur in areas
´ ´ ´
coastal zone (Soberon-Chavez and Yanez-Aracibia with poor sanitation and inadequate sewerage sys-
´
1985; Ray 1996; Sklar and Browder 1998; Chıcharo tems. They are transmitted by contaminated water
259
and food. Crop irrigation with untreated sewage In some of the tropical regions of the developing
can cause higher Salmonella infection rates among world, the incidence of malaria has increased in
children living in the wastewater-spreading area recent years as the mosquito and the malaria para-
(Melloul and Hassani 1999). site it spreads have evolved more resistance to
Also, eutrophication is depriving lakeside resi- DDT alternatives and to the medications used
dents of good water quality in many densely popu- to prevent or treat the disease (Thurow 2001).
lated areas of the world (UNEP 1999). Malaria now strikes more than 300 million people
In the developed countries, Cryptosporidium and per year and kills about a million of those affected.
Giardia are two of the most common agents of In some regions (e.g., South Africa) DDT is again
surface water-associated infections and disease increasingly used to control mosquitoes. In gen-
outbreaks. These parasitic infections, primarily eral, land claim for agriculture, deforestation and
causing diarrhoea, are difficult to control due to changes in land use most probably bear the greatest
the high resistance of the oocysts and cysts to blame for the climatic and habitat changes produ-
environmental stress and disinfectants. The level cing this development. Yet, it was primarily the
of contamination is particularly dependent on drainage of wetlands for agricultural purposes
the land use in the water-catchment area, on waste- that contributed most significantly to the eradica-
water discharge, and on weather conditions tion of malaria in Europe (Reiter 2000).
(Exner et al. 2001). It is today often erroneously thought that larvae
For decades, a great deal of research has concen- of malaria-transmitting mosquitoes can only
trated on the direct influence of contaminants on develop in freshwater. Furthermore, malaria is
aquatic biota, their bioaccumulation in trophic not restricted to the tropics. Only in 1975 did
webs, and their detrimental effect on humans. the World Health Organization declare that
Examples of threats to human health arising from Europe was free of malaria. About 200 years ago
the degradation ofmarine, freshwater and watershed malaria was a leading cause of death in many
ecosystems are summarised in Table 2. Informa- marshlands communities along the coast of south-
tion about these topics is available in the literature ern England (Reiter 2000). There, extensive salt-
and will not be further related here. marshes provided high-quality grazing for sheep
and cattle, but they were also a favoured habitat
for a highly effective malaria vector, Anopheles
atroparvus, which prefers to breed in brackish
Examples of health threats derived from
water along river estuaries and in the presence of
management of aquatic resources
abundant algae. Until the 19th century, malaria
was a major mortality factor in The Netherlands.
Wetlands
By the end of that century transmission had
These ecosystems filter floodwaters and support
dropped precipitously in the more prosperous
the genetic, species and functional group diversity
countries of North Europe. A major factor contri-
that ensures resilience to stress and resistance to
buting to this decline was that the mosquito habitat
pests and pathogens (Epstein 1999). Mangroves,
was eliminated by improved drainage and extensive
marshes and floodplains have been devastated in
land claim. However, major epidemics occurred in
the last centuries and decades. In the 20th century
Russia and Poland in the 1920s, with high death
alone, some 10 million square kilometres of wet-
rates reaching regions near the Arctic Circle.
lands have been drained across the globe, an area
Today, malaria is again common in many parts
about the size of Canada (Chivian 2002). This has
of Central America, northern South America, tro-
brought losses of habitat and retention capacity of
pical and subtropical Asia, some Mediterranean
water and sediments, as well as the filtering func-
countries and many of the republics of the former
tion for waterborne nutrients. Interrelated issues
USSR. Thus, policies on wetland creation or
affecting aquatic ecosystems such as loss of biolo-
restoration must carefully consider, besides the
gical diversity, endangered physical coastal stabi-
benefits of the re-establishment of lost ecological
lity, floods, and eutrophication make it a priority
services, the potential consequences of increased
to adequately restore these environments and the
areas of slow-flowing or stagnant waters on disease
‘ecological services’ they offer.
260
Table 2. Examples of threats to human health arising from the degradation of marine, freshwater and watershed ecosystems. Modified
from UNEP (1999).
Driving forces Changing ecological pattern Influence on human health/example
Marine
Pollution from oil, industry, naval Deterioration of marine ecosystems from Decrease in life expectancy, skin and eye
operations and sewage discharge imbalances due to dense ship traffic diseases (Black/Azov seas, Caspian Sea)
Biological/bacterial contamination due Effects on fish and algae Typhoid, malaria, diphtheria (Central
to hydrological changes Asia)
Rising sea levels Ecosystem destruction and pasture See main text in the frame of climate
degradation; changes in depth and change
amplitude of water fluctuations lead to
reduction of wildlife habitats
Biological contamination of surface Gastroenteritis, eye and skin infections
water with waste water (UK, France, S. Africa)
Biological and chemical contamination: Shellfish poisoning, wildlife mortalities, Poisonings, diarrhoea, dehydration,
harmful (toxic and nontoxic) algal sunlight penetration prevention, oxygen headaches, confusion, dizziness, memory
blooms from the rapid reproduction shortages, reservoirs for bacteria loss, weakness, gastroenteritis, bacterial
and localized dominance of infections, swimming related illnesses,
phytoplankton neurological diseases, deaths (Florida,
Gulf States, S. America)
Contamination: cholera contaminated Coastal shellfish and fish contamination Cholera (Peru and other 16 countries)
sea plankton due to contaminated
ships’ hulls
Freshwater
Dam construction Inundating of lands Schistosomiasis epidemics
(Senegal River: Manantali and
Diama dams)
Changes in hydrogeological cycle Habitat alteration, new breeding sites for Intestinal nematode infections
mosquitoes (Yangtze River)
Changes in water flow: High levels of suspended soils and faecal Rift valley fever (Senegal)
scarcity and degradation of water bacterial content Faecal infections, intestinal diseases
resources contaminated river systems (S. Africa)
Pollution
Pesticides, herbicides, fertilizers and Water quality deterioration Health problems (general), infant
defoliants. Airborne toxic salts mortality and morbidity (Central Asia)
Water contamination with heavy metals Accumulation in biochains Progressive and irreversible kidney
(Cadmium) damage (Japan)
Faecal water pollution Diarrhoea, Hepatitis (Bangladesh, India)
Declines in ground water levels and Declines have been implicated in the Poisoning (Bangladesh, India)
groundwater contamination widespread arsenic contamination of well
water
Degradation, e.g., fallen water tables, Severe increase in salinity Cholera outbreak (1995), infectious
chemical contamination diseases, intestinal parasites (Gazan
aquifer, Palestine, Israel)
Catchment
Deforestation Erosion, changes in hydrological cycles Malaria (S. Africa)
Firewood shortage, land degradation Trauma, allergic reactions, infections,
‘‘slash and burn’’ use, increased wood respiratory diseases, cancer (Gambia,
transport and processing infrastructure Kenya, South Africa, Zimbabwe)
New breeding grounds for insects Yellow fever (Ghana, Nigeria, Zaire,
Sudan, Liberia, Cote d’Ivoire)
Agroecosystem development: e.g., land New breeding grounds for insects Japanese encephalitis (Japan, China,
inundation for rice growing India)
Intensified agriculture, irrigation Insecticide resistance Malaria (Central America)
schemes damaging ecosystems
261
vector proliferation, particularly under a scenario water of ships. As many as 500,000 people were
of increasing temperatures, which could allow dis- afflicted; there were 5000 deaths.
ease causing organisms to significantly extend their
range.
Habitat degradation
The degradation of ecosystems, such as forests,
Aquaculture wetlands and coastal waters, is the single most
Threats to human health from aquaculture pro- important factor behind the current mass extinction
ducts can arise from the misuse of therapeutic of species. Compared with the knowledge of the
drugs (e.g., chloramphenicol), chemicals (e.g., tri- impact of human pressure on aquatic ecosystems,
butyltin), fertilizers (e.g., raw chicken manure con- almost nothing is known about the effects of
taining salmonella), and viruses from shrimp farms diversity loss in these systems on human health. A
spreading to wild stocks (US EPA 1998). The aban- new category of extremely serious threats to eco-
donment of small aquaculture ponds in tropical system and human health arises from the so-called
countries leads to an extension of mosquito habi- ‘emerging infectious diseases’ (EIDs), which in sev-
tats and concomitant increases in malaria. Escape eral cases arise from human-induced habitat
of exotic species and animal pathogens into the destruction or invasion. EIDs link health and the
environment can have a tragic impact on native fate of humans and wildlife; EIDs are ‘one of
aquatic species. The FAO and the WHO recom- the most significant, yet underestimated, anthro-
mend that the Hazard Analysis and Critical pogenic threats to biodiversity conservation’
Control Points (HACCP) concept be applied to (Daszak et al. 2000). In turn, disturbances in bio-
fresh water aquaculture programs to control food- diversity are probably a driving force in major
borne digenetic nematode infections in humans emerging disease threats to humans. These include
(Garret et al. 1997; and references therein). Hantavirus, influenza, AIDS, Lyme’s disease,
Ebola virus – they are all zoonotic or have resulted
from pathogens switching hosts from wild animal
Examples of health threats derived from global
reservoirs to humans.
changes
If knowledge of the above field is just beginning
for terrestrial ecosystems, much less is known
Global warming
about human-relevant EIDs in aquatic ecosystems.
The effect of global warming on aquatic ecosys-
Mass mortalities due to disease outbreak have
tems, mainly concerning deforestation of water-
recently affected major marine taxa. Emerging dis-
sheds and plankton blooms, is considered one of
eases are associated with algal toxins, and bacteria
the driving forces behind re-emerging diseases such
and viruses are affecting fish, shorebirds, and
as cholera and malaria. Strong evidence links the
mammals. Of great concern also are diseases that
outbreak of cholera in recent years to warmer seas
attack coral and sea grasses, essential habitats that
caused by El Ni~o. There is a biological basis for a
n
sustain mobile species (Epstein 1999). An increase
link between sea surface temperature (SST), marine
in the severity of coral bleaching in 1997–1998 is
ecology, and human cholera (Colwell 1996).
coincident with high El Ni~o temperatures. Such
n
Zooplankton, which feed on algae, accumulate
climate-mediated physiological stresses may com-
Vibrio cholerae and other enteric pathogens. This
promise host resistance and increase frequency of
may explain why cholera follows seasonal warming
opportunistic diseases. New diseases typically have
of SST that can enhance plankton blooms. Vibrio
emerged through host or range-shifts of known
spp. in general are influenced by temperature and
pathogens. Both climate and human activities
salinity (Lipp and Rose 1997). Many others factors
may have also accelerated global transport of spe-
promote algal growth, the reservoir of cholera,
cies, bringing together pathogens and previously
including pollution, eutrophication, the loss of
unexposed host populations. There is hitherto no
mangrove filtration systems, and declining fisheries
evidence of switch in the host-pathogen interac-
in offshore waters. The result was an epidemic
tions at the level of aquatic fauna and humans,
between 1991 and 1993, which started in Asia and
yet this may be ‘absence of evidence, rather than
may have spread to South America in the bilge
262
evidence of absence of a (climate) effect’ (Kovats Human impacts in estuaries from upland
et al. 2001). activities
Direct human impacts on estuaries arise from sev-
Changes in hydrological cycles
eral sources. They range from the consequences
Hydrological change is a main threat to human
of engineering works (e.g., damming of rivers,
health (WHO 2000). Through the joint WHO/
abstraction of water for irrigation, dredging, land
UNICEF programme ‘Health Map’, specific GIS
claim of wetlands and harbour constructions) on
software was developed for this purpose, combin-
the estuarine water residence time and erosion and
ing a standardized geographic database, a data
sedimentation patterns, to the effects of wastewater
manager and a mapping interface. However, human
discharges on the health of biota. Anthropogenic
health is not usually included in interdisciplinary
discharges include an array of chemical conta-
research projects dealing with coastal wetlands or
minants, such as metals, organometals, and petro-
river basin management programs. Increasingly,
leum hydrocarbons, organic compounds from
human health needs to become a major issue to
pesticides, industrial waste products and nutrients.
be considered in management strategies.
Dams built in uplands for water storage or power
There are many pathways through which hydro-
generation can result in lower river flows and raised
logically relevant events can affect health; notably
estuarine salinities. Indirect human impacts in estu-
when a river or stream inundates the flood plains
aries can be just as severe (see below) as a result of
(see the arrow labelled H in Figure 3). This pro-
poor land-use management in the river catchment
duces changes in mosquito abundance (malaria,
and of damming rivers.
dengue fever), or contamination of surface water
with human or animal waste, for example, rodent
urine (leptospirosis). Conversely, droughts can
Impacts from land clearing, soil erosion and
produce changes in vector abundance, for example,
damming
a vector breeding in stagnant river ponds. Flooding
may become more intense with climate change and
Land clearing, overgrazing and other poor farming
can affect health through the spread of disease
practices considerably increase soil erosion
(Noji 1997; Menne et al. 1999). Thus, the elabora-
(Wolanski and Spagnol 2000). Most of the eroded
tion of high-resolution topographic models (digital
soil is fine, cohesive sediment. Much of this mud is
elevation models) of basins is necessary for a better
not retained in reservoirs and flows right through to
prediction of the dispersal of inundation waters
the estuary. When the excess sediment is deposited
and the location of stagnant waters. Precisely in
in the coastal waters, changes in coastal properties
tropical coastal areas, where the impact of climate
may be very rapid, occurring within a few decades.
change on vector-transmitted diseases is of high
Examples of this are found in some bays along the
concern, there is frequently a lack of topographic
Great Barrier Reef, which have become perma-
information with an adequate resolution for
nently muddy in only a few decades following land
low-lying sectors. In vulnerable regions, the combi-
clearing and accelerated soil erosion (Wolanski and
nation of risks to both food and water can exacer-
Duke 2002). The coral reefs are also being degraded
bate the health impact of even minor weather
by this excess fine sediment (Wolanski et al. 2003a).
extremes such as floods and droughts (Webb and
An example is the La Sa Fua River, in the wet,
Iskandarani 1998). The only way to reduce vulner-
tropical island of Guam, Mariana Islands (Table 1).
ability is to ensure the infrastructure for removal of
This river drains a steep sloping drainage basin,
solid waste and wastewater and to supply potable
1.5% that of the Cimanuk River. Due to poor
water. No sanitation technology is ‘safe’ when cov-
land-uses practices in this catchment of highly erod-
ered by floodwaters, as faecal matter mixes with
ible volcanic soils, the sediment yield is twice that of
floodwaters and is spread wherever the floodwaters
the Mekong and Yangtze. The river has no delta
run. Similarly, groundwater quality deterioration
and discharges directly to a shallow, coral-fringed
by saline intrusion due to climate change and
bay where coral reefs are smothered by mud
sea-level rise must be considered (Sherif and Singh
(Wolanski et al. 2003b).
1999).
263
Much of the eroded soil remains trapped in estu- per year before construction of the Aswan High
aries, resulting in increased siltation and flooding, Dam (Khafagy et al. 1992; Fanos 1995; Stanley
increased turbidity, decreased primary productiv- and Warne 1998). About 90% of this sediment is
ity, and a loss of aesthetics resulting in an economic now trapped in the reservoirs. As a result, coastal
loss for the tourism industry. As a result, the estu- erosion is intense – the Rosetta and Damietta pro-
montories are eroding at the rates of 106 m yearÀ1
ary can be severely degraded. An example is the
and 10 m yearÀ1, respectively. Similarly, the
Cimanuk River, Indonesia (Table 1). Compared to
the Mekong (Table 1), the Cimanuk River is very Ribarroja-Mequinenza dam on the Ebro River,
small. However, due to human activities, its sedi- Spain, traps about 96% of the riverine sediment.
ment yield is thirty times larger. The catchment is This has led to coastal recession at the river mouth
yielding sediment at a rate exceeding 6000 ton mud area – stopping the previous seaward progradation
kmÀ2 yearÀ1, possibly the highest recorded erosion ´ ´
of the delta (Jimenez and Sanchez-Arcilla 1993;
rates in the world (Milliman and Meade 1983; Guillen and Palanques 1997). The story is repeated
Wolanski and Spagnol 2000). The river is now for the Colorado River, USA and Mexico, which
once supplied 150 Â 106 ton of sediment per year to
profoundly degraded and little more than a drain
for eroded soil and human waste. As a result of the Gulf of California before damming (Meckel
siltation, flooding is common. To protect the farms 1975). Damming has resulted in coastal recession.
from flooding, levees have been constructed; these The Mississippi River suspended sediment load has
need to be raised at a rate of 0.05–0.1 m yearÀ1 and decreased by about 40% between 1963 and 1989,
reach 4 m in height. Other costs involve hard engi- and this may be the major cause for the recession of
neering structures along the river to combat ero- the Mississippi deltaic coast (Coleman et al. 1998;
sion of the levees and maintenance-dredging to Streever 2001). It is also likely that the Three
combat siltation. Gorges Dam in China, under construction, will
The Fly Estuary, Papua New Guinea, is rapidly generate coastal erosion and recession, thus rever-
silting due to increased sediment input from activ- sing the present trend of progradation of the river
ities in the river catchment, as well as from sedi- delta (Chen et al. 1985; Xiqing 1998; Yang et al.
ment imported from coastal waters by tidal and 2001). Water diversion from China’s Luanhe River
wind-driven currents (Wolanski et al. 1998b). The has decreased the riverine sediment load by 95%
Jiaojiang River estuary, China, is infilling at a rate and resulted in its delta’s recession at a rate of
of about 0.13 m yearÀ1, requiring continuous dred- about 17.4 m yearÀ1 (Qian 1994). These problems
ging to maintain navigability (Guan et al. 1998). occur for both small and large rivers. Indeed, similar
In the Mekong Estuary, Vietnam, some of the coastal problems due to water regulation in the
sediment that is flushed to the coastal zone during catchment of small rivers are degrading coastal
freshwater floods may return to the estuary during embayments (Poulos et al. 1994; Jay and Simenstad
the low-flow season. Over time-scales of years, the 1996). The on-going, rapid shoreline retreat in sev-
natural system may be at quasi-equilibrium in the eral segments of Portugal is mainly caused by human
sense that the size, shape and depth of the system activity and not natural fluctuations (Granja and
may change only very slowly, evolving at time- Soares-de-Carvalho 1999; Dias et al. 2000).
scales of decades. This is important because it In semi-arid regions, river floods are short-lived
gives humans the time to adapt to changing estu- and can be suppressed by large dams. The balance
aries and coasts. However, there are plans for 100 between scouring of the estuary during occasional
hydroelectric dams and water diversion structures river floods, and the regular import into the estuary
in the Mekong River and tributaries in riparian of coastal zone sediment by tidal pumping, is then
countries upstream of Vietnam, and this will dis- disturbed by dams. The estuary may then silt, such
turb this quasi-equilibrium (Wolanski et al. 1996). as did indeed happen in the Ord Estuary, Australia,
which has silted by 30 Â 106 m3 over the last 30 years
Many estuaries are dammed, and these dams
trap much of the non-cohesive sediment (e.g., since the river was dammed (Wolanski et al. 2001).
sand). As a result, many temperate estuaries are Mangrove creeks are self-scouring; this effect is
generally suffering from sediment starvation. The proportional to the mangrove area. If the latter
Nile River carried about 110 Â 106 ton of sediment decreases because of mangrove land claim, then
264
tidal creeks will silt in a few years (Wattayakorn recover from an earlier collapse due to overfishing
et al. 1990). (Jackson et al. 2001).
Human populations and economic activities in
Changes to estuarine biology and variability the coastal zone are increasing in most countries.
At the same time, more rivers are dammed world-
Changes in river flows due to irrigation, damming wide. Deforestation is continuing and in many
and water diversion change the entire food web – countries, accelerating, while urban and rural
even up to the level of fisheries – with significant activities are growing. Thus, even though sewage
negative socio-economic consequences (see above). discharge from cities may increasingly be treated,
Longer-term climatic variability is also an diffuse sources remain unchecked, overall the river
important factor because it affects the density of flow is increasingly modified, and the load of sedi-
important species and influences predator–prey ment and nutrients is increasing. No solution has
relationships. The ultimate survival of many yet been offered of how, using only limited finan-
estuarine species depends both on the environ- cial resources, to preserve, restore and manage cri-
mental state of the individual estuaries where they tical estuarine habitats, and reduce the load of
occur and critically on the continued existence of a nutrients and pollutants in waters in the face of
chains of habitats (estuaries) to enable them to increasing human population and its aspirations.
respond to long-term climatic changes. A solution that managers advocate the most at
Global warming, sea level rise and changes in present is to reduce the amount of waste and to rely
precipitation and freshwater runoff to estuaries on a technological fix, namely to construct sewage
that result from greenhouse-gas emissions will treatment plants and to modify farming practices
alter not only the physical system but also its biol- and technology. This technological approach is
ogy. At this stage it is difficult to predict these logical; nevertheless it often fails for reasons out-
changes, partly because the predictions of eco- lined in the section on ‘Introduction: degraded
nomic and social development within the catch- estuaries and coastal waters’.
ment and of climate change remain uncertain. An ecohydrology-based solution is needed to
restore the health of estuaries. Ecologically, the estu-
ary is part of the river basin. It is the end-user of the
river where incremental small-scale impacts become
Ecohydrology as a solution for estuarine and
cumulative, and where the summation of minor
coastal management
environmental stresses becomes a critical issue. In
practice, people have commonly not considered this
Estuary and coastal water degradation
integrative, end-member aspect of estuaries. Criti-
cally, in most countries, land-use managers dealing
Estuary and coastal water degradation occurs
with development within the river catchment, water-
worldwide and is made apparent by the fact that
resource managers dealing with hydrology and
coastal fisheries are collapsing worldwide. Large
water quality, city councils dealing with growth
indicator species, such as sea turtles, sea lions,
and waste disposal in residential areas, and coastal
dugongs and manatees, are disappearing and
and fisheries managers do not have the forum or
critical habitats such as saltmarshes, mangroves,
mechanism to cooperate as a result of their various
coral reefs and seagrass beds are also declining
socio-economic and political imperatives.
(Pauly et al. 1998; Lindeboom 2002; Hughes et al.
2003; Diop 2003). In many cases, particularly in
less developed countries, it is not that traditional
Ecohydrology as a solution
management has failed but that management is
piecemeal or that there is no management at all;
In comparison with purely technological solutions,
overexploitation is the rule. In countries where
ecohydrology works from within ecosystems.
management policies and strategies are now in place,
Implementation of ecohydrologic solutions requires
but did not exist in the past, the destruction of
a profound knowledge of the interdependences
habitats and decreased water quality may be an
between water and biota. By manipulating these
important reason for the failure of fisheries to
265
countries it has been, degraded by human activities
that alter the hydrologic regime, increase the nutri-
ent load from the catchment, and destroy wetlands.
Some of the aquatic ecosystem functions can be
restored by a combination of engineering, ecohy-
drology, and phytotechnology (i.e., using plants)
solutions. As described in the previous sections
(see a summary in Figure 3), the estuary ecosystem
is more robust than the river ecosystem, especially
if the latter is impounded (Figure 2). However,
while the system is more robust, it is more difficult
to restore the system after it has been degraded,
Figure 5. Sketch of the relationship between productivity (and
because the range of options is more limited. Under
also biodiversity) and the nutrient inflow in an estuary.
human influences, the level of degradation of the
Extra nutrients in small quantities may generate an increase
estuarine ecosystem will depend on two key factors,
of biological productivity without dramatic modification of
biodiversity. More commonly the load of nutrients and namely (1) the ratio between the nutrient flux and
pollutants is so high that it degrades the water quality, the
the flushing time, and (2) the robustness of the
ecological services, biodiversity, and productivity of coastal
system. The robustness of the estuary is controlled
waters, leading in extreme cases to toxic algae blooms.
both by a number of parameters, including the
residence time, the estuarine food webs within
the water column, and the buffer effect and the
interdependences, it is possible to control ecosys- habitats provided by the fringing wetlands (princi-
tem characteristics and increase system robustness. pally mudflats, saltmarshes and mangroves). In the
Estuarine food webs show a strong dependence estuary it is generally not possible to use engineer-
on river flow, the ecosystem shifting as a result of ing solutions to solve environmental degradation
changing river inflow and salinity regimes. problems, though in some cases it is possible to
Therefore, it is necessary for estuary managers to diminish the residence time by dredging, opening
take into account in their decision-making the new river mouths, and, for dammed rivers, creating
dependence of food webs on natural (hydrologic artificial river floods. Generally estuarine environ-
and oceanic) factors, as well as on human-induced mental restoration can only be carried out by
changes of river inflows of water, nutrients, pollu- enhancing the biotic integrity (Figure 3); restoring
tants and eroded soils. Key processes are sketched intertidal wetlands is one such key option. For non-
in Figures 3 and 5. Robust systems are less likely to robust estuaries (Figure 5) none of these solutions
become degraded than non-robust systems from will be adequate and the health of the estuary can
disturbances such as fertilisation and changes in only be restored by adopting a basin-wide ecohy-
hydrology. Changes in estuarine and coastal zones drology solution. This solution requires (1) regulat-
biota will affect humans by degrading water quality ing basin-wide human activities that impact on the
and reducing its uses, and also by impacting on river, and (2) manipulating the river ecosystem to
fisheries, with consequent socio-economic impacts. decrease its impact on the estuary (Figure 6). The
The ecologically sustainable solution to estuary coastal zone is the most robust (Figure 2) and the
management is to adopt ecohydrology as the only option available to restore environmental
underpinning principle to guide the management health of the coastal zone is to adopt a basin-wide
of the entire river basin from the headwaters down ecohydrology solution (Figure 6).
to the coastal zone (Figure 6). Environmental A holistic, basin-scale approach is necessary
degradation can only be remedied by restoring because the whole river basin is a functioning,
some of the working of the ecosystem and helping inter-related ecosystem (Zalewski 2002). UNEP
the partially restored system improve itself natu- recently advocated an integrated approach to the
rally. In the freshwater part of the river and espe- hydrological environment, emphasising links
cially in impoundments, the aquatic ecosystem is between river and coastal systems (Coccossis et al.
the least robust (Figure 2). It can be, and in most 1999; Burt 2003). The Mersey Basin Campaign,
266
Figure 6. Sketch of the ecohydrology-based management of a river basin that is necessary to enable sustainable development of an
estuary.
UK, is one of a number of recent examples in quality by converting excess nutrients into plant
Europe, North America and Australasia where biomass, and by sequestering pollutants.
this basin-wide approach has been taken in envir- The interplay between hydrology and biota, and
onmental management, but even here coastal associated fluxes and exchanges of sediment, nutri-
waters are not considered explicitly in the cam- ents and pollutants, can be used in the development
paign’s main objectives. Similarly, ecohydrology of sustainable ecological solutions for water man-
principles may be adopted in the restoration of agement. A number of practical steps are possible,
specific tracts, watercourses and habitats, but they the first of which is to reduce the discharge to the
do not guide the over-arching ethos of the manage- river of excess nutrients, sediment and pollutants
ment strategy. Until this is changed, estuaries and from rural and urban areas through environmen-
coastal waters will continue to degrade. tally sensitive land-use and lifestyles in addition
Significantly, ecohydrology is a low-cost, uni- to high cost technology. More importantly,
formly applicable principle. Ecohydrology is a ecohydrology-based management relies on the use
crucial element of future river basin and coastal of feedback mechanisms between hydrology and
management where growing human populations biota as a management tool by using and manip-
are putting increasing pressure on resources and ulating plants and trees within the rivers, lakes,
the environment, particularly in the developing wetlands and flood plains, and, indeed, the coastal
world, and where, in addition, the coast is also zone, to address specific problems of water, sedi-
threatened by climate change and sea-level rise. ment, nutrients and pollutants. For example, wet-
Ecohydrology relies on using our understanding lands can be used as a remediation tool to sequester
of biota and process-interaction to moderate all excess nutrients into biomass, that is, production of
elements of catchment and estuary robustness bio-energy or timber, to limit their delivery to the
(e.g., flow rates, residence times and productivity) estuary. Similarly, aquatic biota may be manipu-
to enhance the ability of the river and coastal lated to control toxic algae. Woodlands and wet-
waters to recover from impacts. At the more spe- lands may even be used to regulate water flow,
cific level, phytotechnology is an element of ecohy- water quality and sediment delivery to the coast.
drology that relies on using plants to improve water The level at which these activities are implemented
267
will be dependent upon the robustness of the estu- annually from wetlands to remove nutrients. Such
ary. Riverine woodlands are another example of wetland manipulations go hand-in-hand with basing
how biota may be used to regulate catchment- remediation measures on biological function and
dependent fluxes; much of which has been lost in productivity. The practical steps that can be taken
lowland Europe and North America to intensive vary from estuary to estuary. Thus, the use of eco-
agriculture and urban development and these need hydrology and phytotechnology in solving practical
to be restored where possible (Peterken and issues in an estuary, depends on the wise choice of
Hughes 1995). Floodplain woodlands have draw- appropriate management practices. These must
backs, such as limited control of floodwaters and reconcile the different spatial and temporal scales
increased evapotranspiration. Nevertheless, they controlling nutrient and pollutant supply, the ro-
offer improved sediment and pollutant retention bustness of the estuary, the hydrologic regime, the
and nutrient sequestration and, indeed, reduced tidal patterns and wave climate, and the biota; these
diffuse discharge as a result of displacing flood- are all factors that can be managed and balanced
plain farmland. They help reduce peak flood flow to enhance the self-purification ability and capa-
and maintain low flow; they increase habitat diver- city of estuaries and coastal zones (Figure 6).
sity and landscape quality, and timber production Also, it is absolutely necessary to consider the
(Kerr and Nisbet 1996). They need restoration to environmental and socio-economic factors of the
improve the health of estuaries downstream. whole river catchment, including the estuary and
In estuaries and coastal waters, ecohydrology- the coastal zone, when proposing developments
based solutions call for the restoration and creation such as land clearance, urbanisation, intensive agri-
of wetlands, including mudflats, mangroves and culture and/or river damming. The environmental
saltmarshes, because of their ability to trap sedi- impacts of such developments must be fully evalu-
ment and pollutants, to convert excess water-born ated and quantified before being included in the
nutrients into plant biomass, to provide habitats final cost-benefit analysis.
for demersal and pelagic species, and, in some The issue of human health is critical when creat-
cases, to protect the coast from increased erosion ing and managing wetlands, to seek equilibrium
following sea-level rise and sediment starvation between the necessary water residence time for effi-
from damming. This is an important element of cient nutrient sequestration and the minimisation
estuarine ecohydrology where the ‘knock-on’ con- of diseases (e.g., bilharzia and malaria) and vector
sequences of catchment land-clearance may be breeding grounds. In intertidal wetlands, this may
exceeding the capacity of intertidal wetlands to be less of an issue than in freshwater wetlands,
‘absorb’ influxes of sediment, nutrients and pollu- depending on the flushing rate – which can, of
tants, which then by-pass the estuary and enter course, be manipulated using ecohydrologic
coastal waters. Wetland creation of this kind neces- principles.
sitates a dynamic coast, using the balance between Thus, the successful management of estuaries
prevailing tidal levels and accommodation space and coastal waters requires an ecohydrology-
to determine the pace at which intertidal wetlands based, basin-wide management, which considers
may be manipulated to address degradation pro- the river basin as the fundamental unit of territorial
blems from the mild to the acute. These wetlands management (Zalewski 2002). This necessitates
must be managed to reduce human health pro- changing present practices by official institutions
blems. In coastal waters, further applications of based on municipalities or counties as an adminis-
ecohydrology are the use of macrophytes to trative unit, or based on managers of specific activ-
enhance the internal consumption rate, and benthic ities (e.g., farming, water resources, fisheries, urban
suspension feeders, such as bivalve molluscs, developments). Generally, at present, the limits of
sponges, tunicates, and polychaetes, to filter and these administrative or usage units do not
pelletize excess nutrients and plankton. There are, coincide with basin boundaries. The ecohydrology
thus, several steps that may be taken in parallel. approach also necessitates a high level of colla-
The ecohydrologic approaches considered above boration and opportunity to share approaches
have an important element of seasonality. Where and experience on a range of forums to develop
possible, plant biomass may need to be harvested best practice. Without these changes, estuaries and
268
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