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Castel et al 96
Hydrobiologia 329 : ix-xxviii, 1996 . ix
P Caumette, J. Castel & R. Herbert (eds), Coastal Lagoon Eutrophication and ANaerobic Processes (CLEAN .).
©1996 Kluwer Academic Publishers . Printed in Belgium .
Eutrophication gradients in coastal lagoons as exemplified by the Bassin
d'Arcachon and the Etang du Prevost
Jacques Castel', Pierre Caumettel & Rodney Herbert 2
1 Laboratoire d'Oceanographie Biologique, University Bordeaux I, 33120 Arcachon, France
2 Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland
Key words : Coastal lagoons, eutrophication, macrophytes, microorganisms, nitrogen cycle, sulphur cycle
Abstract
The conditions of eutrophication are described in three lagoon systems differing by their structure, their catchment
area and their connection with the sea : the Bassin d'Arcachon on the Atlantic coast, SW France, the semi-artificial
fish ponds of the Bassin d'Arcachon, and the Etang du Prevost on the Mediterranean . The Bassin d'Arcachon is a
shallow semi-enclosed bay, strongly influenced by climatic factors and tidal currents . The Bassin receives significant
inflow of freshwater and the waters are only partially renewed . The greatest part of the primary production is due to
the seagrass Zostera noltii . Although the ecosystem remains on the whole in steady state, some evidence of potential
eutrophication are visible . For instance, the flux of nitrogen into the Bassin d'Arcachon has increased by more than
50% during the last 25 years . The most significant change among primary producers is the massive development
since 1988 of the green alga Monostroma obscurum . The fish `reservoirs' of the Bassin d'Arcachon are man-made
enclosures designed for extensive aquaculture and where the water renewal is only possible during certain periods
of time . Thus, because of the shallowness and the confined nature of these fish ponds, acute eutrophication is
sometimes observed in summer . The Etang du Prevost is extremely eutrophic due to agricultural and urban run-off .
Red waters occur periodically during the warm summer months as a consequence of ecological events beginning in
the early spring with a bloom of green macroalgae (Ulva sp.) . In summer, the algal biomass is degraded by aerobic
heterotrophic bacteria ; the oxygen demand encompasses the oxygen production, leading to the predominence of
anaerobic processes and dystrophic crisis . From the comparison of the selected sites, three stages of eutrophication
are recognized according to the conceptual model of Nienhuis (1992) describing the relation between the relative
dominance of primary producers connected to the availability of nutrients . Such macroscopic observations should,
now, be explained by the study of microbiological processes including meiofauna, protozoa, bacteria and all the
components of the microbial loop .
Introduction eutrophication are amongst the major problems faced
by those responsible for the management of these sen-
Littoral ecosystems such as lagoons, estuaries, and salt sitive ecosystems.
marshes are highly productive environments but how Lagoons have been historically important as sites
they function is still not fully understood (Lasserre, for human settlement providing access to both the land
1979a ; Lasserre & Postma, 1982) . Most of them and the sea . Not only they are important for trans-
are subjected to continuous natural modifications . portation, they also provide natural food resources,
Lagoons, in particular, may undergo periods of dis- such as oysters, clams, shrimps, fish, as well as pro-
order due to excessive eutrophication which can lead viding a convenient place to dump urban and industrial
to dystrophic crisis, the so-called malaigue in Mediter- waste . Some of these multiple uses are compatible ;
ranean lagoons (Caumette & Baleux, 1980) . The con- others are not . It is a key objective of the manage-
trol of production and consequently the control of ment of these systems that the maximum benefit can
x
Table 1 . Comparison of the characteristics of
seawater and lagoonal water. Example from the
The continental inputs in the coastal lagoons are
Bassin d'Arcachon, SW France (August 1986, mainly characterized by river waters and, sometime, by
after Escaravage, 1990) . Seawater = open water ground water or rain water that drain the surrounding
in the Bassin d'Arcachon, fish ponds = shallow soils . These waters not only carry large amounts of
lagoonal enclosures .
particulate material in the form of clay particles and
Seawater Fish ponds organic detritus but also dissolved material in the form
of dissolved organic matter and nutrients arising from
P04 (µm011 -1 ) 0.25 1
human activity in the vicinity of the lagoons (fertilizers,
NH4 (µm011 -1 ) 0.78 5.62
domestic and industrial effluents . . . ) . Most of these
N03 (µmol 1 -1 ) 1 .14 0.30
materials are deposited and concentrate in the lagoons .
NO2 (µm011 -1 ) 0.26 0.15
Nutrient and organic inputs, together with shal-
pH 8 .5 7 .3
low water conditions, good light penetration and good
02 (% saturation) 87 35
mixing, lead to high primary production . It is widely
accepted that the rates of primary production in coastal
lagoons are among the highest measured for natural
be derived from these exploitations without jeopardis- ecosystems (Lasserre, 1979a) . The bulk of the primary
ing their long-term future . production is due to macrophytes : phanerogams (Rup-
pia, Zostera) and macroalgae (Enteromorpha, Ulva) .
However, significant production by microphytes (epi-
Ecological structure and biogeochemical cycles of phytic and/or epibenthic) have also been recorded.
eutrophic coastal lagoons The pelagic compartment (phyto- and zooplankton)
is not the most important component in lagoon ecosys-
Basic model of lagoon ecosystem tems . The shallowness of the water column associat-
ed with the stagnation of the water normally does not
Coastal lagoon ecosystems are directly related to allow the planktonic community to develop to any great
the physical and chemical environment, i .e . coastal extent. Most of the micro-invertebrates encountered in
lagoons are dynamic, open systems where functions the water column are phytophilous or epibenthic .
are dominated and controlled by physical processes In lagoons, the benthic compartment largely con-
(Figure 1) . trols the functioning of the ecosystem . The superficial
The driving forces of these systems are character- film of sediment on the lagoon bed is generally cov-
ized by : ered by a blanket of unicellular organisms (cyanobacte-
- flux vectors (currents, tide, solar energy, rain), ria, peridinians, dinoflagellates, diatoms) . Through the
-marine inputs (sediment, coastal waters and asso- intense browsing action of herbivorous fauna such as
ciated elements such as nutrients, plankton), small gastropods and crustaceans, this vegetal mate-
-continental inputs (rivers, groundwater, nutrients, rial is converted into increasingly fine debris (Fig-
sediment, organic matter) . ure 2) which is quickly colonized by a large number
Because of their position as an interface between of microorganisms (microalgae, bacteria) and small-
the terrestrial and marine environments, they are sub- sized invertebrates (meiofauna, crustaceans, annelids) .
ject to both continental and marine influences . Thus This colonized detritus constitutes an important food
they are highly dynamic environments . source for higher trophic levels (shrimps, fish, espe-
The lagoon ecosystem receives from the marine cially mullet) .
environment oxidized forms of inorganic nitrogen and The excessive primary production leads to high
phosphorus which are restored as reduced compounds . rates of production in the rest of the biological food
Ammonia is a preponderant form of nitrogen (Table 1). web in these ecosystems . Thus eutrophication is char-
Its concentration in the sediment is high, due to the acteristic of many coastal lagoons which should be
mineralization of large amount of organic matter . In considered as frontal ecosystems, which whilst very
contrast, nitrate concentrations are generally low. Low productive are unstable and sensitive to changes in
phosphate concentrations recorded in lagoon waters physical and chemical conditions . Recent research,
are related to adsorption by particulate matter and the however, suggests that their biological communities
sediments . However, frequent release of P04 leads to are well adapted to such perturbations and are able to
high variation in the measured concentrations . function as apparently stable ecosystems .
xi
tides
currents sun
winds
natural upland
systems
forests, marshes
4
offshore LAGOON
fish, plankton agriculture
internal runoff
nutrients, salt
recycling
sediment characteristics
systems
beach processes dams /
reinforcement structures water diversion
tide gates irrigation urban`
1-1 sewage,
fishing dredging
Figure 1 . The lagoon as concentrating mechanism and controllable interface between land and sea (redrawn from Lasserre, 1979a) .
Microbial communities in coastal lagoons ton, filamentous algae and Ulva sp . benefit from the
increased levels of nutrients and will grow in volume
Many shallow coastal lagoons exhibit well defined and dominance at the expense of flowering plants and
salinity gradients from seawater salinity near the con- other algae. They commonly dominate in many coastal
nection with the sea to low salinity in the continental lagoons .
areas of the lagoon receiving river inputs, or to brines In the sediment, the microbial community is more
in solar evaporation lagoons . Such variable conditions complex depending on the gradients of oxygen pene-
of salinity have a very important impact on the dis- trating the sediment and subsequently on the interface
tribution and the selection of the components of the between oxic and anoxic phases . This interface is gen-
biological communities of coastal lagoons . erally found within the first 2 mm of the sediment.
In the water column bacterial communities are Therefore many different kinds of semi-aerobic and
dominated by aerobic heterotrophic bacteria, either anaerobic bacteria coexist, living by fermentation or
with strictly oxidative metabolism or facultatively fer- anaerobic respiration . In most coastal lagoons, it is
mentative metabolism. These bacteria are halotolerant known that the anaerobic bacterial community is dom-
organisms and are mainly of terrestrial origin (Car- inated by sulphate-reducing bacteria that can transform
mouze & Caumette, 1985) . The distribution of marine up to 50% of the organic material (Jorgensen, 1983)
bacteria depends on the influence of tides (Erchen- but produce H2S, a toxic compound that accumulates as
brecker & Stevensen, 1975) . Although many authors FeS2 (pyrite) in the anoxic environment . If light reach-
have reported on their distribution and physiology (see es the anoxic layers, phototrophic sulphide-oxidizing
references in Caumette, 1989a), little is known about bacteria can grow and sometimes populate these envi-
their activity and role in coastal lagoons . They miner- ronments by forming dense purple to brown colored
alize organic material either in free living conditions, masses (Caumette, 1989b ; Van Gemerden, 1983) . In
or associated with clays or particulate organic matter . addition to sulphur bacteria, anoxic layers of coastal
They contribute to the release of inorganic nutrients . It lagoons are densely populated by nitrogen bacteria and
is known that fast growing plants such as phytoplank- methanogenic bacteria (Caumette, 1989a). However,
xii
r. In coastal lagoons, the trophic structure is charac-
terized by a number of different primary food types
50
and a highly connected food web of generalist feeders .
40
Much of the emergent and submergent plant materi-
30 al enters the food web as detritus . Because of the net
20 nutrient uptake which occurs during the fermentation
10 of detritus, this material may have a higher nutritional
value than the original plant material . For both detri-
tus and epiphytes, it is not just plant material, but an
entire community which is consumed, including bac-
40
teria, fungi, microalgae and protozoans .
7
30
The increased pool of autochthonous particulate
OCTOBER
matter results in intense microbial activity in the
20 L
surface sediment. This might be further enhanced
10
by benthic-pelagic coupling in the topmost layer of
510 20 30 40 50 60 µm 10 the sediment. Due to the good light conditions, the
high primary production resulting from eutrophication
5 10 20 30 40 50
7 exceeds the capacity of the heterotrophic decomposers
60
50
T to completely mineralize the biomass . As a conse-
MAY NOVEMBER
40
quence, the pool of particulate organic matter in the
sediment is increasing due to the imbalance between
30
production and mineralisation .
20
In coastal lagoons, the excess detritus is accumu-
10 10
lated from late autumn through winter . The eutrophic
r
5 10 20 30 40 50 60 µm 5 10 20 30 40 50 60 µm conditions prevailing in spring and summer contribute
Figure 2 . Seasonal variation of the particle size (first mm of the to a rapid decomposition of detritus . As a result decom-
sediment) in a semi-enclosed lagoon ecosystem : the fish ponds of poser production and the microbial loop are also stimu-
the Bassin d'Arcachon (Castel, unpublished) . lated . This provides an abundant food source on which
luxurious development of the meiofauna can occur. In
this case, the ratio of detritus to biomass results in
although many papers describe these bacteria and their an imbalance if the energy released from the detri-
role in marine or coastal estuarine environments (Her- tus by the meiofauna is dissipated too rapidly . These
bert, 1975 ; McFarlane & Herbert, 1984 ; Marty et al ., high decomposition rates result in an increased oxygen
1990), little is known on their distribution and role in demand and may lead to a temporary disappearance of
coastal lagoons . dissolved oxygen in the water column (Lasserre et al .,
Lagoon environments display very high densities of 1976) . This phenomenon causes mass mortality of the
meiofauna composed mainly of nematodes, harpacti- benthic macrofauna and fish.
coid copepod, oligochaetes, ostracods, turbellarians, These summer dystrophic crises lead to the for-
and polychaetes (Castel, 1992) . These meiofauna com- mation of high bacterial biomasses in the lagoons as
ponents may consume much more energy in the form a consequence of high bacterial activity involved in
of organic matter (detritus + microflora + bacteria) mineralization (both aerobic and anaerobic bacteria).
than do the macrofauna (Lasserre et al ., 1976) . Fur- Such biomass can further be used as a food source by
thermore respiration rates when related to biomass are the meiofauna. In many coastal lagoons, particularly
very high, indicating the importance of the micro- and at the sediment surface, bacteria play an important role
meiofaunal communities in coastal lagoons (Table 2) . as a food source for higher organisms via the detrit-
One gram of meiofauna respires a rate at least ten ic food chain (Coull, 1973) . However the quantitative
times greater than an equivalent macrofaunal biomass. importance of bacteria was not appreciated until rela-
In shallow eutrophic areas, the ratio macrofauna, meio- tively recently (Fenchel & Jt rgensen, 1977) although
fauna, microfauna, in term of biomass is approximately some early studies (see Zobell, 1946) emphasized the
100, 10, 1 ; whereas from a metabolic point of view it importance of bacteria in marine food chains .
is 4, 2, 1 .
X111
Table 2 . Total in situ oxygen consumption and relative uptake by micro- and meiofauna (in
percent) in man-made lagoons (fish ponds) and salt marshes in the Bassin d'Arcachon (from
Lasserre et al ., 1976 and Lasserre, 1979b) .
Biotope Compartment Oxygen consumption % Micro- and meiofauna
-1 )
(m102 m -2h
July December July December
Fish ponds Phytal 218 44 17 .9 20 .5
Benthos 108 28 58 .3 42 .9
Salt marshes Phytal 99 23 11 .1 21 .7
Benthos 81 16 16 .0 25 .0
Recent studies have shown very high bacterial pro- redox state of the sediments and overlaying water col-
ductivity in coastal lagoons (references in Torreton, umn . It is well established that their oxidation is mainly
1991) but few describe accurately the interrelationships mediated by chemotrophic prokaryotes that use oxy-
between such biomass and the consumers in the water gen as terminal electron acceptor.
column and benthos (Gophen et al ., 1974 ; Caumette Such organisms live around the redoxcline of the
et al ., 1983 ; Rieper, 1982 ; Carman & Thistle, 1985 ; lagoon ecosystem, depending on reduced compounds
Decho & Castenholz, 1986 ; Souza-Santos et al ., 1996) . from the anoxic layer and oxygen from the overly-
Fenchel & Jorgensen (1977) calculated that bacteria in ing oxic layer. Thus reduced nitrogen compounds are
the detritic food chain provided between 60-80% of the generally oxidized via the activity of ammonia- or
diet of the meiofauna . On average, meiofauna graze at nitrite-oxidizing bacteria . In contrast, reduced sulphur
a rate of 1 % of the standing stock of both heterotrophic compounds can be oxidized under anoxic conditions .
bacteria and autrotrophic microalgae per hour (Mon- Chemotrophic bacteria that perform anaerobic respi-
tagna, 1995) . Meiofauna grazing is therefore broadly ration such as Thiobacillus denitrificans generate a
in equilibrium with microbial production . However, proton electrochemical gradient as electron flow from
new more accurate data are now required to clarify our reduced sulphur compounds to nitrate, nitric or nitrous
understanding of the functioning of coastal lagoons, in oxides . Moreover, in the upper part of the anoxic layers
terms of carbon turnover, organic matter fluxes via the reached by light, phototrophic sulphur oxidizing bac-
biogeochemical cycles, and the mineralization and the teria are able to oxidize reduced sulphur compounds
production of organic material through the food chain . to sulphate as a consequence of their anoxygenic pho-
tosynthesis . Thus, in coastal lagoons, both cycles are
Biogeochemical nitrogen and sulphur cycles in functioning in similar conditions : reduction of nitro-
coastal lagoons gen or sulphur compounds occurs under anoxic con-
ditions whereas oxidation of the reduced compounds
The nitrogen and sulphur cycles include a diverse range mostly occur at the interface between oxic and anoxic
of oxidation and reduction reactions (Figures 3 & 4) . zones, i .e . the redoxcline . Almost all eutrophic shallow
These can be divided into two categories : dissimi- water coastal lagoons have a redoxcline occurring at
latory reactions that are found principally amongst the sediment surface . Therefore most of these metabol-
prokaryotes and assimilatory reactions that occur in ic processes coexist within the first millimeters of the
both prokaryotes and eucaryotes (Ferguson, 1988) . sediment. In eutrophic coastal lagoons, ammonia is the
In both cycles, the production of reduced com- major nitrogen compound produced in anoxic layers
pounds such as NH3, NO2 , H2S, S ° are either the (Koike & Hattori, 1978 ; Nishio et al ., 1982,1983 ; Mar-
end products of dissimilatory anaerobic respiration ty et al ., 1990) . However, production of reduced nitro-
or derive from aerobic and anaerobic degradation of gen compounds is lower compared to that of reduced
organic material (Herbert, 1982 ; Caumette, 1986 ; Her- sulphur compounds which are derived from sulphate
bert & Nedwell, 1990) . These compounds accumulate reduction which is the most important anaerobic respi-
in the anoxic layers mainly in the sediment of eutroph- ratory process occurring in coastal lagoons (Caumette,
ic coastal lagoons . Their oxidation is dependent on the 1986) and coastal marine environments (Jorgensen,
xiv
Figure 3. Nitrogen cycle.
IOxicconditions
Anoxic conditions
Figure 4 . Sulphur cycle.
xv
1982, 1983) . The bacterial oxidation of reduced nitro-
gen or sulphur compounds at the sediment surface pre-
vents their diffusion into the overlying oxic layer (the
water column) . Little is known on the oxidation of
nitrogen compounds in coastal lagoons and field work
as well as laboratory work are required to determine
and quantify the oxidative pathways of reduced nitro-
gen compounds . In contrast, it is established that in
13
most of the coastal sediments studied so far, between
50 to 95% of the H2S produced is reoxidized, at the
interface between the oxic and anoxic zones . However,
sulphide does not usually reach the oxic zone unless
sulphate reduction is very intensive .
Thus, it appears that the abiotic and biotic equi-
librium in coastal lagoons is primarily dependent on
the balance between oxidation and reduction activi-
ties in the biogeochemical cycles . When reduction of
nitrate or sulphate in the anoxic layers is enhanced, the
reduced compounds produced can diffuse into the oxic
layers leading to the establishment of anoxic condi-
tions and the release of reduced nitrogen and/or sulphur
compounds to the atmosphere . In recent years, atten-
tion has been paid to volatile methylated sulphur com-
pounds and their metabolism at the sediment surface of
coastal environments . In coastal lagoons, their inten-
sive production and transformations may have impor-
tant consequence on the global sulphur cycle and the
atmospheric behaviour of sulphur. It is known that
these conditions develop when organic matter accu-
mulates and the subsequent activity of mineralization Figure 5. Location of the Bassin d'Arcachon along the Atlantic
coast, showing the drainage basin and the most important inputs of
processes are stimulated in the water column and sed- freshwater.
iments . However, detailed field experiments on the
oxidation and reduction processes taking place in both
cycles at the sediment surface are required in order to Examples of eutrophication gradients in coastal
understand the development of such drastic events in lagoons
coastal lagoons .
In many shallow coastal lagoons, much of the sur- The Bassin d'Arcachon, a moderately eutrophic
face light irradiance reaches the sediment surface lead- lagoon
ing to the development of photosynthetic benthic com-
munities which in turn may lead to the development General hydrography
of microbial mats . These mats, which are characteris- The Bassin d'Arcachon (44°40' N, 1° 10' W) is a trian-
tics of many coastal lagoons, are composed of differ- gular shaped embayment on the South-West Atlantic
ent laminated layers of oxygenic and anoxygenic pho- coast of France (Figure 5) . Channels and intertidal
totrophic bacteria (cyanobacteria and different kinds of areas cover 155 km 2 but only 40 km2 of the bay are
purple or green sulphur oxidizing bacteria) depending subtidal . Seventy percent of the lagoon is composed
on oxygen and sulphide microgradients in the upper of intertidal flats called 'crassats' which are used for
sediment . It has been recently shown that methylated oyster farming . The intertidal areas, especially in the
sulphur compounds are very important in the sulphur eastern half of the Bay are covered by Zostera noltii
cycle that takes place in microbial mats, at the oxygen Hornem . whilst Zostera marina L . is found in the chan-
sulphide interface (Visscher et al ., 1991) . nels . The oyster beds cover an area of 10 km2 . The
decaying eelgrass and detritus from the oysters pro-
xvi
vides a rich supply of organic nutrients for the intertidal
sediments leading to an enhancement of the abundance
of the small invertebrate fauna (Castel et al ., 1989) . The
Bassin d'Arcachon is connected to the Atlantic Ocean
at the South-West end by two narrow channels (4-5 m
deep at low water) . The presence of sandbanks at the
entrance to the lagoon have a major effect on water
exchange between the bassin and the Atlantic . On a
spring tide 370 x 106 m3 of water are exchanged whilst
on a neap tide this is reduced to 130 x 10 6 m3 . These
volumes are similar to those calculated by Caspari in
1863 (c it. in Labourg, 1985) . This shows how stable the
tidal influence has been over more than a century . The
exchange of such large volumes of water induces con-
siderably high water flow : the mean discharge through
the channels is 17 x 103 ms s -1 (comparable to the
average discharge of the St Lawrence) . These waters
carry a great amount of sand in suspension particular-
ly on the ebb during spring tides . Sediment transport
has been estimated to be as much as 11 500 t through
the channels on a spring tide . In contrast, in the inner
bay, resuspension is much less ; on average the concen-
1-1 .
tration of suspended matter is around 3-7 mg At Figure 6 . Distribution of the water masses in the Bassin d'Arcachon
Arcachon Eyrac, the tidal range is 4 .9 m on a spring according to salinity (modified from Bouchet, 1968) . Fish ponds are
tide and 1 .1 m on a neap tide . Tidal currents are strong, shallow brackishwater enclosures .
reaching velocities of 2 m s - ' in the channels .
The oceanic water entering the bassin is diluted by
-Inner neritic waters : temperature : 1-25 ° C, salini-
freshwater inflow, mainly from the northern and east-
ty : 22-32%o
ern parts of the bassin (1 .8 x 10 6 m a d -1 ) . Furthermore,
ground waters provide approximately 106 M3 d - . As These water masses move according to the tidal
a consequence, salinity and temperature variations are state and their renewal is only partial . In the North-
directly proportional to the distance `upstream' from East and eastern sectors of the lagoon, water exchange
the inlet . In summer the water temperature reaches with the Atlantic is occasional, probably once or twice
21-22 °C in August, falling to 6-8 °C in mid-winter . a year. The inner waters are eliminated from the Bassin
Salinity ranges from 30-33%o at high water but occa- as a laminar flow or nappes at the surface . The interme-
sionally decreases to 20%o after periods of heavy rain . diate body of water oscillates and tend to mix with the
In spring and summer the temperatures are notably inner waters whilst the water mass associated with the
higher in the inner bay than in the outer channels : the deep channel is well mixed with Atlantic water during
converse is observed in autumn and winter . Temper- each tidal cycle .
atures are uniform in the bassin in February-March Whilst the Bassin d'Arcachon is continuously
and in September-October. During these periods water changing, the system as a whole remains in a steady-
temperatures of the Bassin d'Arcachon and the Bay state . The lagoon is subject to erosion but this is bal-
of Biscay tend to be similar . This has strong ecologi- anced by sedimentation . The most important changes
cal implications, especially in spring when migrating occur near the inlet and the adjacent coast area :
species (cephalopods, fish) enter the bassin . - slow advance and sporadic erosion of the Cap Fer-
Three distinct water masses have been recognised ret spit
in the Bassin d'Arcachon (Bouchet, 1968 ; Figure 6) : - migration of the navigation channel and associated
-External neritic waters : temperature : 9 .5-21 °C, sand banks to the South
salinity : 34-35%0 -erosion of the southern coast .
-Intermediate neritic waters : temperature 6- These changes tend to reduce exchange between
22 .5 °C, salinity : 26 .8-33 .2% the Bassin and the Atlantic . In the inner bay the sed-
xvll
Table 3 . Annual flux of mineral nitrogen
(in tonnes) originating from the drainage
basin in the Bassin d'Arcachon (Auby et
al ., 1994) .
Origin Years
1970 1980 1990
Forests 263 257 252
Agriculture 281 415 575
Lakes 17 66 27
TOTAL 561 738 854
12-
10
Outer bay
® Inner bay
8 Eyre river
O - N N ~? N 10 r t0 0) O +- N N Figure 8. Location of the seagrass beds of Zostera noltii and Z.
O m O O a1 O m O O UD 01 07 07 0!
01 W 00 0) O 07 0! marina in the Bassin d'Arcachon (redrawn from Auby, 1993) .
w
Figure 7. Long-term distribution of nitrite + nitrates in the Bassin
d'Arcachon (outer and inner water masses) and in the river Leyre
(modified from Auby et al ., 1994) .
from the river input to the outer region of the Bassin,
imentation rate is quite low (10 cm per century today the nutrient concentrations have clearly increased in
cf. 1 m per century 1700 years ago) . a large part of the Bassin d'Arcachon (by a factor 2
in the inner Bassin from 1977-1981 and 1990-1993) .
Evidence of eutrophication Such increases could have stimulated the primary pro-
The catchment of the Bassin d'Arcachon covers an duction and thus eutrophication . Measurements made
area of 4140 km 2 (Figure 5) . Approximately a quarter since 1976 (Castel, unpublished ; Robert et al., 1987)
of the catchment can be considered to indirectly con- indicate however that the standing stock of phytoplank-
tribute nutrients to the Bassin through coastal lakes . ton (expressed in chlorophyll a) is not particularly high
Since the 1970's intensive agriculture in the region and remains at a relatively constant level from one year
(especially maize) has increased . In 1970 agriculture to another : the baseline value is around 2 pg 1 -1 and
was responsible for 50% of the annual flux of nitrogen the spring bloom does not exceed 15 pg 1 -1 .
to the Bassin d'Arcachon ; at present it is responsi- The most abundant primary producer in the Bassin
ble for 66% of this flux (Table 3) . The annual flux of d'Arcachon is the seagrass Zostera noltii (Table 4) .
total phosphorus remains quite stable (25-30 t yr -1 ) . The seagrass beds occupy 7000 ha in the intertidal zone
Urban sewage has significantly decreased during the (Figure 8) which represents the largest area in Europe .
last twenty years (from 127 t total N yr -t to 40 t total In contrast to other places in Europe where the biomass
N yr-1 ) . of Z. noltii has declined in recent years, the seagrass
Most of the freshwater flux comes from the riv- meadows in Arcachon have remained almost constant
er Leyre (see Figure 5) thus it is not surprising that over the last 30 years . Mean annual biomass is 70-
nutrient concentrations have increased during in recent 100 g DW m -2 for the leaves and 70-160 g DW m-2
years (Figure 7) . Although there is a dilution effect for the roots and rhizomes (Auby, 1991) .
xvii'
Table 4. Estimated annual production of the different primary producers in the Bassin d'Arcachon
(from Auby et al ., 1993) .
Taxa Total production Carbon Nitrogen Phosphorus
(t d.w . yr 1) (t yr t ) (t yr ') (t yr- ' )
Halophilous phanerogams 7612-9098 3045-3636 537-686 73-93
Zostera noltii 30, 790-43,700 9275-13,300 660-960 70-100
Zostera marina 6213 2003 157 15
Monostroma obscurum 7600 2508 342 23
Other macroalgae unknown unknown unknown unknown
Microphytobenthos 4930-12,270 860-2140 120-290
Phytoplankton 3540 625 85
The subtidal eelgrass Zostera marina occupies
4 .26 km2 in the channels (Figure 8) . It constitutes a
unique refuge for invertebrates as well as fish .
The most significant change among primary pro-
ducers is the development, since 1988, of the green alga
91st
Monostroma obscurum (= Ulvaria obscura Kutzing) .
This species was first described in 1843 on the Basque
coast. It is a cosmopolitan species found in North-
ern Europe as well as in the Pacific . Since the ear-
1030t
ly 1980's, fishermen from Arcachon have exploited
the natural deposits of oysters near the Adour estuary,
close to the site where this species was first described.
The oysters were transported to Arcachon for growth,
without cleaning the shells . It is probable that some
thalli were imported together with the oysters . The
rapid development of Monostroma shows that this alga
finds environmental conditions in the Bassin ideal for
growth. Although no causal relationship can be demon-
strated, it is interesting to note that the development
of Monostroma coincided with the increased nutri-
ent input into the Bassin d'Arcachon . In spring, the
total biomass of Monostroma has been estimated to be
between 18 000 to 21 000 t . The alga mainly colonizes
the inner region of the Bassin, in the intertidal zone
as well as in the channels (Figure 9) . The maximum
density is around 6 kg W W m -2 , which is comparable
to other macroalgal blooms (Etang du Prevost, Ulva :
Figure 9. Total biomass (tonnes wet weight) of the green alga
5 kg W W m-2 ; Baie of St Brieuc, Ulva : 8 kg W W
Monostroma obscurum in the Bassin d'Arcachon in June 1993
m -2 ) although it is less than the massive development (redrawn from Auby et al ., 1994).
of Enteromorpha and Ulva observed in the lagoon of
Venice (30 kg W W m -2 ) . Arcachon is the only place
in the world where a bloom of Monostroma has been
the human activities, however, it is clear that some
observed .
species have disappeared and some appeared during
Changes in animal populations and other algae have
the last century. The barnacle Elminius modestus Dar-
also been observed (Labourg, 1985) . It is not always
win, originating from Australia, after colonizing the
possible to differentiate between the impact of nat-
Mediterranean coasts, appeared in Arcachon in 1960.
ural variation of the environment and the effect of
The Mediterranean balist Balistes capriscus L. was
xix
reported in the bassin in 1962 . These are example of and outflow are regulated by sluice gates (Figure 11) .
species having extended their area of distribution . Oth- The fish ponds or fish reservoirs have a characteristic
er species have appeared since 1968 as a consequence shape : channels or ditches, 1 .5 to 2 m in depth, feed
of the importation of the Pacific oyster Crassostrea large expanses known as `fiats', each of which covers
gigas Thunberg, including the tunicate Styella clava an area ranging from 1000 to 10 000 m= . In the canals
Herdman, some Bryozoans and Annelids . More recent- the fish are `penned' during the winter months to coun-
ly, the large brown alga Sargassum muticum (Yen- teract the colder temperatures prevailing in the shallow
do) Fensholt accidentally introduced in Great Britain areas . These flats, 20-50 cm deep, are separated from
with the Pacific oyster, invaded the coast of Britan- each other by ridges formed from the mud removed
ny and appeared some years ago in Arcachon . Even during the digging of the ditches . All inflows and out-
small organisms (< 1 mm) such as the Harpacticoid flows can be regulated by sluices located at intervals
Copepod Stenhelia latioperculata Ito (originally from along these embankments .
Japan) are likely to have been introduced with oys- At high tide, the sluices are manoeuvred in such a
ters . Conversely, some species have disappeared due way that a current is created and the fish, both imma-
to either modifications of hydrological conditions (e.g . ture and adults, are drawn into the reservoirs . The fish
the clam Venus verrucosa Linne) or to overexploitation are prevented from returning to the sea by a system of
(e .g . the bivalve Chlamys varia Linne) . mesh frames . These operations are usually carried out
during the cooler seasons, from March to November .
Fish ponds of the Bassin d'Arcachon, a moderately Every two weeks, when high tides occur at the time of
eutrophic lagoon system the full or new moon, water from the sea is allowed to
flow into the reservoir through the sluices . This oper-
General description ation lets in the young fish and natural mineral salts,
The `fish reservoirs' of the Bassin d'Arcachon (Fig- and replenishes the reservoir by exchanging the water
ure 10) are man-made enclosures created in the lagoon- held in the ponds and fresh seawater. This operation
al marshes (wetlands) and where a number of eury- is carried out at low tide . The young fish are free to
haline fish are farmed : grey mullet (Chelon labrosus move and to grow inside the complicated network of
Risso, Liza ramada Risso), sea bass (Dicentrarchus basins, in which the salinity may vary from very dilute
labrax Linne), eels (Anguilla anguilla Linne) and gilt brackish water to full seawater .
head bream (Sparus aurata Linne) . Such fish reservoirs Because of the shallowness and the confined nature
are also known from the Charente and Vendee along of the fish ponds, the salinity regime is extremely vari-
the Atlantic coast. These structures, designed for tradi- able both in time and space . The salinity ranges from
tional extensive (without food supply) aquaculture, are almost freshwater to hypersaline (60%o) . The salinity
comparable to the 'valli', situated along the Adriatic regime of the fish reservoirs is strongly linked to (1) the
coast, and also to the tropical 'tambaks' in Indonesia . relative location of the ponds between the sea and the
They constitute mixohaline and shallow (0 .2-1 .5 m continental freshwater inputs, and (2) the renewal of
depth) environments where the rich input of detritus the water through the sluices . The man-induced renew-
plays a prominent role in the food chain . Originally al of water is supposed to maintain salinity compatible
they were salt-pans exploited since the end of the Mid- with biological activities . However, the maintenance
dle Ages . Progressively, during the late eighteenth cen- of a given salinity requires a careful and periodic oper-
tury the exploitation of the salt decreased and the salt- ating of the sluices since, due to the shallowness of
pans were converted into fish ponds . These fish reser- the ponds, the salinity tends to vary widely (Figure 12)
voirs schemes flourished all through the nineteenth between periods of renewals . The same situation can
century up to the end of the Second World War but they also be observed for nutrients (Escaravage, 1990) . In
have since gone into decline because of the labour costs such ponds, the greatest part of the regulation of nutri-
and the relatively low yield (50 kg ha - ') of this type of ent concentrations is controlled by in situ biochemi-
extensive aquaculture. These semi-enclosed lagoonal cal processes, especially at the benthic level . These
systems cover a surface area of approximately 940 ha mechanisms are hardly affected by the renewal of the
in the Bassin d'Arcachon . The old salt-evaporation overlaying water.
areas and the salt-pan runoff ditches are separated by
embankments from the sea and through which inflow
xx
Figure 10. Map of the fish ponds of the Bassin d'Arcachon (Certes reservoirs) .
Figure 11 . Schematic diagram of the fish ponds of the Bassin d'Arcachon . Left : salt marsh comprising the 'slikke' (SI) = mudflat, the 'schorre'
(Sch) = high marsh and 'estey' (es) = small channel . The ponds are separated from the salt marsh by a dyke (d) along which a sluice (ec) is
established . Right: typical structure of the reservoirs, Pr = channel (width : 3-4 m, depth : 0.8-1 .5 m), PI = shallow basins (width : 10-40 m,
length : 100-800 m, depth: 0 .2-0 .5 m) . Redrawn from Lasserre (1979a) and Castel (1989) .
From eutrophy to dystrophy Cladophora vadorum (Aresch .) Kutz at the water sur-
As in many inshore areas, the rich input of detritus, face, and mats of green algae (Lamprothamnium papu-
organic matter, bacteria and benthic microflora play losum J . Groves, Chaetomorpha aerea (Dillwyn) Kiitz,
a prominent role in the food chain of the fish ponds . or cyanobacteria at the sediment surface. The biomass
The most important primary producers are sea grasses of Ruppia cirrhosa has been estimated to 126 g W W
(Ruppia cirrhosa Petag), and filamentous green algae : M -2 for the above-ground and to 51 g W W
m-2 for the
xxi
29- °C a
28
27
26
25
24
23
22
21
20-
19-
18-
0 April May June July August Sept . Oct .
Figure 12 . Temporal variation of salinity in a fish pond of the Bassin
d'Arcachon (Le Teich in Figure 6, April-October 1974) . Measure-
ments were made 100 m far from a sluice . The bars on the x-axis
indicate the periods of water renewal (Castel, unpublished) .
below-ground structures (Soriano-Sierra, 1988) . This
biomass apperas to be relatively stable from year to
year and has on average standing stock of 149-186 g
W W M -2 (Labourg, 1979) . Green macroalgae are
strongly dominated by Chaetomorpha area with an
average biomass of 79 g W W m -2 . Macro-epiphytes
(Cladophora vadorum, Rhizoclonium kernerii Stock-
mayer) are found on the stalks and blades of R. cir-
Figure 13 . a) Daily variation of temperature (mean, maximum, min-
rhosa . Their average biomass has been estimated at imum) during the period 19 June - I 1 September 1974 in a fish pond
68 g W W m-2 (Soriano-Sierra, 1988) . of the Bassin d'Arcachon (Certes in Figure 6). Measurements were
Temperature is probably one of the principal factors made 20 m far from a sluice at 0.40 m depth. b) Daily variation of the
tide coefficient (proportional to the height of water) during the same
conditioning the eutrophication processes and further period . Periods of HZS production are indicated by vertical shaded
the dystrophic crisis . In the fish ponds, the variations in bars . The horizontal bars on the x-axis indicate the period of water
temperature depend both on the sun light and renewal renewal (redrawn from Castel, 1978) .
of the waters . Each time seawater is allowed to enter
the pond the temperature significantly decreases (Fig-
ure 13) . In contrast, on a neap tide, when there is no (Labourg, 1975 ; Castel, 1978) . Several tonnes of
entry of seawater, the temperature rapidly increases, dead fish have been collected during particularly acute
reaching sometimes values close to 30 °C . Generally, crises . However, such dramatic events do not occur
production of hydrogen sulfide is observed for tem- every year. Generally, dystrophic crises in the fish
perature reaching 24-25 °C, at the end of the neap ponds of the Bassin d'Arcachon are less acute and less
tide period, when the water is stagnant . Concurrently extensive in space than in the Mediterranean lagoons .
(even before the production of H2S) the oxygen con-
centration drops to very low values . pH values also The bang du Prevost, a strongly eutrophic lagoon
tend to decrease due to a hyper production of acids
(organic acids, sulfate, sulphur) . A biological indica- General hydrography
tion of eutrophication is the presence of Peridinians in The Prevost Lagoon (43 ° 30' N, 3°54' E), located on
the plankton, usually dominated by diatoms in the fish the French Mediterranean coast, belongs to a lagoon
reservoirs (Castel, 1978) . system that extends along the coast, between Sate
In some years, during warm summer, true dys- and Montpellier. This littoral zone is formed by a
trophic crises may occur, with the formation of white succession of coastal brackish ponds (`etangs'), sur-
waters due to the precipitation of carbonates . During rounded by marsh and separated from the sea by a
such periods, high mortalities of fish may be observed low, sandy beach allowing only limited amount of
Montpellier
etang de Perols
Palavas
etang du Prevost
Mediterranean Sea
Figure 14 . Location of the principal lagoons along the Languedocian Mediterranean coast .
water exchanges . The Etang du Prevost is situated in 'Le Lez', and seawater through the only connection
a lagoonal complex delimited to the South-West by with the sea 'Le Grau du Prevost' . Although the lagoon
the Etang de Thau and the Sete promontory and to the is situated in a nontidal zone, the water level can vary
North-East by the wetlands of the Camargue and Rhone from -0 .30 m to +0.40 m relative to the mean level
delta (Figure 14) . These ponds are of recent geological because of the strong winds . In the meantime, the
formation . The sandy and detrital zones in this region current speed can reach 0 .40 m s -1 in the lagoon and
explain their continental formation . The creation by 0 .80 m s -1 in the 'grau' . When the wind is blowing,
the currents of offshore bars ('cordons littoraux') has 20 to 25% of the water can be renewed in one day.
allowed the transformation of the coastline and the for- In contrast, during calm periods, the water remains
mation and evolution of these coastal lagoons since the stagnant . This is particularly true in summer. The wind
quaternary period . action may induce very strong and rapid variations of
The water in these lagoons is slowly renewed, the salinity (Figure 15) . In summer, because of the
via small channels ('graus') communicating with the evaporation, the salinity can reach 40%o .
Mediterranean sea . These shallow bodies of water are The system is extremely eutrophic due to agricul-
subjected to continental inputs . They are directly influ- tural and urban run-off. For the Etang du Prevost, 85-
enced by urban or industrial effluents . 90% of the input of N and P are of domestic origin
In contrast to the Bassin d'Arcachon, the Etang du (Table 5) . The input per volume unit of lagoonal water
Prevost is a shallow lagoon with an average depth of is approximately 24 .6 t 106 m -3 for the total nitrogen
m-3
about 0 .8 m and a surface area of 380 ha (Figure 14) . It and 4 .3 t 106 for the total phosphorus (Anony-
receives fresh water and sewage carried by a small river
XX111
Lux
S04
water
H2S
sulfide oxidation
sulfate reduction "'
+ -, S ; H 2 sediment
fermentation
Lux
water
- S04
_ C02 4 2 H 2 S+2H2 O . 2So . C red water
B
phototrophic bacteria
..... . ........ ............ .. .......... ..................... ......
.
plant
degradation sediment
Lux
white water
plant
degradation : sediment
Figure 16 . Schematic description of the biogeochemical processes
occurring during a dystrophic crisis (redrawn from Baleux et al .,
1979) . A : Equilibrium state of the sulphur cycle, B : Dystrophic crisis
with formation of `red waters', C : Dystrophic crisis with formation
of `white water' .
Figure 15. Salinity fields (%o) in the Prevost lagoon during two
different climatic conditions : wind blowing from the Northeast
(22 .02 .1973) and wind blowing from the Southwest (10.03.1973) Etang du Provost. Its flora is essentially composed of
(redrawn from Guelorget & Perthuisot, 1992) . green algae (Ulva and Enteromorpha spp .) . The bio-
mass of these algae may reach 5 kg W W m-2 and
Table 5 . Annual input and stocks of nitrogen and phos- sometimes accumulates at densities up to 30 kg W
phorus, expressed in tonnes, in the Etang du Prevost
W M-2 . In summer, this biomass is rapidly degraded
(anonymous, 1991).
by aerobic heterotrophic bacteria whose numbers and
Origin Nitrogen Phosphorus activities increase rapidly . During this period the water
turns anoxic and becomes rich in sulphide, which leads
Domestic input 60 .2 11 .1
to severe dystrophic crisis .
Agriculture 5 .5 0.5
Industry 1 .4 0.2
Sediment stock (10 cm) 475 140
The dystrophic crisis
Annual flux in sediment 22 2 .4 The Etang du Prevost is regularly subject to dystrophic
Ulva stock 20 .6 1 .8 crises, locally called the `malaigues' . Such dramat-
ic events appear every year during the warm summer
months for a period of about 15 days . They have a dra-
matic consequence for aquaculture, particularly oyster
mous, 1991) . As a comparison, the input of nitrogen culture that takes place in the lagoon . Macroscopical-
in the Bassin d'Arcachon is only 3 t 106 m _3 . ly, the dystrophic crises are characterized by coloured
Such large inputs of nutrients lead to the devel- waters (white, red or black waters) . They originate
opment of high biomass of primary producers in the from a perturbation of the sulphur cycle .
Xxiv
O
O ~ O
N 0 l0 0 0 0 M 0 ~ 0
0
0 0
r' m0 Ln00' O
Ln0
N ~ ~ O r+
N 0 N 4- M 0 01 O O
~
N 0 Ln \0 0 ML
I T O N a ca • ~~0
N0M O
0
•
~ 0'~ o0
-4
` C- 0
L1) 0 -4 M ~O 0
uT N 0 N
~' II
Oil a
to to
En 0Nas to Il 01 a N
U) W 0 ra to
U
ti
N. 4 N
o
r _ 0
"~ 0 U-0 N 0
O ~
UN O~ 0 0 „ 0
. N
o 8
N O N° O
0
Ln 0
~ M 0 -400 ,`
N ~~ MO 0o0a'
0 I ' \ M~ 0 0
.~
M" O '-+ 0, 0
N
' . . 0
0) O~
-4 C0 O
to 0 ~ N M 0 . 61
-4 ~01 O C' O O II
I O11 N 0i
.
to t/) 0 PO to
~ 11 U)
0 M U
N
)
W 3 ~
3
O N
0
: appearance of anoxic water
Figure 17 . The Prevost lagoon during a dystrophic crisis (June-August 1977) . I : before the dystrophic crisis, 11
. SO4 ,
with presence of sulfide, III : appearance of red and white waters, IV : after the dystrophic crisis . 1, 2, 3 and C refer to sampling stations
. Bacterial counts are expressed in numbers m1 -1 .
S - and 02 are expressed in mg 1-1 . B = phototrophic bacteria, SR = sulfate reducers
xxv
In normal conditions, there is a steady-state at the
benthic level between sulfate-reducing bacteria pro-
ducing hydrogen sulfide and bacteria oxidizing this
reduced sulphur compounds or elemental sulphur (Fig-
ure 16) . The reactions of sulphate reduction lead to the
production of hydrogen sulfide which is then rapidly Enteromorpha
0 Me
oxidized to sulphate in the presence of oxygen . After O Monostroma
an increase in organic load these oxido-reduction reac- 0 Ruppia
tions accelerate . The S04/Cl ratio increases consider-
o Zostera
ably indicating a solubilization of the sulphate . This
ratio, which is typically around 11-12, can exceed 16
during a period of eutrophication . Under anoxic con- 1,111 Is.~ S
ditions white waters can be observed together with the M J S D
central
M J S D M J S D M J S D
inner Certes Provost
production of H2S . They usually occur when the light Bassin d'Arcachon
intensity is low . Concentrations of sulphur and carbon- Figure 18. Biomass of living, above-ground parts of macrophytes in
the Bassin d'Arcachon (central and inner parts), fish ponds of the
ates are high which imparts a white colour to the water . Bassin d'Arcachon and Provost lagoon, in March, June, September
The pH is generally low (< 7 .5) due to the presence and December 1993 (modified from Auby et al ., 1993 and Bachelet
of considerable quantities of sulphur . This induces pre- et al ., 1994) .
cipitation of carbonates in the form of white suspension
(Figure 16) .
Under certain conditions, red waters can be Conclusions : the eutrophication gradient
observed . They are due to the development of pho-
totrophic sulphur oxidizing bacteria (e .g . Thiocapsa) . The three lagoon systems : Bassin d'Arcachon, fish
The conditions for their development are : a sufficient reservoirs and etang du Prevost clearly differ by their
light intensity, a pH value near 8 and a H2S concentra- degree of eutrophication . An evidence of eutrophi-
tion < 1 ppm . In anoxic conditions, in the light, only cation gradient is the increase in nutrient concentra-
phototrophic bacteria are able to oxidize sulfide and to tions between the Bassin d'Arcachon and the Etang du
use it as an electron donor for reducing carbon dioxide . Prevost (Table 6) . As a consequence, the biomass of
Chronologically, white water is observed before primary producers also increases .
red water. In favourable conditions, phototrophic bac- This is shown by a recent study (Auby et al ., 1993)
teria metabolize the toxic compounds generated by where species composition and biomass of macro-
anaerobic bacteria during fermentative processes and phytes have been studied in spring and summer 1993 .
reduction of sulfate . In a reduced environment, rich One station (station A) was located in a seagrass bed
in hydrogen sulfide, they are able to induce a new (Zostera noltii) on a sandy mudflat in the central part of
steady-state of the sulphur cycle by oxidizing the sul- the Bassin d'Arcachon . Total living biomass (leaves +
phur, under anaerobic conditions, using light energy . roots + rhizomes) of seagrass amounted to 141-167 g
In the Etang du Prevost lagoon a succession of such AFDW m' 2 (173-211 g DW m -2 ), which is close to
events is commonly observed in summer (Amanieu et values reported in an earlier study (Auby, 1991) in the
al ., 1975 ; Caumette & Baleux, 1980) . Coloured waters Bassin d'Arcachon (140-260 g DW m -2 ) . The above-
can invade most of the lagoon within a few days and ground biomass did not show any clear seasonal trend
disappear almost as quickly (Figure 17) . In contrast to (Figure 18) . In the inner part of the Bassin d'Arcachon
the Etang du Prevost, in the fish ponds of Arcachon (station B) living biomass was dominated by the green
only white waters have been observed . This is proba- alga Monostroma obscurum . It declined from March
bly due to a lower light intensity, and the absence of m-2,
to September (29-13 g AFDW or 43-15 g DW
complete anoxia in the water column (some Ruppia are m-2) and never reached the high values found for oth-
always present) . er Ulvaceae in the Etang du Prevost (Figure 18) . A
high biomass of Zostera debris was measured in the
sediments (92-186 g AFDW m -2) . The shallow fish
ponds of Certes (station Cl) were colonized by Rup-
pia cirrhoses . There was a clear seasonal trend with
the highest biomass (63 g AFDW m -2 ) occurring in
xxvi
Table 6 . Dissolved nitrate and ammonia in the water of the Bassin d' Arcachon
(central and inner parts), fish reservoirs of Certes and Etang du Prevost . Values
are given for the summer 1993 (Sloth et al ., 1993) except for the fish reservoirs
(summer 1985, Castel, unpublished) .
Lagoon system NO3 (µM) NH4 (µM)
Bassin d'Arcachon, central part (station A) 0.32-2 .0 2 .0- 2 .4
Bassin d'Arcachon, inner part (station B) 0 .32-1 .3 1 .5- 2 .5
Fish reservoirs (station Cl) 0 .7 -1 .01 4 .6- 8 .3
Etang du Prevost (station 11) 1 .2 -4 .9 5 .7-10.5
This model applies to the three types of lagoon envi-
Relative dominance
primary producers ronments described here . In `healthy' lagoons sea-
and availability grasses dominate . Nitrogen load and concentrations
of nutrients are low and the relative importance of phytoplank-
ton in the shallow seagrass beds is insignificant . The
Bassin d'Arcachon is an example of phase I. In brack-
ish waters were eutrophication increases, revealed by
higher nitrogen loads and nitrogen concentrations and
generally lower, unstable salinities, seagrasses are out-
competed by macroalgae . Epiphyte growth on seagrass
and algae increases considerably together with the rel-
I II M ative dominance of phytoplankton . The fish ponds of
Eutrophication phase the Bassin d'Arcachon are an example of phase II . In
Figure 19 . Tentative model depicting the relation between primary
hypereutrophicated systems (phase III) nutrient con-
producers and nutrients, and the successive stages in the process of centrations are continuously high . Dense uncontrolled
eutrophication (after Nienhuis, 1992) . phytoplankton blooms alternate with mass growth of
macroalgae and rooted plants have completely disap-
peared . Bottom sediments suffer from permanent anox-
June . Some Monostroma were also collected in Sep- ia. The Etang du Prevost is an example of phase III .
tember. In the Prevost lagoon (Station 11) the March Such macroscopic observations should, now, be
samples contained a small amount of Enteromorpha explained by the study of microbiological processes
flexuosa (Wulfen exRoth) J. Agardh and E. intestinalis including meiofauna, protozoa, bacteria and all the
(Linn .) Link (12 g AFDW m -2 ) . These algae disap- components of the microbial loop .
peared in June when they were replaced by Ulva sp .
(270 g AFDW m -2 ) which filled the whole water col-
umn and induced anoxia in the benthos . In September Acknowledgements
algal biomass (Enteromorpha + Ulva) decreased to a
low value (19 g AFDW m -2 ) . From these observations This paper provides the opportunity to J C and P C
it appears that macrophytes did not show any season- to express their gratitude to Prof . M . Amanieu and
al trend in the Bassin d'Arcachon . Some evidence of P. Lasserre who have promoted ecological research of
eutrophication were visible in the Certes lagoons . The lagoon systems in France. This is a contribution to
most obvious changes in macrophyte biomass occurred the E .C . Environment programme (CLEAN contract
in the Prevost lagoon, especially in its inner part, with a EV5V CT92-0080) .
massive development of green algae in June, followed
by their complete disappearance in September.
A tentative model has been developed by Nien- References
huis (1992) describing the relation between the rela-
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XXvii
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