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Bosire 2003
Aquatic Botany 76 (2003) 267–279
Colonization of non-planted mangrove species
into restored mangrove stands in
Gazi Bay, Kenya
J.O. Bosire a,b,∗ , F. Dahdouh-Guebas a , J.G. Kairo a,b , N. Koedam a
a Laboratory of General Botany and Nature Management, Mangrove Management Group,
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
b Kenya Marine and Fisheries Research Institute (KMFRI), P.O. Box 81651, Mombasa, Kenya
Received 18 July 2001; received in revised form 5 February 2003; accepted 2 April 2003
Abstract
Recruitment of non-planted mangrove species into Rhizophora mucronata, Sonneratia alba and
Avicennia marina reforested stands (all of them 5 years old) was investigated to assess possibili-
ties for natural colonization. Corresponding bare (denuded or open without mangroves) and natural
(relatively undisturbed) sites were used as controls. Interstitial water salinity and temperature (mea-
sured at low tide) were lower, whereas sediment organic matter content was higher in the areas with
mangrove cover. Also, the bare sites were more sandy, whereas those with mangrove cover had
more clay and silt. There was no apparent recruitment of non-planted mangrove species into the
bare areas, but the reforested stands of S. alba, A. marina, and R. mucronata had 5400, 4000 and
700 recruits ha−1 , respectively of different mangrove species. The results therefore suggest that
mangrove reforestation has facilitated natural colonization of sites, most likely by altering local
hydrodynamics.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Mangrove reforestation; Colonization; Non-planted species; Environmental variables; Kenya
1. Introduction
Mangrove forests are among the most productive ecosystems and offer a wide range
of resources and services including shoreline stabilization (Teas, 1977; Snedaker, 1987;
Field, 1995), habitat, nursery and breeding ground for many fish species and other fauna
(Teas, 1977; Collete, 1983; Ahmad, 1984; Kurian, 1984; Robertson and Duke, 1987; Ngoile
∗ Corresponding author. Tel.: +254-11-475154; fax: +254-11-475157.
E-mail address: jbosire@kmfri.co.ke (J.O. Bosire).
0304-3770/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-3770(03)00054-8
268 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
and Shunula, 1992; Sasekumar et al., 1992; Rönnbäck, 2001), wood for fuelwood, timber,
poles, boats (Ahmad, 1984; Burbridge, 1984; Fredericks and Lampe, 1984; Aksornkoae,
1987; Hirsh and Mauser, 1992; Dahdouh-Guebas et al., 2000) among other products. Un-
fortunately, development and demographic pressure in many areas have led to widespread
overexploitation of the world’s mangrove forests, at a rate faster than they are being regen-
erated (Field, 1999). In Kenya in particular, mangroves were heavily exploited in the 1970s
due to indiscriminate cutting of trees leading to extensive bare lands in some areas along
the coastline (Kairo, 1992, 1995; Bosire, 1996).
The realization that in some parts of the world mangrove ecosystems are being de-
stroyed, with a consequent loss of inherent services has prompted an upsurge in the number
of rehabilitation projects (Field, 2000). Examples of such mangrove rehabilitation projects
are reported from, e.g. Thailand (Aksornkoae, 1996), Pakistan (Qureshi, 1996), Australia
(Saenger, 1996), Bangladesh (Siddiqi and Khan, 1996), Sri Lanka (SFFL, 1997) and Kenya
(Kairo, 1995). However, monitoring of such replantation sites has been restricted to as-
sessment of early development and growth performance and consequently very little is
known about concomitant natural developments in these stands, such as re-colonization by
non-planted mangrove species. Walters (2000), for example found no post-planting recruit-
ment of non-planted mangrove species into reforested stands of 50 and 60 years old in The
Philippines.
The purpose of this study was to assess the potential of reforested mangrove stands
for re-colonization of non-planted mangrove species, using bare mangrove areas (denuded
or open without mangroves) and natural stands (relatively undisturbed) as controls. We
quantified several physico-chemical parameters and determined the density of non-planted
mangroves in reforested monospecific stands as compared to bare and natural mangrove
areas.
2. Study area
The study was conducted at Gazi (Maftaha) Bay (Fig. 1), on the southern coast of Kenya
about 50 km from Mombasa in Kwale district (4◦ 25 S and 39◦ 30 E). The Bay is sheltered
from strong waves by the presence of the Chale peninsula to the east and a fringing coral reef
to the south. The mangrove is not continuously under direct influence of fresh water because
the two rivers (Kidogoweni in the north and Mkurumji in the south) draining into the Bay are
seasonal and temporal depending on the amount of rainfall inland. Groundwater seepage is
also restricted to a few points (Tack and Polk, 1999). Generally freshwater influx via rivers
and direct rainfall in the Bay accounts for a volume of 305 000 m3 per year of which 20%
is lost due to evapotranspiration, which is also responsible for a salinity maximum zone of
38 PSU in the upper region of the Bay covered by mangroves (Kitheka, 1997). High tidal
flushing rates are coupled with short residence times (3–4 h), which are a function of wide
shallow entrance, lack of topographic controls and the orientation of the bay with respect
to dominant water circulation patterns. River discharge is important during the wet season,
which enhances weak stratification in the upper parts of Kidogoweni, whereas in the dry
season, well mixed homogenous water is found in most regions of the Bay (Kitheka, 1996,
1997). All the nine mangrove species occurring in Kenya are found in this Bay: Avicennia
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 269
Fig. 1. Map of the Kenyan coast showing the study area (Gazi Bay) and the location of the bare, reforested
and natural sites of the R. mucronata, A. marina and S. alba forests (Ruwa, 1997 and Dahdouh-Guebas et al.,
2001). KEY: ( ) mangroves, ( ) sampling sites, (1) R. mucronata bare site, (2) R. mucronata natural site, (3)
R. mucronata reforested site, (4) S. alba bare and reforested sites, (5) S. alba natural site and (6) A. marina bare,
reforested and natural sites.
marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lamk., Ceriops tagal (Perr.) C.B. Rob.,
Heritiera littoralis Dryand., Lumnitzera racemosa Willd., Rhizophora mucronata Lamk.,
Sonneratia alba J. Smith, Xylocarpus granatum Koen. and Xylocarpus moluccencis (Lamk.)
Roem. (nomenclature after Tomlinson, 1986).
The climate in Gazi is typical of the Kenyan coast and principally influenced by monsoon
winds. Total annual precipitation varies between 1000 and 1600 mm showing a bimodal
pattern of distribution. The long rains fall from April to August under the influence of
the southeast monsoon winds, while the short rains fall between October and Novem-
ber under the influence of the northeast monsoon winds. It is normally hot and humid
with an average annual air temperature of about 28 ◦ C with little seasonal variation. Air
temperature in Gazi Bay varies between 24 and 39 ◦ C (data recorded by the Meteoro-
logical Department). Relative humidity is about 95% due to the close proximity to
the sea.
2.1. Site history
The mangrove forests of Gazi have been exploited for many years especially for wood
used for industrial fuel (in the calcium and brick industries in the 1970s) and building poles
(Kairo, 1995; Dahdouh-Guebas et al., 2000). The clear-felling due to the industrial extrac-
tion left some areas along the coastline completely denuded. Experimental reforestation
(plantation trials) was initiated at Gazi Bay in 1991 (Kairo, 1995) where five sites were se-
lected and saplings of S. alba, R. mucronata, and C. tagal were replanted. Results (in terms
270 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
of growth and survival rates) obtained after 3 years indicated that the performance of the
replanted mangroves depended on planting material type (saplings/propagules), elevation
(height above datum) of the forests and the size of the saplings during transplanting. Saplings
did better than propagules and as for elevation, Rhizophora did better at inundation classes
3 and 4 than at classes 1, 6 and 7. Submergence and excessive drought thus were major
constraints. In inundation class 1, profuse barnacle infestation also caused high mortality
of saplings. Ceriops did best in inundation class 5 compared to saplings in class 1 which
died after 2–3 months due to long hours of submergence. The information obtained was
used in an extended experimental reforestation program in 1994, which was done through
community participatory forestry in the rehabilitation of deforested mangrove areas of Gazi.
During this replanting, saplings of R. mucronata, B. gymnorrhiza, A. marina, S. alba and
C. tagal were planted in denuded mangrove areas of Gazi Bay from March to May 1994
in monospecific stands. The sites which were replanted had been clear-felled in the 1970s
and did not show any natural regeneration almost 25 years later when the reforestation was
done (Kairo, 1995). The reforested stands (among those planted above) used in this study
had R. mucronata (6.74 ha), S. alba (0.4 ha) and A. marina (0.25 ha). The three stands were
of the same age (5 years).
The bare and natural sites used as controls were chosen based on physical proximity,
tidal inundation and similarity in site history as the criteria, so as to minimize environmen-
tal variation and maximize paired matching. The denuded controls for reforested S. alba
and R. mucronata stands, were of the same inundation classes following Watson (1928),
i.e. class 1 (at 2 m above datum, with the “zero” datum level at the lowest astronomical
tide level for the Kenyan coast as a reference) and class 2 (2.5 m above datum), respec-
tively, were also clear-felled in the 1970s (Kairo, 1995) and were closest to the respective
reforested sites. The control site for the S. alba forest was just adjacent, whereas that
of the R. mucronata forest was about 1 km away but it was the closest site of the same
inundation class and site history. The control site for the A. marina forest was also of
the same inundation class (class 2) as the reforested site but had no mangroves before
they were planted. The natural control sites were adjacent to the reforested stands in all
cases.
3. Materials and methods
3.1. Environmental factors
Sediment interstitial water samples were randomly collected by digging a hole into the
soil of 10–15 cm (depending on the inundation class, 10 cm for class 1 and 15 cm for class
2) using a machete. Salinity was measured using an optical refractometer (Atago brand),
whereas temperature and pH were taken using a pH meter (WTW pH 320/set-1). Three
subsamples were taken per quadrat for three 10 m × 10 m quadrats randomly chosen per
site. The same experimental protocol was repeated for the controls (bare and natural sites).
All measurements were taken at low tide. Sediment samples were taken down to a depth
of 5 cm using a hand corer. Three replicates were taken per site (one replicate per quadrat).
These samples were oven-dried at 80 ◦ C for about 3 days until constant dry weight was
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 271
obtained and stored in labeled plastic bottles for granulometric analysis. About 20 g was
weighed for each sample and transferred into their prelabeled beakers. The organic matter
in the samples was removed by digestion using an excess of 30% diluted technical H2 O2
as an oxidising agent after which the samples were rinsed with demineralised water until a
more or less stable suspension was obtained (Wartel et al., 1995). The samples were then
re-dried for 24 h at 105 ◦ C and weighed. The difference in weight gave an estimate of the
organic matter content. Grain size analysis was done using a combination of dry sieving
and sedigraph method as outlined by Wartel et al. (1995). The sedigraph determines the
size distribution of particles dispersed in a liquid assuming settling of particles according
to Stokes’ law (Arnold, 1986). For grain size ranges, the unified soil classification system
was used (Robert et al., 1997).
3.2. Vegetation structure and recruitment
Three quadrats of 10 m × 10 m randomly taken in each of the natural and reforested
forests were measured giving a total of nine quadrats for the reforested sites and a sim-
ilar number for the natural controls. Tree height and the diameter D130 (Brokaw and
Thompson, 2000; formerly referred to as DBH, the diameter at breast height) for all the trees
greater than 2.5 cm diameter were measured using a Suunto hypsometer (or a graduated
rod where the forest was thick) and forest calipers, respectively. Density, basal area and
absolute frequency (presence of a mangrove species in quadrats within a site) were then
computed. From these three parameters, relative density ((density of individual species/total
density of all species) × 100), relative dominance ((dominance of a species/dominance of
all species) × 100) and relative frequency (absolute frequency of a species/total absolute
frequency of the stand) were computed and the latter three then summed respectively to get
the importance value (IV) of every mangrove species for all the stands according to Cintron
and Schaeffer-Novelli (1984). This IV combines the three absolute indices (density, basal
area and absolute frequency) from which the relative values are derived to show which man-
grove species contributes relatively more to the structure of a stand. A complexity index
was calculated according to Holdridge et al. (1971). This index combines all the measured
stand structural attributes (stem density, DBH calculated into basal area, height and number
of a species) to show how complex or structurally developed a stand is. The density of juve-
niles (seedlings less than 2.5 cm in diameter and less than 1 m in height) recruited into the
reforested and natural stands were also counted. All juveniles in the reforested stands were
less than 1 m in height. Important to emphasize is that there was no natural regeneration at
the reforested sites during mangrove replanting and thus the areas were completely bare at
that time (Kairo, 1995).
3.3. Statistical analysis
Statistical analyses of environmental factors, vegetation structural indices and juvenile
densities data were done using two-way ANOVA (fixed effect with replication). Multiple
comparisons were done using Tukey’s Honest Significant Difference (HSD) test. In all
cases, the quadrats mentioned above were treated as replicates and the three sites (bare,
reforested and natural) within forests as treatments.
272 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
Table 1
Two-way ANOVA of the sediment characteristics in A. marina, R. mucronata and S. alba forests (stands)
Variable Factor d.f. SS (%) P
Organic matter Mangrove stand 2 21 0.01
Cover type 2 37 0.01
Interaction 4 16 0.06
Error 18 26
Salinity Mangrove stand 2 11 0.09
Cover type 2 32 0.01
Interaction 4 22 0.06
Error 18 36
Temperature Mangrove stand 2 4 0.19
Cover type 2 66 0.01
Interaction 4 8 0.21
Error 18 21
pH Mangrove stand 2 23 0.04
Cover type 2 1 0.82
Interaction 4 21 0.19
Error 18 56
Clay Mangrove stand 2 47 0.01
Cover type 2 15 0.01
Interaction 4 5 0.65
Error 18 33
The different cover types (bare, reforested and natural) were used as treatments.
4. Results
4.1. Environmental factors
With the exception of the S. alba forest, the bare sites in the other forests had higher
interstitial salinities (P < 0.05) than the corresponding reforested and natural sites (Tables 1
and 2). Salinity was similar in all sites of the S. alba forest. pH did not vary significantly
among sites in all the forests. In A. marina and R. mucronata forests, interstitial temperature
was highest (P < 0.05) at bare sites and lowest at natural sites. However, in the S. alba
forest, the bare and reforested sites had similar (P > 0.5) and higher temperature than the
natural site. The bare sites had the lowest organic matter content and higher proportion of
sand than the reforested and natural sites The clay content was not significantly different
among sites within respective mangrove stands.
4.2. Vegetation structure and recruitment
Within its natural stand, R. mucronata was the most dominant (Table 3) compared to the
other species (X. granatum and B. gymnorrhiza). All the reforested sites were monospecific
for the adult trees. The natural stand of S. alba was also monospecific. The natural stand
of A. marina had the highest number of mangrove species with A. marina mostly being
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
Table 2
Site averages (mean ± S.E.) of sediment characteristics in plots with matched natural and reforested (Ref.) stands as well as bare controls for A. marina, R. mucronata
and S. alba forests
Parameter A. marina R. mucronata S. alba
Bare Ref. Natural Bare Ref. Natural Bare Ref. Natural
Salinity (‰) 43 ± 1 a 38 ± 1 b 35 ± 0.7 c 44 ± 3 a 34 ± 0.6 b 35 ± 0.6 b 35 ± 0.5 a 36 ± 0.4 a 35 ± 0.4 a
pH 7±0 7±0 7 ± 0.1 7 ± 0.1 7 ± 0.1 7 ± 0.1 8 ± 0.1 8 ± 0.1 7±0
Temperature (◦ C) 30 ± 0.4 a 27 ± 0.1 b 26 ± 0.3 c 30 ± 0.6 a 27 ± 0.2 b 26 ± 0.1 c 30 ± 0.3 a 29 ± 0.7 a 27 ± 0.1 b
Organic matter (%) 3 ± 0.2 a 19 ± 8 b 25 ± 11 b 4 ± 0.1 a 20 ± 4 b 40 ± 2 c 1 ± 0.3 a 11 ± 2 b 5±1b
Clay (%) 7±3 23 ± 13 20 ± 9 17 ± 4 37 ± 8 42 ± 9 0 5±2 5±2
Tukey multiple comparisons within each forest are also presented. Sites within forests bearing same letters were not significantly different. pH and percent clay did not
differ significantly among sites within forest stands, hence no letters were assigned to them (n = 3).
273
274
Table 3
Absolute (and relative) adult tree density, basal area (and derived % dominance) and absolute (as well as relative) frequency of mangrove species in natural and reforested
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
stands of A. marina, R. mucronata and S. alba forests
Stand Site Species Abs. density (rel.) Basal area (dom.) Abs. frequency IV Mean stand Complexity
(n ha−1 ) (m2 ha−1 ) (rel.) (%) height (m) indexa
A. marina Natural A. marina 3530 ± 730 (83) 26 ± 0.4 (96) 100 (43) 222
C. tagal 430 ± 26 (11) 1 ± 0.1 (3) 67 (29) 43
R. mucronata 130 ± 13 (4) 0 (1) 33 (14) 19
S. alba 70 ± 60 (2) 0 (1) 33 (14) 17 6.1 ± 0.1 27.4
Reforested A. marina 4530 ± 420 (100) 8 ± 0.1 (100) 100 (100) 300 4.5 ± 0.1 1.6
R. mucronata Natural R. mucronata 2570 ± 410 (69) 34 ± 0.3 (80) 100 (43) 192
B. gymnorrhiza 1130 ± 410 (29) 8 ± 0.3 (20) 100 (43) 92
X. granatum 70 ± 61 (2) 0 (0) 33 (14) 16 7.5 ± 0.2 35.6
Reforested R. mucronata 3330 ± 921 (100) 3 ± 0.1 (100) 100 (100) 300 2.9 ± 0.1 0.3
S. alba Natural S. alba 4300 ± 1221 (100) 35 ± 0.9 (100) 100 (100) 300 8.3 ± 0.6 12.5
Reforested S. alba 7640 ± 600 (100) 12 ± 0.3 (100) 100 (100) 300 2.6 ± 0.04 2.4
The relative values are expressed as percentage, while averages are given as mean ± S.E.
a Complexity index is the product of number of species, stem density, mean stand height and basal area divided by 105 (Holdridge et al., 1971).
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 275
Table 4
Density (n ha−1 ) of juvenile mangrove trees in plots within sites of the reforested and natural stands of A. marina,
R. mucronata and S. alba given as mean ± S.E.
Stands Recruits Total
A. marina S. alba R. mucronata C. tagal B. gymnorrhiza
Reforested stands
S. alba 600 1700 1600 1200 300 5400 ± 1100
R. mucronata 0 100 100 100 400 700 ± 100
A. marina 1800 0 600 1200 400 4000 ± 300
Natural stands
S. alba 400 0 1000 100 0 1500 ± 300
R. mucronata 2000 0 4900 1100 0 7000 ± 300
A. marina 2600 0 4000 100 0 6700 ± 200
Table 5
Two-way ANOVA of juvenile densities in the reforested and natural sites of the A. marina, R. mucronata and S.
alba forests (stands)
Factor d.f. SS (%) P
Mangrove stand 2 11 0.01
Cover type 1 12 0.00
Interaction 2 70 0.00
Error 12 8
dominant. Due to their higher mean heights and basal areas, all the natural stands had higher
complexity indices than their respective reforested stands (Table 3).
The monospecific reforested A. marina and R. mucronata stands had seedling recruits of
four species each, whereas in the S. alba stand we found five non-planted mangrove species
(Table 4). The S. alba reforested stand had the highest density of newly recruited individ-
uals (5400 recruits ha−1 ), followed by the A. marina stand (4000 recruits ha−1 ) and the R.
mucronata stand (700 recruits ha−1 ). All the three mangrove species recruited themselves
with A. marina recruiting itself most, followed by S. alba and R. mucronata (1800, 1700
and 100 recruits ha−1 , respectively). A maximum of three mangrove species (A. marina,
R. mucronata and C. tagal) was found growing in each of the natural stands. R. mucronata
had the highest density of seedlings (7000 ha−1 ), followed by A. marina (6700 ha−1 ) and
S. alba (1500 ha−1 ) in natural stands. There were highly significant differences (Table 5) in
seedling recruitment among the reforested and natural sites within respective forest stands
(P = 0.001) and among the three forest stands (P = 0.005).
5. Discussion
We found no re-colonization in any of the bare sites, whereas a number of species had
recruited into the comparable reforested and natural stands with tree cover. These findings
276 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
suggest that mangrove regeneration has modified site conditions, in a way that facilitates
settling and establishment of propagules.
Save for the S. alba forest, salinity and interstitial water temperature were lower in all
reforested and natural sites as compared to bare sites probably due to the shading by the
canopy above, as in Frith et al. (1976) and Frith and Brunnenmeister (1980). The similar
salinity and temperature in the S. alba sites may be attributed to the fact that the forest is
under water during all high tides (inundation class 1). Salinity was similar to that of seawater
in this forest. The tree cover in the Rhizophora and Avicennia reforested sites has probably
helped in reducing the effect of desiccation as a potential threat to propagule survival.
No colonization occurred in the S. alba bare site and yet this site had similar salinity and
temperature as the comparable reforested and natural sites. Possible causes of this failure in
seedling recruitment may include: a limited influx of propagules (FAO, 1994; Panapitukkul
et al., 1998), propagule predation (Jones, 1984; Smith, 1988; Dahdouh-Guebas et al., 1997,
1998; Lee, 1998), high wave energies, hydrodynamic trapping or damage of propagules by
floating debris (Walter, 1971; Snedaker, 1978; Cintron, 1996; Delgado et al., 2001; Stieglitz
and Ridd, 2001; Thampanya et al., 2002), as well as the low tidal position of the sandflat,
with associated strong tidal currents. However, with reproductive stands adjacent, propagule
supply can be ruled out as a cause. Osborne and Smith (1990) found that propagules are
more vulnerable to herbivores beneath closed canopies than in gaps, which may make
propagule predation less probable as an important limiting factor. We suggest that daily
tidal inundation exposes potential recruits to both wave action and tidal currents. The high
sand content (100%) in this site may be indicative of the impact of these hydrodynamic
processes.
Natural colonization varied among sites and mangrove species. The reforested S. alba
stand had a higher number of recruits as compared to the natural stand, which may suggest
a higher natural regeneration potential in the former. Even when compared with the other
two natural stands, the natural S. alba stand had the lowest density of recruits. The low
recruitment in this stand may be attributed to the harshness of the habitat for seedling
survival due to exposure to stronger wave attack and higher tidal velocities. The higher
densities of seedlings in the A. marina and R. mucronata natural stands can also be attributed
to the presence of other adult tree species within these stands, which were the most likely
propagule sources of the seedlings recruited contrary to the S. alba natural stand which was
monospecific even for the adult trees.
6. Conclusion
The findings of this study suggest that clear-felling of mangroves greatly impairs natural
regeneration most likely due to the resulting unfavourable site conditions. Mangrove refor-
estation however, appears to facilitate natural colonization of sites, most likely by altering
local hydrodynamics and other physico-chemical factors. The aerial roots of established
trees help in breaking waves, slowing tidal currents and trapping floating mangrove propag-
ules assuring the establishment of a sapling bank (Ellison, 2000). With severely limited
propagule retention, regeneration of any mangrove vegetation may not occur in the absence
of human intervention.
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 277
Acknowledgements
Special thanks go to Prof. Daro for her assistance in the field, the Royal Belgian Institute
of Natural Sciences through Prof. Wartel and Frederik Francken for assistance in grain
size analysis, and all the colleagues at KMFRI who assisted in both the field work and lab
work to make this work a success. The second author is a Postdoctoral Researcher of the
Fund for Scientific Research (FWO, Vlaanderen). The research was also financed by the
European Community (Contract IC18-CT96-0065) and the Institute for the Promotion of
Innovation by Science and Technology in Flanders (IWT). This work was in part presented
on the 15th Biennial International Conference of the Estuarine Research Foundation (ERF)
in New Orleans, USA, 1999, with financial support of the ERF and the Western Indian
Ocean Marine Science Association (WIOMSA).
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Colonization of non-planted mangrove species
into restored mangrove stands in
Gazi Bay, Kenya
J.O. Bosire a,b,∗ , F. Dahdouh-Guebas a , J.G. Kairo a,b , N. Koedam a
a Laboratory of General Botany and Nature Management, Mangrove Management Group,
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
b Kenya Marine and Fisheries Research Institute (KMFRI), P.O. Box 81651, Mombasa, Kenya
Received 18 July 2001; received in revised form 5 February 2003; accepted 2 April 2003
Abstract
Recruitment of non-planted mangrove species into Rhizophora mucronata, Sonneratia alba and
Avicennia marina reforested stands (all of them 5 years old) was investigated to assess possibili-
ties for natural colonization. Corresponding bare (denuded or open without mangroves) and natural
(relatively undisturbed) sites were used as controls. Interstitial water salinity and temperature (mea-
sured at low tide) were lower, whereas sediment organic matter content was higher in the areas with
mangrove cover. Also, the bare sites were more sandy, whereas those with mangrove cover had
more clay and silt. There was no apparent recruitment of non-planted mangrove species into the
bare areas, but the reforested stands of S. alba, A. marina, and R. mucronata had 5400, 4000 and
700 recruits ha−1 , respectively of different mangrove species. The results therefore suggest that
mangrove reforestation has facilitated natural colonization of sites, most likely by altering local
hydrodynamics.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Mangrove reforestation; Colonization; Non-planted species; Environmental variables; Kenya
1. Introduction
Mangrove forests are among the most productive ecosystems and offer a wide range
of resources and services including shoreline stabilization (Teas, 1977; Snedaker, 1987;
Field, 1995), habitat, nursery and breeding ground for many fish species and other fauna
(Teas, 1977; Collete, 1983; Ahmad, 1984; Kurian, 1984; Robertson and Duke, 1987; Ngoile
∗ Corresponding author. Tel.: +254-11-475154; fax: +254-11-475157.
E-mail address: jbosire@kmfri.co.ke (J.O. Bosire).
0304-3770/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-3770(03)00054-8
268 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
and Shunula, 1992; Sasekumar et al., 1992; Rönnbäck, 2001), wood for fuelwood, timber,
poles, boats (Ahmad, 1984; Burbridge, 1984; Fredericks and Lampe, 1984; Aksornkoae,
1987; Hirsh and Mauser, 1992; Dahdouh-Guebas et al., 2000) among other products. Un-
fortunately, development and demographic pressure in many areas have led to widespread
overexploitation of the world’s mangrove forests, at a rate faster than they are being regen-
erated (Field, 1999). In Kenya in particular, mangroves were heavily exploited in the 1970s
due to indiscriminate cutting of trees leading to extensive bare lands in some areas along
the coastline (Kairo, 1992, 1995; Bosire, 1996).
The realization that in some parts of the world mangrove ecosystems are being de-
stroyed, with a consequent loss of inherent services has prompted an upsurge in the number
of rehabilitation projects (Field, 2000). Examples of such mangrove rehabilitation projects
are reported from, e.g. Thailand (Aksornkoae, 1996), Pakistan (Qureshi, 1996), Australia
(Saenger, 1996), Bangladesh (Siddiqi and Khan, 1996), Sri Lanka (SFFL, 1997) and Kenya
(Kairo, 1995). However, monitoring of such replantation sites has been restricted to as-
sessment of early development and growth performance and consequently very little is
known about concomitant natural developments in these stands, such as re-colonization by
non-planted mangrove species. Walters (2000), for example found no post-planting recruit-
ment of non-planted mangrove species into reforested stands of 50 and 60 years old in The
Philippines.
The purpose of this study was to assess the potential of reforested mangrove stands
for re-colonization of non-planted mangrove species, using bare mangrove areas (denuded
or open without mangroves) and natural stands (relatively undisturbed) as controls. We
quantified several physico-chemical parameters and determined the density of non-planted
mangroves in reforested monospecific stands as compared to bare and natural mangrove
areas.
2. Study area
The study was conducted at Gazi (Maftaha) Bay (Fig. 1), on the southern coast of Kenya
about 50 km from Mombasa in Kwale district (4◦ 25 S and 39◦ 30 E). The Bay is sheltered
from strong waves by the presence of the Chale peninsula to the east and a fringing coral reef
to the south. The mangrove is not continuously under direct influence of fresh water because
the two rivers (Kidogoweni in the north and Mkurumji in the south) draining into the Bay are
seasonal and temporal depending on the amount of rainfall inland. Groundwater seepage is
also restricted to a few points (Tack and Polk, 1999). Generally freshwater influx via rivers
and direct rainfall in the Bay accounts for a volume of 305 000 m3 per year of which 20%
is lost due to evapotranspiration, which is also responsible for a salinity maximum zone of
38 PSU in the upper region of the Bay covered by mangroves (Kitheka, 1997). High tidal
flushing rates are coupled with short residence times (3–4 h), which are a function of wide
shallow entrance, lack of topographic controls and the orientation of the bay with respect
to dominant water circulation patterns. River discharge is important during the wet season,
which enhances weak stratification in the upper parts of Kidogoweni, whereas in the dry
season, well mixed homogenous water is found in most regions of the Bay (Kitheka, 1996,
1997). All the nine mangrove species occurring in Kenya are found in this Bay: Avicennia
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 269
Fig. 1. Map of the Kenyan coast showing the study area (Gazi Bay) and the location of the bare, reforested
and natural sites of the R. mucronata, A. marina and S. alba forests (Ruwa, 1997 and Dahdouh-Guebas et al.,
2001). KEY: ( ) mangroves, ( ) sampling sites, (1) R. mucronata bare site, (2) R. mucronata natural site, (3)
R. mucronata reforested site, (4) S. alba bare and reforested sites, (5) S. alba natural site and (6) A. marina bare,
reforested and natural sites.
marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lamk., Ceriops tagal (Perr.) C.B. Rob.,
Heritiera littoralis Dryand., Lumnitzera racemosa Willd., Rhizophora mucronata Lamk.,
Sonneratia alba J. Smith, Xylocarpus granatum Koen. and Xylocarpus moluccencis (Lamk.)
Roem. (nomenclature after Tomlinson, 1986).
The climate in Gazi is typical of the Kenyan coast and principally influenced by monsoon
winds. Total annual precipitation varies between 1000 and 1600 mm showing a bimodal
pattern of distribution. The long rains fall from April to August under the influence of
the southeast monsoon winds, while the short rains fall between October and Novem-
ber under the influence of the northeast monsoon winds. It is normally hot and humid
with an average annual air temperature of about 28 ◦ C with little seasonal variation. Air
temperature in Gazi Bay varies between 24 and 39 ◦ C (data recorded by the Meteoro-
logical Department). Relative humidity is about 95% due to the close proximity to
the sea.
2.1. Site history
The mangrove forests of Gazi have been exploited for many years especially for wood
used for industrial fuel (in the calcium and brick industries in the 1970s) and building poles
(Kairo, 1995; Dahdouh-Guebas et al., 2000). The clear-felling due to the industrial extrac-
tion left some areas along the coastline completely denuded. Experimental reforestation
(plantation trials) was initiated at Gazi Bay in 1991 (Kairo, 1995) where five sites were se-
lected and saplings of S. alba, R. mucronata, and C. tagal were replanted. Results (in terms
270 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
of growth and survival rates) obtained after 3 years indicated that the performance of the
replanted mangroves depended on planting material type (saplings/propagules), elevation
(height above datum) of the forests and the size of the saplings during transplanting. Saplings
did better than propagules and as for elevation, Rhizophora did better at inundation classes
3 and 4 than at classes 1, 6 and 7. Submergence and excessive drought thus were major
constraints. In inundation class 1, profuse barnacle infestation also caused high mortality
of saplings. Ceriops did best in inundation class 5 compared to saplings in class 1 which
died after 2–3 months due to long hours of submergence. The information obtained was
used in an extended experimental reforestation program in 1994, which was done through
community participatory forestry in the rehabilitation of deforested mangrove areas of Gazi.
During this replanting, saplings of R. mucronata, B. gymnorrhiza, A. marina, S. alba and
C. tagal were planted in denuded mangrove areas of Gazi Bay from March to May 1994
in monospecific stands. The sites which were replanted had been clear-felled in the 1970s
and did not show any natural regeneration almost 25 years later when the reforestation was
done (Kairo, 1995). The reforested stands (among those planted above) used in this study
had R. mucronata (6.74 ha), S. alba (0.4 ha) and A. marina (0.25 ha). The three stands were
of the same age (5 years).
The bare and natural sites used as controls were chosen based on physical proximity,
tidal inundation and similarity in site history as the criteria, so as to minimize environmen-
tal variation and maximize paired matching. The denuded controls for reforested S. alba
and R. mucronata stands, were of the same inundation classes following Watson (1928),
i.e. class 1 (at 2 m above datum, with the “zero” datum level at the lowest astronomical
tide level for the Kenyan coast as a reference) and class 2 (2.5 m above datum), respec-
tively, were also clear-felled in the 1970s (Kairo, 1995) and were closest to the respective
reforested sites. The control site for the S. alba forest was just adjacent, whereas that
of the R. mucronata forest was about 1 km away but it was the closest site of the same
inundation class and site history. The control site for the A. marina forest was also of
the same inundation class (class 2) as the reforested site but had no mangroves before
they were planted. The natural control sites were adjacent to the reforested stands in all
cases.
3. Materials and methods
3.1. Environmental factors
Sediment interstitial water samples were randomly collected by digging a hole into the
soil of 10–15 cm (depending on the inundation class, 10 cm for class 1 and 15 cm for class
2) using a machete. Salinity was measured using an optical refractometer (Atago brand),
whereas temperature and pH were taken using a pH meter (WTW pH 320/set-1). Three
subsamples were taken per quadrat for three 10 m × 10 m quadrats randomly chosen per
site. The same experimental protocol was repeated for the controls (bare and natural sites).
All measurements were taken at low tide. Sediment samples were taken down to a depth
of 5 cm using a hand corer. Three replicates were taken per site (one replicate per quadrat).
These samples were oven-dried at 80 ◦ C for about 3 days until constant dry weight was
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 271
obtained and stored in labeled plastic bottles for granulometric analysis. About 20 g was
weighed for each sample and transferred into their prelabeled beakers. The organic matter
in the samples was removed by digestion using an excess of 30% diluted technical H2 O2
as an oxidising agent after which the samples were rinsed with demineralised water until a
more or less stable suspension was obtained (Wartel et al., 1995). The samples were then
re-dried for 24 h at 105 ◦ C and weighed. The difference in weight gave an estimate of the
organic matter content. Grain size analysis was done using a combination of dry sieving
and sedigraph method as outlined by Wartel et al. (1995). The sedigraph determines the
size distribution of particles dispersed in a liquid assuming settling of particles according
to Stokes’ law (Arnold, 1986). For grain size ranges, the unified soil classification system
was used (Robert et al., 1997).
3.2. Vegetation structure and recruitment
Three quadrats of 10 m × 10 m randomly taken in each of the natural and reforested
forests were measured giving a total of nine quadrats for the reforested sites and a sim-
ilar number for the natural controls. Tree height and the diameter D130 (Brokaw and
Thompson, 2000; formerly referred to as DBH, the diameter at breast height) for all the trees
greater than 2.5 cm diameter were measured using a Suunto hypsometer (or a graduated
rod where the forest was thick) and forest calipers, respectively. Density, basal area and
absolute frequency (presence of a mangrove species in quadrats within a site) were then
computed. From these three parameters, relative density ((density of individual species/total
density of all species) × 100), relative dominance ((dominance of a species/dominance of
all species) × 100) and relative frequency (absolute frequency of a species/total absolute
frequency of the stand) were computed and the latter three then summed respectively to get
the importance value (IV) of every mangrove species for all the stands according to Cintron
and Schaeffer-Novelli (1984). This IV combines the three absolute indices (density, basal
area and absolute frequency) from which the relative values are derived to show which man-
grove species contributes relatively more to the structure of a stand. A complexity index
was calculated according to Holdridge et al. (1971). This index combines all the measured
stand structural attributes (stem density, DBH calculated into basal area, height and number
of a species) to show how complex or structurally developed a stand is. The density of juve-
niles (seedlings less than 2.5 cm in diameter and less than 1 m in height) recruited into the
reforested and natural stands were also counted. All juveniles in the reforested stands were
less than 1 m in height. Important to emphasize is that there was no natural regeneration at
the reforested sites during mangrove replanting and thus the areas were completely bare at
that time (Kairo, 1995).
3.3. Statistical analysis
Statistical analyses of environmental factors, vegetation structural indices and juvenile
densities data were done using two-way ANOVA (fixed effect with replication). Multiple
comparisons were done using Tukey’s Honest Significant Difference (HSD) test. In all
cases, the quadrats mentioned above were treated as replicates and the three sites (bare,
reforested and natural) within forests as treatments.
272 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
Table 1
Two-way ANOVA of the sediment characteristics in A. marina, R. mucronata and S. alba forests (stands)
Variable Factor d.f. SS (%) P
Organic matter Mangrove stand 2 21 0.01
Cover type 2 37 0.01
Interaction 4 16 0.06
Error 18 26
Salinity Mangrove stand 2 11 0.09
Cover type 2 32 0.01
Interaction 4 22 0.06
Error 18 36
Temperature Mangrove stand 2 4 0.19
Cover type 2 66 0.01
Interaction 4 8 0.21
Error 18 21
pH Mangrove stand 2 23 0.04
Cover type 2 1 0.82
Interaction 4 21 0.19
Error 18 56
Clay Mangrove stand 2 47 0.01
Cover type 2 15 0.01
Interaction 4 5 0.65
Error 18 33
The different cover types (bare, reforested and natural) were used as treatments.
4. Results
4.1. Environmental factors
With the exception of the S. alba forest, the bare sites in the other forests had higher
interstitial salinities (P < 0.05) than the corresponding reforested and natural sites (Tables 1
and 2). Salinity was similar in all sites of the S. alba forest. pH did not vary significantly
among sites in all the forests. In A. marina and R. mucronata forests, interstitial temperature
was highest (P < 0.05) at bare sites and lowest at natural sites. However, in the S. alba
forest, the bare and reforested sites had similar (P > 0.5) and higher temperature than the
natural site. The bare sites had the lowest organic matter content and higher proportion of
sand than the reforested and natural sites The clay content was not significantly different
among sites within respective mangrove stands.
4.2. Vegetation structure and recruitment
Within its natural stand, R. mucronata was the most dominant (Table 3) compared to the
other species (X. granatum and B. gymnorrhiza). All the reforested sites were monospecific
for the adult trees. The natural stand of S. alba was also monospecific. The natural stand
of A. marina had the highest number of mangrove species with A. marina mostly being
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
Table 2
Site averages (mean ± S.E.) of sediment characteristics in plots with matched natural and reforested (Ref.) stands as well as bare controls for A. marina, R. mucronata
and S. alba forests
Parameter A. marina R. mucronata S. alba
Bare Ref. Natural Bare Ref. Natural Bare Ref. Natural
Salinity (‰) 43 ± 1 a 38 ± 1 b 35 ± 0.7 c 44 ± 3 a 34 ± 0.6 b 35 ± 0.6 b 35 ± 0.5 a 36 ± 0.4 a 35 ± 0.4 a
pH 7±0 7±0 7 ± 0.1 7 ± 0.1 7 ± 0.1 7 ± 0.1 8 ± 0.1 8 ± 0.1 7±0
Temperature (◦ C) 30 ± 0.4 a 27 ± 0.1 b 26 ± 0.3 c 30 ± 0.6 a 27 ± 0.2 b 26 ± 0.1 c 30 ± 0.3 a 29 ± 0.7 a 27 ± 0.1 b
Organic matter (%) 3 ± 0.2 a 19 ± 8 b 25 ± 11 b 4 ± 0.1 a 20 ± 4 b 40 ± 2 c 1 ± 0.3 a 11 ± 2 b 5±1b
Clay (%) 7±3 23 ± 13 20 ± 9 17 ± 4 37 ± 8 42 ± 9 0 5±2 5±2
Tukey multiple comparisons within each forest are also presented. Sites within forests bearing same letters were not significantly different. pH and percent clay did not
differ significantly among sites within forest stands, hence no letters were assigned to them (n = 3).
273
274
Table 3
Absolute (and relative) adult tree density, basal area (and derived % dominance) and absolute (as well as relative) frequency of mangrove species in natural and reforested
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
stands of A. marina, R. mucronata and S. alba forests
Stand Site Species Abs. density (rel.) Basal area (dom.) Abs. frequency IV Mean stand Complexity
(n ha−1 ) (m2 ha−1 ) (rel.) (%) height (m) indexa
A. marina Natural A. marina 3530 ± 730 (83) 26 ± 0.4 (96) 100 (43) 222
C. tagal 430 ± 26 (11) 1 ± 0.1 (3) 67 (29) 43
R. mucronata 130 ± 13 (4) 0 (1) 33 (14) 19
S. alba 70 ± 60 (2) 0 (1) 33 (14) 17 6.1 ± 0.1 27.4
Reforested A. marina 4530 ± 420 (100) 8 ± 0.1 (100) 100 (100) 300 4.5 ± 0.1 1.6
R. mucronata Natural R. mucronata 2570 ± 410 (69) 34 ± 0.3 (80) 100 (43) 192
B. gymnorrhiza 1130 ± 410 (29) 8 ± 0.3 (20) 100 (43) 92
X. granatum 70 ± 61 (2) 0 (0) 33 (14) 16 7.5 ± 0.2 35.6
Reforested R. mucronata 3330 ± 921 (100) 3 ± 0.1 (100) 100 (100) 300 2.9 ± 0.1 0.3
S. alba Natural S. alba 4300 ± 1221 (100) 35 ± 0.9 (100) 100 (100) 300 8.3 ± 0.6 12.5
Reforested S. alba 7640 ± 600 (100) 12 ± 0.3 (100) 100 (100) 300 2.6 ± 0.04 2.4
The relative values are expressed as percentage, while averages are given as mean ± S.E.
a Complexity index is the product of number of species, stem density, mean stand height and basal area divided by 105 (Holdridge et al., 1971).
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 275
Table 4
Density (n ha−1 ) of juvenile mangrove trees in plots within sites of the reforested and natural stands of A. marina,
R. mucronata and S. alba given as mean ± S.E.
Stands Recruits Total
A. marina S. alba R. mucronata C. tagal B. gymnorrhiza
Reforested stands
S. alba 600 1700 1600 1200 300 5400 ± 1100
R. mucronata 0 100 100 100 400 700 ± 100
A. marina 1800 0 600 1200 400 4000 ± 300
Natural stands
S. alba 400 0 1000 100 0 1500 ± 300
R. mucronata 2000 0 4900 1100 0 7000 ± 300
A. marina 2600 0 4000 100 0 6700 ± 200
Table 5
Two-way ANOVA of juvenile densities in the reforested and natural sites of the A. marina, R. mucronata and S.
alba forests (stands)
Factor d.f. SS (%) P
Mangrove stand 2 11 0.01
Cover type 1 12 0.00
Interaction 2 70 0.00
Error 12 8
dominant. Due to their higher mean heights and basal areas, all the natural stands had higher
complexity indices than their respective reforested stands (Table 3).
The monospecific reforested A. marina and R. mucronata stands had seedling recruits of
four species each, whereas in the S. alba stand we found five non-planted mangrove species
(Table 4). The S. alba reforested stand had the highest density of newly recruited individ-
uals (5400 recruits ha−1 ), followed by the A. marina stand (4000 recruits ha−1 ) and the R.
mucronata stand (700 recruits ha−1 ). All the three mangrove species recruited themselves
with A. marina recruiting itself most, followed by S. alba and R. mucronata (1800, 1700
and 100 recruits ha−1 , respectively). A maximum of three mangrove species (A. marina,
R. mucronata and C. tagal) was found growing in each of the natural stands. R. mucronata
had the highest density of seedlings (7000 ha−1 ), followed by A. marina (6700 ha−1 ) and
S. alba (1500 ha−1 ) in natural stands. There were highly significant differences (Table 5) in
seedling recruitment among the reforested and natural sites within respective forest stands
(P = 0.001) and among the three forest stands (P = 0.005).
5. Discussion
We found no re-colonization in any of the bare sites, whereas a number of species had
recruited into the comparable reforested and natural stands with tree cover. These findings
276 J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279
suggest that mangrove regeneration has modified site conditions, in a way that facilitates
settling and establishment of propagules.
Save for the S. alba forest, salinity and interstitial water temperature were lower in all
reforested and natural sites as compared to bare sites probably due to the shading by the
canopy above, as in Frith et al. (1976) and Frith and Brunnenmeister (1980). The similar
salinity and temperature in the S. alba sites may be attributed to the fact that the forest is
under water during all high tides (inundation class 1). Salinity was similar to that of seawater
in this forest. The tree cover in the Rhizophora and Avicennia reforested sites has probably
helped in reducing the effect of desiccation as a potential threat to propagule survival.
No colonization occurred in the S. alba bare site and yet this site had similar salinity and
temperature as the comparable reforested and natural sites. Possible causes of this failure in
seedling recruitment may include: a limited influx of propagules (FAO, 1994; Panapitukkul
et al., 1998), propagule predation (Jones, 1984; Smith, 1988; Dahdouh-Guebas et al., 1997,
1998; Lee, 1998), high wave energies, hydrodynamic trapping or damage of propagules by
floating debris (Walter, 1971; Snedaker, 1978; Cintron, 1996; Delgado et al., 2001; Stieglitz
and Ridd, 2001; Thampanya et al., 2002), as well as the low tidal position of the sandflat,
with associated strong tidal currents. However, with reproductive stands adjacent, propagule
supply can be ruled out as a cause. Osborne and Smith (1990) found that propagules are
more vulnerable to herbivores beneath closed canopies than in gaps, which may make
propagule predation less probable as an important limiting factor. We suggest that daily
tidal inundation exposes potential recruits to both wave action and tidal currents. The high
sand content (100%) in this site may be indicative of the impact of these hydrodynamic
processes.
Natural colonization varied among sites and mangrove species. The reforested S. alba
stand had a higher number of recruits as compared to the natural stand, which may suggest
a higher natural regeneration potential in the former. Even when compared with the other
two natural stands, the natural S. alba stand had the lowest density of recruits. The low
recruitment in this stand may be attributed to the harshness of the habitat for seedling
survival due to exposure to stronger wave attack and higher tidal velocities. The higher
densities of seedlings in the A. marina and R. mucronata natural stands can also be attributed
to the presence of other adult tree species within these stands, which were the most likely
propagule sources of the seedlings recruited contrary to the S. alba natural stand which was
monospecific even for the adult trees.
6. Conclusion
The findings of this study suggest that clear-felling of mangroves greatly impairs natural
regeneration most likely due to the resulting unfavourable site conditions. Mangrove refor-
estation however, appears to facilitate natural colonization of sites, most likely by altering
local hydrodynamics and other physico-chemical factors. The aerial roots of established
trees help in breaking waves, slowing tidal currents and trapping floating mangrove propag-
ules assuring the establishment of a sapling bank (Ellison, 2000). With severely limited
propagule retention, regeneration of any mangrove vegetation may not occur in the absence
of human intervention.
J.O. Bosire et al. / Aquatic Botany 76 (2003) 267–279 277
Acknowledgements
Special thanks go to Prof. Daro for her assistance in the field, the Royal Belgian Institute
of Natural Sciences through Prof. Wartel and Frederik Francken for assistance in grain
size analysis, and all the colleagues at KMFRI who assisted in both the field work and lab
work to make this work a success. The second author is a Postdoctoral Researcher of the
Fund for Scientific Research (FWO, Vlaanderen). The research was also financed by the
European Community (Contract IC18-CT96-0065) and the Institute for the Promotion of
Innovation by Science and Technology in Flanders (IWT). This work was in part presented
on the 15th Biennial International Conference of the Estuarine Research Foundation (ERF)
in New Orleans, USA, 1999, with financial support of the ERF and the Western Indian
Ocean Marine Science Association (WIOMSA).
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