Taxonomic Diversity and Structure of Benthic Macroinvertebrates in Aby Lagoon (Ivory Coast, West Africa)
The benthic macroinvertebrates of Aby lagoon (West Africa:
Ivory coast) was studied during four seasons (high dry season, high rainy
season, low dry season and low rainy season, respectively) from June 2006
to March 2007. The distribution of the benthic macroinvertebrates species
was recorded at 13 stations on the whole of the lagoon. A total of 62
taxa of benthic macroinvertebrates belonging to 28 families and 10 orders
were listed. The molluscs and crustaceans dominate qualitatively by adding
up 51 and 24%, respectively of the total number of organisms. Five taxa
(Corbula trigona (20%), Pachymelania aurita (12%), Clibernhardius
cooki (7%), Oligochaeta (7%) and Crassostrea gasar (6%) accounted
for 52% of total abundance. Classification analysis used to perform the
characterisation of the lagoon on the basis of benthic macroinvertebrates
showed the existence of four main clusters in which the seasonal pattern
in benthic macroinvertebrates were very similar in the four seasons. In
contrast the species richness and diversity indices were significantly
different. Furthermore these indices where higher in the stations closer
to the sea and surrounded by mangrove trees (southern area) compared to
the inland ones.
Littoral ecosystems such as lagoons and estuaries environments serve
as important nursery habitats for host of fisheries species, including
other vertebrates and invertebrates, many of which feed on benthic macrofauna
(Bazaïri et al., 2003). Because of such ecological characteristics
there are many literatures on benthic ecology (Mistri et al., 2002;
Marzano et al., 2003; Sconfietti et al., 2003). In Ivory
Coast, such ecosystems are rather well represented. Indeed, three large
lagoons in communication with the Atlantic Ocean can be noticed. It is
about the Ebrié lagoon, the vastest (566 km2) localised
in the central part of the littoral frontage, the Aby lagoon (424 km2)
in the eastern and the Grand-Lahou lagoon (190 km2) in the
west. To date, most of the studies on the benthic macrofauna were only
devoted to the Ebrié lagoon. Among these studies, the most recent
(Zabi and Leloeuf, 1993) is approximately 15 years old. The benthic macroinvertebrates
of the two other lagoons remains unknown. According to Albaret (1994),
the Aby lagoon for example, is known for its high halieutic productivity.
However, it is subjected to natural variations of the climatic conditions
(lowers pluviometry) and of the increasing of anthropic disturbances (Sankaré
et al., 1999). What could have an influence on the faunistic composition
in general and particularly the benthic macroinvertebrates often used
like good indicator of pollution and water quality (Borja et al.,
2000; Inglis and Kross, 2000).
The present study fills the gap by making an inventory, examining the
population structures (abundance and diversity) of benthic macroinvertebrates
through seasons and spaces in order to appreciate the medical state of
the Aby lagoon.
MATERIALS AND METHODS
Study area: The Aby lagoon (Fig. 1) located
in the far east of Ivory Coast, forms a natural boundary with Ghana and
is composed primarily of 3 lagoons which are Aby, Ehy and Tendo. The lagoon
covers an area of 424 km2 and stretches for 24.5 km north to
south and 56 km east to west with a maximum width of 15.5 km (Avit et
al., 1996). The maximum depth is 17 m in its broadest part. The lagoon
is connected to the sea in its southern part by an artificial channel
and receives freshwater primarily of Bia and Tanoe rivers in the north
and the east, respectively.
|| Map of Aby lagoon showing the sampling stations
Bottom sediments are mostly composed by sand towards the banks and mud
with high organic content and shell fragments in the central part of the
basin (Sankaré et al., 1999).
The vegetation around this lagoon is dominated by a mangrove forest (Rhizophora
racemosa, Avicennia germinans, Conocarpus erectus),
the palm tree (Elaeis guineensis) and coconut (Cocos nucifera)
The climate is an equatorial type with an annual rainfall ranged between
1500 to 1800 mm. It is characterised by two rainy seasons and two dry
seasons (Durand and Skubich, 1982). The High Rainy Season (HRS) extends
from April to July, the Low Dry Season (LDS) from August to September,
the Low Rainy Season (LRS) from October to early December and the High
Dry Season (HDS) from December to March.
Sampling and data analysis
Sampling: Thirteen stations abbreviated A1 to A13 (Fig.
1) were sampled seasonally from June 2006 to March 2007 (June 2006,
September 2006, November 2006 and March 2007). Soft bottoms were collected
using a Van Veen grab and each sample had a surface area of 0.30 m2.
The samples were sieved in situ through a 1 mm mesh. The materiel
retained on the mesh was fixed in formaldehyde 10%. In the laboratory,
the macroinvertebrates were sorted, identified at the lowest taxonomic
level possible and counted.
Water parameters (dissolved oxygen, salinity, temperature and transparency)
were recorded on field at each site.
Data analysis: The macroinvertebrates structure was described
through the species richness (S), the Shannon-Weaver diversity index (H`)
and abundance (A).
The taxonomic similarity between stations was elucidated by cluster analysis
based on the faunal composition (Ward linkage method, Euclidian distance)
using the abundance matrix. The abundances were square root transformed
to limit the influence of the most dominant taxa. Significant differences
in species richness and diversity indices were performed using Kruskal-Wallis
test followed by Rank multiple comparison tests. All these analysis were
carried out using the STATISTICA 7.1 software computer.
Abiotic parameters: The average values of the abiotic water variables
measured on the surface during the study were mentioned in Table
1. Temperature ranged between 26.3 (station A8, LDS) and 32.0 °C
(station A5, HDS), salinity between 0 PSU (stations A10 to A13, HRS; A1,
A12, A13, LDS; A1, A2, A10 to A13, LRS) and 12.3 PSU (station A8, HDS),
dissolved oxygen between 2.89 mg L-1 (station A13, LDS) and
13 mg L-1 (station A7, HDS) and transparency between 30 cm
(station A13, HDS) and 110 cm (station A9, LDS).
Taxonomic composition: A total of 62 taxa of benthic macroinvertebrates
belonging to 28 families and 10 orders were collected during this study
(Table 2). The orders are: Mesogastropoda and Eulamellibranchia
for the Mollusca, Amphipoda, Isopoda and Decapoda for the Crustacea, Diptera,
Coleoptera and Trichoptera for the Insecta, Nereidiformia and Capitelliformia
for the Polychaete.
Molluscs and Crustaceans with 64% of the taxonomic richness dominate
qualitatively this biotic structure.
The dominance of both classes also appears on the quantitative level
by adding up, respectively 51 and 24% of the total number of organisms.
Five of the taxa collected accounted for 52% of total abundance. This
group comprised Corbula trigona (20%), Pachymelania aurita
(12%), Clibernhardius cooki (7%), Oligochaeta (7%) and Crassostrea
gasar (6%). Only a species (Corbula trigona) is common to all
Spatial and seasonal pattern: Species richness and diversity indices
have great variability between the stations (Fig. 2).
The species richness and diversity indices displayed two gradients. The
first gradient from the north (mouth of Bia river in the lagoon) to the
south towards the point of contact of the lagoon and the sea. The second
is from the eastern part (entrance of Tanoe river in the lagoon) to the
centre of the lagoon. In both cases, species richness and diversity indices
increase. Stations A2, A3 in the north and A9, A10, A11 in the east displayed
the low values of the species richness and diversity indices. In several
cases the highest average species richness and diversity indices were
recorded in station A5. In addition, these indices have a similar pattern
in the four seasons.
||Average data of temperature, water salinity, oxygen
percentage saturation and transparency (Secchi disk) measured on the
surface during the sampling campaigns
|HRS: High Rainy season; LDS: Low Dry season; LRS: Low
Rainy Season; HDS: High Dry Season
||Taxonomic list of benthic macro invertebrates found
at the thirteen stations in Aby lagoon during the present investigation
||Community indices of benthic macroinvertebrates in the
thirteen stations and the four sampling campaigns in Aby lagoon, (a)
HRS: High Rainy Season, (b) LDS: Low Dry Season, (c) LRS: Low Rainy
Season and (d) HDS: High Dry Season
Cluster identification: The cluster analysis (Fig.
3) showed 4 main groups (Euclidian distance of 30). Cluster I is mostly
consisted of samples of the southern stations in the south of the lagoon.
This part of the lagoon is influenced by seawater and the shoreline is
fringed by mangrove forest. Only two stations (A4, A5) constituted Cluster
II. Both stations were closed to the town of Adiaké and the village
of Ehoussou respectively. In Cluster III, except the sample of station
A4 in the high dry season (A44), all the other samples (A94,
A103, A102, A33, A34, A91,
A93, A23, A31 belonged exclusively to
the stations found in the central part of the lagoon. These stations were
characterised by high deep and muddy substrate. Finally, in Cluster 4,
stations were mostly located at the mouths of freshwaters and the lagoon
in its northern and eastern part. These stations were shallow and floating
macrophytes such as Eichhornia crassipes and Pistia stratiotes
were present. Overall, the species richness and diversity indices were
significantly different between clusters (Kruskal-Wallis test, p<0.001).
Species richness was significantly higher in cluster 1 and 2 compared
to cluster 3 and 4 (Rank Multiple Comparison Test, p<0.001) (Fig.
4). In the same way, the diversity indices were significantly lower
in the clusters 3 and 4 compared to the two ones (Rank Multiple Comparison
tests, p<0.001) (Fig. 4).
||Cluster analysis of benthic macroinvertebrates showing
similarities between the sampling stations in Aby lagoon. C1: Cluster
1, C2: Cluster 2, C3: Cluster 3, CIV: Cluster IV; A1 to A13: stations;
in index: 1 = HRS (High Rainy Season), 2: LDS (Low Dry Season), 3:
LRS (Low Rainy Season), 4: HDS (High Dry Season)
||Box-plots showing differences in species richness and
diversity indices between the clusters. The box is corresponding to
50% of the values, the point in the box to the median and vertical
bars to the minimum/maximum values. The various alphabetical letters
on the box-plots indicate a significant difference (p<0.05; multiple
comparison test) between clusters; there is no significant difference
between the box having an alphabetical letter in common (p>0.05)
The taxonomic structure of benthic macroinvertebrates of Aby lagoon is
characterised by the gastropoda, the bivalvous, the amphipoda, the isopoda
and the polychaetes. This taxonomic list is common to the traditional
ones in the lagoons environment as suggested by Mistri et al. (2002)
and Bazaïri et al. (2003). In addition to the traditional
groups, insects and oligochaetes were listed in Aby lagoon.
The taxonomic list obtained (62 taxa) is the first of the kind in the
Aby lagoon. Compared to the list of other lagoons (Mergaoui et al.,
2003; Munari et al., 2003; Marchini et al., 2004; Chaouti
and Bayed, 2005), the Aby lagoon appears rich in benthic macroinvertebrates.
The high number of species may be explained by the fact that the Aby lagoon
is connected to the sea and receives freshwaters from the Bia and Tanoé
rivers. According to Zabi and Leloeuf (1993), such a lagoon is favourable
to provide a habitat for the three groups of organisms in a lagoon. They
are sea, brackish and freshwater species.
The spatial distribution of these macroinvertebrates is variable according
to the sectors of the lagoon. The southern part of the lagoon (cluster
1) is characterised by a more diversified settlement of macroinvertebrates.
That could be explained by the presence of the mangroves forests used
as habitats and nurseries for many species (Sankaré et al.,
1999) and the influence of seawater from where, the lagoon receives salt
by the surge of the waves. Indeed, according to Menif and Ben Hassine
(2003), the influence of the sea results in high species richness following
the intrusion of the marine species such as fishes, polychaetes, moulds
and crabs in the lagoon.
The highest abundances were obtained at the stations A4 and A5 (cluster
2) during the high rainy season. That could be due to a disturbance dependent
in the vicinity of the agglomerations. As shown by Marzano et al.
(2003), a moderate disturbance can favour the growth of a complex community
to the profit of another.
The high depths and the substrate primarily characterised by the mud
in the central part of the lagoon (cluster III) could explain the low
values of the species richness and the diversity indices observed in this
sector. Indeed, Chantraine (1980) and Metongo (1985) showed that water
at the bottom of these places is not renewed and remains anoxic all the
year. In addition, the substrate primarily composed of mud is not favourable
for the transfer of certain groups to another place.
The stations of the northern sector and the east of the lagoon (Cluster
4) revealed low species richness and an abundance of stagnant water species
such as the larvae of Diptera (chironomidae). That could be explained
by the influence of freshwaters. According to Menif and Ben Hassine (2003),
these organisms support significant variations of salinity, dissolved
oxygen and temperature. In addition, they are remarkably resistant to
significant rates of pollution (Diomandé et al., 2000).
In conclusion, this study on the taxonomic diversity and structure of
benthic macroinvertebrates in Aby lagoon is the first of the kind. It
has contributed to identify 62 taxa of benthic macroinvertebrates dominated
by Molluscs and Crustaceans. Compared to the taxonomic list of other lagoons,
the Aby lagoon appears rich. However, the increasing of anthropic disturbances
on this lagoon should affect the benthic fauna at long term. Therefore,
some investigations should be performed in order to know the ecology of
the taxa for a conservation program.
This study was supported by the government of Ivory Coast and the Agence
Universitaire de la Francophonie (6313PS657). This is MARE contribution
n °XXX. We sincerely thank Seu-Anoï Netto M. and Konan Mexmin
K. (Laboratoire d`Environnement et de Biologie Aquatique, University of
Abobo-Adjamé, Ivory Coast) for their help during the sampling and
their comments on the manuscript. Thanks to Dr. Bony Kotchi Y. (University
of Abobo-Adjamé, Ivory Coast) and Dr. Chantal Salen P. (University
of Mediterranean, Marseille) for their help in Molluscs and Polychaetes
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