Available online at http://ajol.info/index.php/ijbcs Int. J. Biol. Chem. Sci. 6(3):1233-1252, June 2012 ISSN 1991-8631 Original Paper http://indexmedicus.afro.who.int Macroinvertebrates communities of a coastal lagoon in southern Benin, West Africa Delphine ADANDEDJAN 1∗, Philippe LALEYE 1 et Germain GOURENE 2 1Laboratoire d’Hydrobiologie et d’Aquaculture, FSA- UAC. Faculté des Sciences Agronomiques, Université d’Abomey-Calavi, 01 BP 526, Cotonou, Benin. 2Laboratoire d’Environnement et de Biologie Appliquée U.F.R.-S.G.E Université d’Abobo- Adjamè, 02 BP 801 Abidjan 02, Côte d’Ivoire. ∗ Corresponding author, E-mail: [email protected] ABSTRACT The macroinvertebrate, assemblage investigated were collected in eight seasonal samplings from July 2007 through June 2009. A total of 4,179 individual macroinvertebrates were gathered, comprising 182 taxa belonging to 25 orders and 114 families. The most predominant groups were Molluscs and Crustacean which make up 71.11% of the taxonomic richness observed. Annelids and Insects were the second most predominant. This restricted biodiversity is based on a limited number of species such as Tympanotonus fuscatus radula , Pachymelania aurita , Neritina glabrata , Corbula sp., Clibanarius spp., Grandidierella africana , Nereis sp. and Chironomus sp.. Site typology based on environmental parameters reveals five assemblages in which no significant difference exists. The community is controlled by a salinity gradient but is also probably affected by hydrologic factors and human economic activities. © 2012 International Formulae Group. All rights reserved. Keywords: Macroinvertebrates, communities, distribution, abundance, diversity, lagoon. INTRODUCTION the ecological state of aquatic ecosystems Diversity in macroinvertebrates (Park et al., 2003; Edia et al., 2007; Fishar and populations reveals remarkable temporal and Williams, 2008). Using macroinvertebrates as spatial variations in coastal wetlands (Akin et indicator, water quality has been found to fit al., 2003; Alfaro, 2006). Monitoring changes on a scale from “clean” to “severely polluted” in water quality in aquatic ecosystems can be (Bazaîri et al., 2003; Alfaro, 2006). However, performed through an investigation of the further investigation is needed to understand macroinvertebrates living therein. Benthic some of the features which characteristize the macroinvertebrates characteristically respond ecology of the macroinvertebrates taxa to anthropic disturbances in an integrated and encountered in those environments. continuous manner thanks to their taxonomic In Africa, particularly in the Republic diversity, their sedentary behaviour, and of Benin, there has been a gradual build-up of relatively long life cycles (Chaouti and Bayed, literature on macroinvertebrates and their use 2005; Edia et al., 2007). As a result, they have as bioindicators. Few inventories of benthic been widely used to assess water quality and organisms exist in Benin and those inventories © 2012 International Formulae Group. All rights reserved. DOI : http://dx.doi.org/10.4314/ijbcs.v6i3.27 D. ADANDEDJAN et al. / Int. J. Biol. Chem. Sci. 6(3):1233-1252, 2012 do not go in-depth in their characterization of the sea via the Boca del Rio delta (Figure 1). macroinvertebrates neither do they extend Ecologically , the lagoon is a sub-unit of the much on their use as water quality indicators. RAMSAR site 1017. Its water has two However, an important step in achieving the origins: a marine origin through tidal actions sustainable management of aquatic and a freshwater origin via the Mono-Sazué ecosystems is a solid understanding of and Couffo-Ahémé-Aho Rivers. Since the community abundance patterns. This is completion of the Nangbéto hydroelectricity because community composition depends on dam on the Mono River, the hydrologic habitat stability which makes possible the regime of the lagoon has changed due to water growth of resident populations. Next, the releases from the dam which considerably structure of aquatic communities is also reduce its salinity. function of various environmental factors The climate is equatorial with two rainy which influence the habitats in both space and seasons and two dry seasons. During the rainy time. As an example, the distribution of season, the lagoon is covered by floating macrobenthic communities is sometimes Eichhornia crassipes, Pistia stratiotes, aquatic correlated with the type of sediment, which in and palustrine weeds (Echinochloa spp., turn is related to a wider set of environmental Panicum sp., etc.) which produce a patchy parameters such as current velocity and distribution of decaying litter. Human organic content of sediments (Snelgrove and activities in the lagoon include fishing, Butman 1994; Gray et al., 2002). These ecotourism, salt production, and oyster complex relationships between community farming. Much of the lagoon is surrounded by variations and environmental disturbances can mangroves, Elaeis guineensis and coconut be studied through community composition trees. Wastes of all kinds (solid household (structure or functional perspective) (Park et wastes, wastewater, etc.) are commonly al., 2003) to establish the patterns of species discarded into the lagoon. biodiversity. The present study aims at analysing the Sampling community patterns of macroinvertebrates Macroinvertebrate and sediment collected in a coastal lagoon in southern Benin samples have been collected seasonally, in 8 and elucidating the association of community phases from July 2007 to June 2009 at 18 sites clusters with the taxa-abundance patterns and (Figure 1). Corresponding environmental its diversity. parameters were recorded. Sites were located by GPS on each sampling occasion. MATERIALS AND METHODS Study area Abiotic variables The study was undertaken in the Salinity, water depth, water southern part of Benin in the Coastal Lagoon transparency and dissolved oxygen were situated between 2°16’ W and 6°20’ N. The measured in situ at each sampling site. The lagoon’s surface area varies from 40 to 52 km² salinity, conductivity and Total Dissolved in low and high waters, respectively. It is Solids (TDS) were determined using a about 60 km long and 0,2 – 1 km wide. It is multiprocessor conductimeter; the dissolved located along the Atlantic Ocean between oxygen concentration (DO) was measured Grand-Popo and Togbin and is bordered to the with an oxymeter probe, while the water north by the towns of Grand-Popo and transparency and water depth were measured Ouidah, and to the east by a swampy area of using a Secchi disk. The bottom sediment Paspalum vaginatum . To the west, it joins the texture was determined in the laboratory by Gbagan River at Anecho City before reaching passing each sample through a series of sieves 1234 D. ADANDEDJAN et al. / Int. J. Biol. Chem. Sci. 6(3):1233-1252, 2012 380° 390° 400° 410° 420° 430° 712° N Benin Africa 707° A h o 13 C h M a o n 2 1 n Ouidah n 12 702° o e 3 l R R i e v p e . r 11 4 8 O 5 f 6 15 14 7 697° 16 T 9 10 O 18 G Boca del Rio Atlantic sea O Grand-Popo 692° City Sampling sites 0 10 km Figure 1: Locations of 18 sample sites in the Coastal Lagoon. Numbers represent sample site. 1 = Togbin 1, 2 = Togbin 2, 3 = Avlékété 1, 4 = Avlékété 2, 5 = Djègbadji 1, 6 = Djégbadji 2, 7 = Djégbadji 3, 8 = Azizakouè 1, 9 = Azizakouè 2, 10 = Djondji, 11 = Aho 1, 12 = Aho 2, 13 = Aho 3, 14 = Hokouè, 15 = Docloboué, 16 = Alongo, 17 = Avlo, 18 = Agonnékanmè. of different mesh sizes; then the percentage microscope and finally preserved in 70% contribution of sand and fine particles (silt and alcohol. clay) in each sediment sample was determined. Data analyses Macroinvertebrates The mean values of abiotic parameters At each sampling site, 8 samples were were calculated. The spatial variations of the collected with an Ekman grab sampler of 225 benthic assemblages were determined using cm² at a depth of approximately 10 cm. The the artificial non-supervised neuron networks, grabs were taken from random spots at each the "Self Organizing Maps (SOM)" or site. Each sample was sieved through two Kohonen maps (Kohonen, 1982). The SOM sieves of 500 µm and 1 mm mesh size in the architecture consisted of two layers of lagoon water and the collected organisms neurons: the input layer made of 70 neurons were cleaned under lagoon water and connected to each vector (line of the matrix) preserved separately in labelled bottles and the output layer made of 36 neurons. We containing 4% formaldehyde solution. Those have chosen 36 neurons because the meshes sizes were used because configuration obtained presented minimum macroinvertebrates are almost always greater values for both quantification and topographic than 1 mm while Gastropods are generally of errors, which are used to appreciate the a size superior to 1 mm while Annelids, insect classification quality (Park et al., 2003). The SOM algorithm calculates the connection larvae and nymphs can be less than 0.5 mm. intensities between input and output layers by So, using a mesh less than the size of the using an unsupervised competitive learning smaller macroinvertebrate would ensure that procedure (Kohonen, 1995). This classifies all the invertebrates are retrieved. Once in the samples in each node according to their laboratory, the invertebrate specimens from similarity in the environment. The analysis each site were sorted and identified. The was carried out using the SOM toolbox for specimens were then counted under Matlab. The relevant groups or sample 1235 D. ADANDEDJAN et al. / Int. J. Biol. Chem. Sci. 6(3):1233-1252, 2012 clusters which characterize the sampling sites sandy substrate and high DO values. Group III assemblages were determined by performing a (od2) comprised 15 samples which were hierarchical classification analysis (Ward’s essentially from the central part of the lagoon. linkage and the Euclidean distance method). It is characterized by high salinity values, The number of clusters was defined by water transparency with a sandy-muddy applying the Bouldin-Davies Index (DBI), in substrate.