Int. J. Aquat. Biol. (2017) 5(6): 413-424DOI: ISSN: 2322-5270; P-ISSN: 2383-0956 Journal homepage: www.ij-aquaticbiology.com © 2017 Iranian Society of Ichthyology Original Article Composition and structure of phytoplankton community in Ouémé River basin, Republic of Benin Arsène Mathieu Houssou*1, 2, Elie Montchowui1, 2, Clément Agossou Bonou3 1Laboratoire de Recherche en Aquaculture et en Biologie et Ecologie Aquatiques, Ecole d’Aquaculture de Vallée, Université Nationale d’Agriculture, Bénin. 2Laboratoire d’Hydrobiologie et d’Aquaculture, Faculté des Sciences Agronomiques, Université d’Abomey-Calavi, Bénin. 3Laboratoire de Recherche en Biologie Appliquée, Ecole Polytechnique d’Abomey-Calavi, Université d’Abomey-Calavi, Bénin. Article history: Received 31 July 2017 Accepted 2 December 2017 Available online 2 5 December 2017 Keywords: Anthropogenic impact Diversity Potamoplankton Specific composition Upstream-downstream gradient Abstract: This study aimed to assess the composition and structure of floating phytoplankton assemblage in Ouémé basin. Phytoplankton samples were collected monthly from October 2014 to September 2015. Quantitative samples were taken with a horizontal Van-Dorn sampler and 20 μm mesh plankton net was used for additional qualitative sampling. Microscopic observation of phytoplankton allowed identification of 208 species including 70 Bacillariophyta species, 58 Chlorophyta species, 24 Charophyta species, 21 Euglenophyta species, 18 Cyanophytes species, 9 Phyrrophyta species, 5 Ochrophyta species and 3 Cryptophyta species. The Shannon diversity index varied from 2.4 bit.ind-1 and 3.1 bit.ind-1 showing a relatively good diversification of the community. The population appears largely dominated by 14 species which represent 83.8% of the total phytoplankton. Aulacoseira granulata and Euglena gracilis were the most predominant species with respectively 40.17% and 15.91% relative abundance. Regarding the horizontal pattern of phytoplankton abundance, downstream stations have the greatest abundances. So, the results suggest that downstream stations are richer in phytoplankton which structure differs from that in upper stations. Introduction Phytoplankton in aquatic environments is an important resource due to maintaining of the food chain and consequently the maintenance of the ecosystem functioning. Sterner and Elser (2002) and Twiss et al. (2010) reported that suspended phytoplankton is highly used in the food chain as a rich source of nitrogen and phosphorus relative to macroalgae, macrophytes and detritus. Phytoplankton studies in Africa, particularly in rivers, still very poor. African potamoplankton is therefore poorly known, whereas Silva et al. (2001, 2005) reported that it is specifically very rich. Potamophytoplankton is sensitive to physico- chemicals factors, climatic factors and river current, and its study appear necessary if needing it as ecological indicator. The unidirectional current imposes a major constraint on the maintenance of its population. Since water is continually transported in *Corresponding author: Arsène Mathieu Houssou DOI: https://doi.org/10.22034/ijab.v5i6.327 E-mail address: arsnehous@yahoo.fr downstream, continuous supply of phytoplankton inoculum is necessary (Reynolds, 2000). Therefore, perennial population is dominated by species which can react rapidly, integrating the short water retention time in the River (Kilham et al., 1986; Reynolds, 2000). Dominance by one or only a few numbers of species may therefore be observed (Quiblier et al., 2008). These species, depending on population structure and control factors, may be used in ecosystem bio-monitoring (Tavassi et al., 2008). To date, Ouémé River’s potamophytoplankton remains little known; while this river is one of the biggest one in West Africa. Its ecosystem comprised of much diversified habitats allowing a rich biological community. In different areas, the river receives various substances (domestic, agricultural, industrial, etc.), which doubtless leads to its enrichment in nitrogenous and phosphorus elements. Phytoplankton in this ecosystem would therefore be very rich and 414 Houssou et al./ Phytoplankton assessment in Ouémé basin diversified. The present study is intended to be a first comprehensive assessment of phytoplankton through- out the Ouémé basin. According to Smayda (1980), the specific composition of the phytoplankton communities, the diversity and dominance of one population in relation to another are all evolving characters and phenomena characterizing succession in the community. The study therefore proposed to evaluate these aspects for the suspended phytoplankton community in Ouémé river basin. Materials and Methods Study area and sampling sites: The study was carried out in Ouémé River basin, which is the longest and largest catchment area in Benin. It is long about 510 km and its catchment (Fig. 1) extends between 6°51' and 10°11' north latitude and 1°29' and 3°24' east longitude. It covers an area equivalent to half of Benin territory (i.e. more than 50000 km²). A total of fifteen stations (Fig. 1, Table 1) were sampled. These are representative of both the River course and its main tributaries (Okpara, Zou, Beffa and Donga rivers). Nine stations were retained on the river. The stations of Affon, Bétérou, Atchakpa- Béthel, Atchakpa-Rejet (wastewater discharge point of the “Sucrerie de Complant du Bénin (SUCOBE)” and Atchakpa-Pompage (water pumping point of SUCOBE) were selected to represent the upper course of the river. The three stations in Atchakpa are also representative of the direct effects of SUCOBE on the Ouémé River. The lower course was represented by stations such as Bétékoukou (Dassa), Zagnanado, Bonou and Agonlon-lowé. The last two stations represented the deltaic zone of the basin (in downstream). Six stations were chosen on the selected tributaries. The Kpassa hydraulic dam and the Kaboua station were representative of Okpara River. Toué and Atchérigbé were retained on Zou River while Vossa (Ouessè) and Donga were chosen respectively on Beffa River and Donga River. Phytoplankton Sampling and processing: Phytoplankton is sampled monthly at each of the fifteen stations between October 2014 and September 2015. The sampling protocol in great Rivers applicable to the European Water Framework Directive (Laplace-Treyture et al., 2010) has been used. Quantitative samples were taken from the first meter of depth using a Van Dorn horizontal sampler (2 liters). At each station, three samples at three different points (horizontal plane) of 2 L each were taken. They were then mixed and Lugol iodine (8 drops per 100 ml of sample) was added (Druart and Rimet, 2008). The mixture was packaged in polyethylene bottle and allowed to sediment for 24 hours (shadow). Then, it was concentrated by removing water to have 100 ml of sample. Additional fixation was done using 5% formalin (Laplace- Treyture et al., 2010). A complementary sample with a qualitative aim was also taken using 20 μm mesh plankton net. The samples were processed in a laboratory under light microscope. Phytoplankton species were identified using guides and specific descriptive works such as Prescott (1954), Compère (1974 and 1975), Vanlandingham (1982), Nogueira and Correia (2000), Tsukii (2005), Kinross (2007), Bellinger and Sigee Figure 1. Location of sampling sites. 415 Int. J. Aquat. Biol. (2017) 5(6): 413-424 (2010), Oyadomari (2011) and Simic et al. (2014). A four-grid counting cell (Burker turk) was used for cells enumeration for each identified species. The current name of each identified species was searched in AlgaeBase, the global algae information database (Guiry and Guiry, 2016). The systematic classification of AlgaeBase was thus used. Minimum of 400 cells of each identified species were counted. In case of very abundant species (more than 400 cells in 1 ml of sample), they were counted in three consecutive 1 ml sub-samples. Rare species were enumerated in the whole sample volume (Houssou et al., 2016). During counting, only cells with an integral structure were taken into account (Houssou et al., 2015). The phytoplankton density per liter of river water was then estimated using the equation below (Eq1). Eq1: 𝐷 = 1 6 ( 𝑁 𝑇𝑑 ∗ 100) Where D is density of the species per liter of river water. N is the number of cells counted and Td is the rate of sample volume corresponding to N. Data analysis: The specific composition of phytoplankton in the study area was evaluated and explored with the occurrence frequency (F). The frequency was calculated according to equation (Eq2). It allowed the assessment of species constancy in a given environment (Dajoz 2000). Depending on F value, three groups of species are distinguished: i-) constant species (F ≥ 50%); ii-) accessory species (25% ≤ F <50%) and iii-) incidental species (F <25%). The community structure was studied through the alpha and beta diversity indices. The Shannon Diversity Index (Eq3), the Evenness (Eq4), the Margalef Index (Eq5) and the Dominance Index Y (Eq6) were calculated. Also, spatial similarity of the zooplankton assemblage was studied with Jaccard index (Eq7). Eq2 (Dajoz, 2000): F=(µi x 100)/µT, Eq3 (Shannon and Wiener, 1949): 𝐻′ = − ∑( 𝑛𝑖 𝑁 ) log 2 ( 𝑛𝑖 𝑁 ) 𝑆 𝑖=1 Eq4 (Buzas and Gibson, 1969): 𝐸𝑣𝑒𝑛𝑛𝑒𝑠𝑠 = e𝐻′ S Eq5 (Margalef, 1958): 𝐷 = S − 1 ln 𝑁 Eq6: 𝑌 = ( 𝑛 𝑁 ) 𝑓𝑖 Eq7 (Jaccard, 1901): NC/ (NA+NB-NC) Where μ is the number of samples in which species i is present, μT is the total number of samples. S is specific richness, ni is the abundance of species i and N is the total abundance of all species. Fi is the frequency of species i in the samples. NA and NB are respectively the number of species present in the sites A and B to be compared. NC is the number of common species to both sites. Table 1. Geographic coordinates of sampling sites. Stations Code River Latitude Longitude Agonlin-Lowé Ag-L Ouémé River 6°39'35.2"N 2°28'38.6"E Bonou Bon Ouémé River 6°54'32.5"N 2°26'57.1"E Zagnanado Zag Ouémé River 7°12'50.9"N 2°28'26.4"E Dassa Das Ouémé River 7°37'17.0"N 2°27'59.1"E Atchakpa-Bethel Atc-Beth Ouémé River 8°00'22.9"N 2°22'39.3"E Atchakpa-Rejet Atc-R Ouémé River 8°03'38.1"N 2°22'33.8"E Atchakpa-Pompage Atc-P Ouémé River 8°04'27.0"N 2°22'12.6"E Bétérou Bét Ouémé River 9°11'55.2"N 2°16'04.6"E Affon Aff Ouémé River 9°57'28.6"N 1°51'45.4"E Kpassa Kpa Okpara River 9°16'59.7"N 2°44'13.4"E Kaboua Kab Okpara River 8°10'49.8"N 2°45'05.5"E Toué Tou Zou River 7°12'22.8"N 2°17'23.3"E Atchérigbé Atc Zou River 7°33'44.8"N 2°07'57.7"E Vossa Vos Beffa River 8°29'34.6"N 2°20'27.1"E Donga Don Donga River 9°42'37.7"N 1°56'41.2"E 416 Houssou et al./ Phytoplankton assessment in Ouémé basin Results Composition of phytoplankton: The identified phytoplankton community is composed of 208 species (Table 2). They belong to 8 phyla such as Bacillariophyta with 70 species in 39 genera, Chlorophyta with 58 species belonging to 32 genera, Charophyta with 24 species in 10 genera, Euglenophyta with 21 species belong 6 genera, Cyanophyta represented by 18 species in 15 genera, Pyrrophyta with 9 species in 7 genera, Ochrophyta with 5 species in 5 genera and Cryptophytes represented by 3 species belonging to 2 genera. The species occurrence frequency showed that 137 species among the 208 identified are constant in the area (F≥50%). These include species such as Microcystis aeruginosa, M. flosaquae, M. protocystis, Anabaenopsis circularis (Cyanobacteria), Aulacoseira ambigua, A. granulata, Gomphonema gracile, G. parvulum, Cocconeis pellucida, Amphora ovalis (Bacillariophyta), Euglena gracilis and Lepocinclis Oxyuris (Euglenophyta). A set of 15 species are accessory to the area (25≤F<50). These include Oscillatoria rubescens, G. vibrio, Nitzschia rostellata and Stephanodiscus sp.. Fifty-five (55) species were accidental in the basin (F<25%). Among these are marine species such as Gyrodinium sp, Prorocentrum denatatum, P. lima, and Prorocentrum sp. (toxic dinoflagellates). These species have been found only in the Ouémé delta. Alpha and beta diversity of phytoplankton community: The phytoplankton specific richness and Margalef index are presented in Figure 2. Higher richness was observed during low flow (from February to July). The temporal highest richness (190 species) was observed in March. The flood period (from October to December) was that of lower specific richness. The lowest richness (118 species) was observed in December. Margalef's index had same pattern as the specific richness. It varied between 6.7 observed in December and 10.8 in July. The phytoplankton community was less diversified during the low flow (Fig. 3). The smallest Shannon index (2.4 bit.cell-1) was observed in March and May. In contrast, the community was more diversified in December (3.1 bit.cell-1). The Evenness had same profile as Shannon diversity with values ranged from 0.06 to 0.12. The Jaccard index (Table 3) showed an important similarity between the phytoplankton communities in all stations (J varying between 0.62 and 1). However, the value of the index was higher between the downstream stations on one hand and between upstream stations (lower limit: Dassa) on the other hand. Value decreases when the communities present in downstream stations are opposite to those present in upstream stations. Dominant phytoplankton species: Only 14 species including 2 Bacillariophyta, 3 Euglenophyta, 6 Chlorophyta, 2 Charophyta and 1 Cyanophyta largely dominated the phytoplankton population (Table 4). They account for 83.8% of the total phytoplankton abundance in the basin. The two Bacillariophyta Figure 2. Temporal variation of the specific richness and Margalef index of phytoplankton community in Ouémé basin. Figure 3. Temporal Variation of Shannon diversity and Evenness index of phytoplankton community in Ouémé basin. 417 Int. J. Aquat. Biol. (2017) 5(6): 413-424 Table 2. Occurrence frequency of identified phytoplankton species in Ouémé River basin. Phyla Genera Species Occurrence Frequency (%) Cyanophyta Anabaena Anabaena oscillarioides 21.1 Anabaenopsis Anabaenopsis circularis 58.3 Aphanocapsa Aphanocapsa elegans 8.9 Asterocapsa Asterocapsa submersa 100 Chroococcus Chroococcus sp. 0.6 Cyanosarcina Cyanosarcina thalassia 7.8 Dolichospermum Dolichospermum spiroides 55 Glaucospira Glaucospira laxissima 75.6 Lyngbya Lyngbya sp. 1.1 Merismopedia Merismopedia glauca 18.3 Microcystis Microcystis aeruginosa 100 Microcystis flosaquae 100 Microcystis protocystis 100 Oscillatoria Oscillatoria rubescens 25 Oscillatoria sp. 92.2 Stigonema Stigonema sp. 76.7 Synechocystis Synechocystis aquatilis 98.9 Tychonema Tychonema bornetii 10 Bacillariophyta Achnanthes Achnanthes felinophila 86.7 Achnanthidium Achnanthidium minutissimum 66.1 Amphora Amphora ovalis 83.3 Amphora sp. 32.8 Amphipleura Amphipleura sp. 2.2 Aulacoseira Aulacoseira ambigua 100 Aulacoseira granulata 100 Caloneis Caloneis undulata 100 Catenula Catenula sp. 77.8 Cocconeis Cocconeis pellucida 96.7 Cocconeis sp. 33.3 Coscinodiscus Coscinodiscus perforatus 1.1 Coscinodiscus radiatus 2.2 Coscinodiscus sp. 6.7 Cyclotella Cyclotella meneghiniana 97.2 Cymbella Cymbella lanceolata 94.4 Cymbella prostrata 65 Cymbella sp. 46.7 Cymatopleura Cymatopleura elliptica 70.6 Diatoma Diatoma sp. 100 Entomoneis Entomoneis alata 100 Eunotia Eunotia bilunaris 100 Fragilaria Fragilaria acus 23.3 Fragilaria capucina 16.7 Fragilaria sp. 100 Gomphonema Gomphonema gracile 100 Gomphonema parvulum 100 Gomphonema sp. 79.4 Gomphonema vibrio 26.7 Grammatophora Grammatophora sp. 25 Gyrosigma Gyrosigma acuminatum 100 Gyrosigma attenuatum 100 Gyrosigma sp. 13.3 Gyrosigma strigilis 8.9 Hyalodiscus Hyalodiscus radiatus 15 Hyalodiscus sp. 7.8 418 Houssou et al./ Phytoplankton assessment in Ouémé basin Phyla Genera Species Occurrence Frequency (%) Hyalosynedra Hyalosynedra laevigata 1.1 Melosira Melosira moniliformis 2.2 Navicula Navicula gregaria 100 Navicula peregrina 6.7 Neidium Neidium sp. 6.7 Nitzschia Nitzschia reversa 100 Nitzschia rostellata 38.9 Nitzschia sp. 68.3 Nitzschia paradoxa 100 Parlibellus Parlibellus protractoides 2.2 Placoneis Placoneis constans 11.1 Placoneis gastrum 100 Pleurosigma Pleurosigma obscurum 100 Pinnularia Pinnularia gibba 10.6 Pinnularia cardinalis var. africana 100 Pinnularia sp. 100 Pinnularia tabellaria 13.3 Pseudo-nitzschia Pseudo-nitzschia seriata 2.8 Rhizosolenia Rhizosolenia setigera 2.2 Sellaphora Sellaphora pupula 19.4 Stephanodiscus Stephanodiscus alpinus 100 Stephanodiscus hantzschii 100 Stephanodiscus sp. 39.4 Surirella Surirella alata 100 Surirella elegans 93.9 Surirella capronii 100 Surirella linearis 92.8 Surirella robusta 100 Synedra Synedra superba 11.1 Tabellaria Tabellaria flocculosa 8.3 Tabellaria sp. 10 Thalassiosira Thalassiosira sp. 57.8 Ulnaria Ulnaria ulna 95.6 Urosolenia Urosolenia eriensis 8.3 Euglénophyta Euglenaria Euglenaria anabaena 8.3 Euglena Euglena gracilis 100 Euglena sp. 6.7 Lepocinclis Lepocinclis acus var. longissima 100 Lepocinclis oxyuris 96.7 Lepocinclis sp. 27.2 Phacus Phacus helikoides 100 Phacus longicauda 88.9 Phacus longicauda var. torta 100 Phacus orbicularis 93.3 Phacus undulatus 75 Strombomonas Strombomonas acuminata 100 Strombomonas confortii 100 Strombomonas ferrazii 100 Strombomonas fluviatilis 100 Strombomonas rotunda 6.7 Strombomonas scabra 9.4 Strombomonas verrucosa 100 Strombomonas verrucosa var. borystheniensis 86.7 Trachelomonas Trachelomonas acanthophora var. speciosa 13.3 Trachelomonas sp. 25 Table 2. Continued 419 Int. J. Aquat. Biol. (2017) 5(6): 413-424 Phyla Genera Species Occurrence Frequency (%) Charophyta Chlorokybus Chlorokybus sp. 7.8 Closterium Closterium acerosum 23.3 Closterium acerosum var. tumidum 100 Closterium braunii 72.2 Closterium gracile 100 Closterium parvulum 100 Closterium setaceum 54.4 Closterium tumidulum 23.3 Cosmarium Cosmarium botrytis 95 Cosmarium contractum 92.8 Cosmarium quinarium 2.2 Cosmarium reniforme 85 Cosmarium sp. 87.8 Euastrum Euastrum ansatum 65 Gonatozygon Gonatozygon brebissonii 2.2 Klebsormidium Klebsormidium sp. 68.3 Micrasterias Micrasterias fimbriata 4.4 Pleurotaenium Pleurotaenium ehrenbergii 2.2 Staurastrum Staurastrum anatinum 100 Staurastrum leptocladum f. africanum 100 Staurastrum longipes 100 Staurastrum natator 100 Staurastrum paradoxum var. parvum 100 Staurodesmus Staurodesmus glaber 100 Chlorophyta Actinastrum Actinastrum hantzschii var. fluviatile 100 Actinastrum hantzschii var. subtile 100 Acutodesmus Acutodesmus acuminatus 100 Ankistrodesmus Ankistrodesmus densus 100 Ankistrodesmus falcatus 100 Ankistrodesmus fusiformis 100 Chlorogonium Chlorogonium sp 10 Chodatella Chodatella quadriseta 4.4 Characium Characium oviforme 26.7 Cladophora Cladophora sp. 100 Coelastrum Coelastrum astroideum 100 Crucigeniella Crucigeniella apiculata 86.7 Crucigenia Crucigenia sp. 31.7 Desmodesmus Desmodesmus abundans 100 Desmodesmus armatus var. bicaudatus 100 Desmodesmus communis 100 Desmodesmus intermedius 100 Desmodesmus intermedius var. balatonicus 100 Desmodesmus magnus 100 Desmodesmus maximus 100 Desmodesmus opoliensis 16.7 Desmodesmus opoliensis var. mononensis 83.3 Dicloster Dicloster acuatus 83.3 Eudorina Eudorina carteri 100 Eudorina sp. 78.3 Eremosphaera Eremosphaera sp. 1.1 Eremosphaera viridis 2.2 Lacunastrum Lacunastrum gracillimum 23.3 Lagerheimia Lagerheimia sp. 4.4 Micractinium Micractinium bornhemiense 100 Monactinus Monactinus simplex var. echinulatum 92.8 Monactinus simplex var. sturmii 100 Table 2. Continued 420 Houssou et al./ Phytoplankton assessment in Ouémé basin species (A. granulate and A. ambigua) occupy respectively 40.17% and 6.28% of the total population (i.e. 46.45% for both species). Euglena gracilis (Euglenophyta) was the second most dominant species (15.91%). The dominance index Y evolved according to species relative abundance. It varied between 0.40 for the most abundant species (A. granulata) and 0.01 for the least abundant species (Acutodesmus acuminatus). The 14 species have a dominance of 0.83 out of a total of 1 for the identified 208 species. Discussion The phytoplankton community recorded in Ouémé basin is composed of 208 species. This specific Phyla Genera Species Occurrence Frequency (%) Chlorophyta Neospongiococcum Neospongiococcum sp. 56.7 Pachycladella Pachycladella zatoriensis 70 Pectinodesmus Pectinodesmus javanensis 100 Pediastrum Pediastrum angulosum 100 Pediastrum boryanum 100 Pediastrum duplex 100 Pediastrum kawraiskyi 76.7 Pediastrum simplex var. biwaense 100 Pediastrum simplex var. duodenarium 100 Quadrigula Quadrigula lacustris 53.3 Scenedesmus Scenedesmus quadricaudata var. biornatus 85 Scenedesmus obtusus 100 Scenedesmus tropicus 100 Selenastrum gracile 100 Stauridium Stauridium privum 86.1 Stigeoclonium Stigeoclonium aestivale 100 Tetradesmus Tetradesmus bernardii 93.3 Tetradesmus obliquus 66.1 Tetrastrum Tetrastrum heteracanthum 16.7 Tetraëdron Tetraëdron incus 62.2 Tetraëdron gracile 91.7 Tetraëdron triangulare 53.9 Tetraëdron trigonum 54.4 Tetraspora Tetraspora sp. 100 Treubaria Treubaria quadrispina 66.7 Volvox Volvox aureus 46.7 Pyrrophyta Ceratium Ceratium carolinianum 70 Gyrodinium Gyrodinium sp. 5 Peridiniopsis Peridiniopsis quadridens 83.3 Peridinium Peridinium bipes 65 Prorocentrum Prorocentrum denatatum 7.8 Prorocentrum lima 2.8 Prorocentrum sp. 1.1 Pyrocystis Pyrocystis sp. 75 Scrippsiella Scrippsiella trochoidea 43.3 Ochrophyta Centritractus Centritractus africanus 40 Dinobryon Dinobryon sertularia 92.8 Gonyostomum Gonyostomum sp. 66.7 Tribonema Tribonema sp. 75 Vaucheria Vaucheria sp. 100 Cryptophyta Campylomonas Campylomonas sp. 94.4 Cryptomonas Cryptomonas ovata 88.9 Cryptomonas sp. 86.7 Table 2. Continued 421 Int. J. Aquat. Biol. (2017) 5(6): 413-424 richness is more or less stable, showing the ecological importance of this ecosystem. The numerous agro- ecological, industrial and residential areas crossed by the Ouémé river and its tributaries justify this specific richness. Good mineralization in water due to exogenous inputs, allows many species survival and multiplication in the environment. The observed specific richness is above that reported (89 species) on the Kwa River in Nigeria (Victor et al., 2013) and 192 species on the coastal river in Ivory Coast (Niamien-Ebrottié et al., 2013). It is also superior to the specific phytoplankton richness (149 species) observed in the subtropical river of the lower Iguaçu in Brazil (Perbiche-Neves et al., 2011). It is below the 265 species of phytoplankton identified in the Australian "Daly" tropical river (Townsend et al., 2012). Geographical differences as well as the various levels of anthropization perfectly explain the deviations from these rivers. Albert (2010) reported that a species distribution reflected in its geographical space (longitude, latitude), its ecological niche defined in environmental space (climate, soil, resource). So even in the absence of a significant difference in climate, soil and resources in the environment are important factors to the biodiversity composition. Compared to African lakes, the phytoplankton richness observed in the Ouémé basin is above the 39 species identified in Hlan Lake (Houssou et al., 2016) Ag-L Bon Zag Tou Atc Das Atc- Béth Atc-R Atc-P Kab Vos Kpa Bét Don Aff Ag-L Bon 0.967 Zag 0.846 0.867 Tou 0.764 0.783 0.893 Atc 0.764 0.783 0.893 1 Das 0.726 0.744 0.858 0.845 0.845 Atc-Béth 0.692 0.709 0.818 0.825 0.825 0.954 Atc-R 0.692 0.709 0.818 0.825 0.825 0.954 1 Atc-P 0.692 0.709 0.808 0.814 0.814 0.941 0.973 0.986 Kab 0.649 0.665 0.851 0.74 0.74 0.857 0.898 0.898 0.911 Vos 0.668 0.685 0.79 0.784 0.784 0.921 0.965 0.965 0.965 0.916 Kpa 0.62 0.635 0.733 0.756 0.756 0.879 0.909 0.909 0.909 0.985 0.914 Bét 0.683 0.7 0.807 0.824 0.824 0.94 0.986 0.986 0.986 0.91 0.979 0.908 Don 0.63 0.645 0.744 0.779 0.779 0.88 0.923 0.923 0.923 0.834 0.915 0.857 0.936 Aff 0.683 0.7 0.807 0.813 0.813 0.94 0.986 0.986 0.972 0.91 0.979 0.908 1 0.936 Sampling sites codes are same as in Table 1 Table 3. Jaccard similarity between the phytoplankton communities of Ouémé River basin Species Mean abundance (x104.cell.L-1) Relative abundance (%) Dominance Y Aulacoseira granulata Bacil 52.82 40.17 0.40 Euglena gracilis Eug 20.92 15.91 0.16 Aulacoseira ambigua Bacil 8.25 6.28 0.06 Lepocinclis oxyuris Eug 5.21 3.96 0.04 Pediastrum duplex Chlo 3.16 2.40 0.02 Pediastrum angulosum Chlo 2.94 2.24 0.02 Desmodesmus intermedius var. balatonicus Chlo 2.85 2.17 0.02 Staurastrum leptocladum cf. africanum Charo 2.76 2.10 0.02 Microcystis aeruginosa Cyano 2.56 1.94 0.02 Desmodesmus intermedius Chlo 1.99 1.51 0.02 Ankistrodesmus densus Chlo 1.95 1.48 0.01 Cosmarium botrytis Charo 1.84 1.40 0.01 Phacus longicauda Eug 1.53 1.16 0.01 Acutodesmus acuminatus Chlo 1.39 1.06 0.01 Total 83.77 0.83 Table 4. List of dominant phytoplankton species in Ouémé basin. 422 Houssou et al./ Phytoplankton assessment in Ouémé basin and 51 species in Azili Lake in Benin (Houssou et al., 2015); these two lakes being strongly influenced by the overflows of Ouémé River. The richness of 111 species of Lake Guiers in Senegal (Ngansoumana, 2006) is also smaller than that of Ouémé. The phytoplankton community in Ouémé basin was during low flow less diversified than during the flood period. The low flow period was that during which phytoplankton is greatly multiplied. This followed the reduction or even the cancellation of the river flow. Weak nutrient diluted associated with high sun exposure have been major factors which increased phytoplankton development. All species have experienced significant population growth which has raised the specific richness. Therefore, rarest species are sampled. Margalef's specific index confirms the profile observed in the specific richness of the basin. The Shannon index and evenness indicated a relatively good diversification of the phytoplankton community (Chen et al., 1994; Sun et al., 2004). However, few species number is dominant during low flow season. This confirms the high mineralization during this season. Regarding the dominance, diatom A. granulata was more predominant (>40% of the population). This fact confirms observations in which diatoms are dominant in terms of abundance in tropical rivers with a predominance of A. granulata and other species of the same genera (Hötzel and Croome, 1996; Decy et al., 2017). According to Reynolds (2000) and Decy et al. (2017), these diatoms are typically of the R strategy. They are able to withstand the nutritional variability associated with variations in water flows and able in achieving net growth within short time imposed by downstream transport. This justifies the dominance of the species even in upstream stations where the River current is more or less continuous throughout the year. Kilham et al. (1986) qualified species of the genus Aulacoseira as species adapted to low light conditions. Chlorophycea species such as those of the genus Ankistrodesmus, Desmodesmus and Pediastrum were also included in the dominant species. This group of species could become predominant in the case of good light penetration in the River (Zalocar de Domitrovic et al., 2014). Euglena gracilis and Lepocinclis oxyuris were also dominant. These two species are known for their selectivity of eutrophic environment, the anthropic impact in the basin then explains their abundance. As regards the similarity between phytoplankton communities in the different sampling stations, a horizontal stratification was observed. The community structure in the three stations in the delta area is similar and clearly differs from all other stations. This confirms the upstream-downstream gradient of mineralization in the basin. In addition to direct exogenous inputs, these stations also receive all substance or particle that is transported by the current making nutrients available for habitats variability in the area. It is also observed that community present in the stations from Dassa to Affon and then on Beffa and Okpara rivers are form equivalent. Conclusion The floating phytoplankton assemblage in the Ouémé basin is composed of 208 species grouped into 8 phyla: Bacillariophyta, Chlorophyta, Charophyta, Euglenophyta, Cyanophyta, Pyrrophyta, Ochrophyta and Cryptophyta. The population is relatively well diversified with lowest diversity during low flow. Fourteen (14) species are dominant with more than 83% of the total phytoplankton population. A. granulata is the most predominant species. Other species such as E. gracilis, A. ambigua and L. oxyuris are also strongly represented. It was also observed an ecological difference between Ouémé delta and all other parts in the basin. Acknowledgement We are grateful to the West Africa Agricultural Productivity Program (WAAPP) which funded this research. We thank all those who have contributed in one way or another to this study. We also thank the reviewers for their comments and contributions. References Albert C. (2010). Variabilité fonctionnelle intraspecifique: quantification in situ et implications dans une vallée 423 Int. J. Aquat. Biol. (2017) 5(6): 413-424 alpine. Ecologie. Environnement. Thèse de doctorat. Université Joseph-Fourier - Grenoble I. Bellinger E.G., Sigee D.C. (2010). Freshwater algae. Identification and use as bioindicators. Wiley- Blackwell. John Wiley and Sons. Ltd. 285 p. Buzas M.A., Gibson T.G. (1969). Species diversity: benthonic foraminifera in western North Atlantic. Science, 163: 72-75. Chen Q.C., Huang L.M., Yin J.Q. (1994). Study on the diversity of zooplanktons in Nansha Islands. Nansha Scientific Exploration Team of the Academy of Sciences of China. Study I on the Diversity of Oceanic Creatures in Nansha Islands and their Ajacent Seas. Ocean Press. Beijing. Compère P. (1974). Algues de la région du lac Tchad. II: Cyanophycées. Cahier O.R.S.T.O.M. Série Hydrobiologie, 8(3-4): 165-198. Compère P. (1975). Algues de la région du lac Tchad. IV: Diatomophycées. Cahier O.R.S.T.O. Série Hgdrobiologie, 9(4): 205-290. Dajoz R. (2000). Précis d’écologie. Edition Dunod. Paris. 615 p. Descy J.P., Darchambeau F., Lambert T., Stoyneva M. P., Bouillon S., Borges A.V. (2017). Phytoplankton dynamics in the Congo River. Freshwater Biology, 62: 87-101. Druart J.C., Rimet F. (2008). Protocoles d’analyse du phytoplancton de l’INRA: prélèvement, dénombrement et biovolumes. INRA-Thonon. Rapport SHL 283. 96 p. Guiry M.D., Guiry G.M. (2016). AlgaeBase. World-wide electronic publication. National University of Ireland. Galway. http://www.algaebase.org; searched on 08 August 2016. Hötzel G., Croome R. (1996). Population dynamics of Aulacoseira granullata (Ehn) Simonsen (Bacillariophyceae. Centrales). The dominant alga in the Murray River. Australia. Archiv fur. Hydrobiologie, 136(2): 191-215. Houssou A.M., Agadjihouédé H., Bonou C.A., Montchowui E. (2016). Composition and seasonal variation of phytoplankton community in Lake Hlan. Republic of Bénin. International Journal of Aquatic Biology, 4(6): 378-386. Houssou A.M., Agadjihouédé H., Montchowui E., Bonou C.A., Lalèyè P. (2015). Structure and seasonal dynamics of phytoplankton and zooplankton in Lake Azili, small Lake of the pond of River Ouémé. Benin. International Journal of Aquatic Biology, 3(3): 161-171. Jaccard P. (1901). Étude comparative de la distribution florale dans une portion des Alpes et des Jura. Bulletin de la Société Vaudoise des Sciences Naturelles, 37: 547-579. Kilham P., Kilham S.S., Hecky R.E. (1986). Hypothesized ressource relationships among African planktonic diatoms. Limnology and Oceanogreaphy, 31: 1169- 1181. Kinross J. (2007). Summary investigation on the phytoplankton of the Rivers Almond and Creran. Available at http://www.algalweb.net/algweb1.htm (accessed August 16th. 2015). Laplace-Treyture C., Chauvin C., Menay M., Dutartre A., Moreau L., Lorraine D. (2010). Protocole standardisé d’échantillonnage et de conservation du phytoplancton en grands cours d’eau applicable aux réseaux de mesure DCE. 19 p Margalef R. (1958). Information theory in ecology of genenal systems. American Journal of Systems and Software, 3: 36-71 Ngansoumana BA. (2006). La communauté phytoplanctonique du lac de Guiers (Sénégal) : types d’associations fonctionnelles et approches expérimentales des facteurs de régulation. Thèse Doc. 3e cycle. Univ. Cheikh Anta Diop Dakar (Sénégal). 155 p. Niamien-Ebrottié J.E., Konan K.F., Edia O.E., Ouattara A., Gourène G. (2013). Composition and spatial and seasonal variation of algal assemblage of coastal rivers of South-eastern Côte d'Ivoire. Journal of Applied Bioscience, 66: 5147-5161. Nogueira N.M.C., Correia F.M.M. (2000). Cyanophyceae/Cyanobacteria in red mangrove forest at mosquitos and coqueiros estuaries. são luís. State of maranhão. Brazil. Brazilian Journal of Biology, 61: 1519-6984. Oyadomari J. (2011). Diatom Gallery. Keweenaw Waterway at MTU Surface plankton two. Available at http://www.keweenawalgae.mtu.edu/index.htm (acce- ssed June 28 th 2015) Perbiche-Neves G., Ferrareze M.F., Serafim-Júnior M., Shirata M.T., Patrícia E.D.L. (2011). Influence of atypical pluviosity on phytoplankton assemblages in a stretch of a large sub-tropical river (Brazil). Biologia, 66(1): 33-41. Prescott G.W. (1954). The freshwater algae. Pictured key. Nature series. Ed. H.E. Jaques. 224 p. Quiblier C., Leboulanger C., Sane´S., Dufour P. (2008). 424 Houssou et al./ Phytoplankton assessment in Ouémé basin Phytoplankton growth control and risk of cyanobacterial blooms in the lower Senegal River delta region. Water Research, 1023-1034 Reynolds C.S. (2000). Hydroecology of river plankton: the role of variability in channel flow. Hydrological Processes, 14: 3119-3132. Shannon C.E., Wiener W. (1949). The Mathematical Theory of Communication. Urbana: University of Illinois Press. 125 p. Silva C.A., Train S., Rodrigues L.C.R. (2001). Estrutura e dinãmica da comunidade fitoplanctõnica a jusante e montante do reservatório de Corumbá. Caldas Novas. Estado de Goiás, Brasil. Acta Scientiarum. Biological Sciences, 23(2): 283-290. Silva C.A., Train S., Rodrigues L.C.R. (2005). Phytoplankton assemblages in a Brazilian subtropical cascading reservoir system. Hydrobiologia, 537: 99- 109. Simic S.B., Komárek J., Dordevic N.B. (2014). The confirmation of the genus Glaucospira (Cyanobacteria) and the occurrence of Glaucospira laxissima (G.S. West) comb. nova in Serbia . Cryptogamie Algologie, 35(3): 259-267. Smayda T.J. (1980). Phytoplankton species succession. In: I. Moms. (Ed.). The physiological ecology of phytoplankton. Blackwell. Oxford. pp: 493-570. Sterner R.W., Elser J.J. (2002). Ecological stoichiometry: the biology of the elements from molecules to the biosphere. Princeton University Press: Princeton. NJ. 464 p. Sun J., Lu D.Y., Gao H.W. 2004. Phytoplankton Community of the Bohai Sea in winter 2001. Periodical of Ocean University of China, 34(3): 413-422. Tavassi M., Barinova S., Glassman H. (2008). Algal communities in the polluted lower Jordan River. Israel. Israel Journal of Plant Science, 56: 111-119. Townsend A.D., Przybylska M., Miloshis M. (2012). Phytoplankton composition and constraints to biomass in the middle reaches of an Australian tropical river during base flow. Marine and Freshwater Research, 63: 48-59. Tsukii Y. (2005). Gomphonema vibrio Ehrenberg. Cell body. Available at http://www.protist.i.hosei.ac.jp/ protist menuE.html (accessed July 21th 2015). Twiss M.R., Ulrich C., Kring S.A., Harold J., Williams M.R. (2010). Plankton dynamics along a 180 km reach of the Saint Lawrence River from its headwaters in Lake Ontario. Hydrobiologia, 647: 7-20. VanLandingham S.L. (1982). Guide to the identification. Environmental requirements and pollution tolerance of freshwater blue-green algae (Cyanophyta). Environmental Monitoring and Support Laboratory. Office of Research and Development. US Environmental Protection Agency. 349 p. Victor O.E., Paul B.E., Andem B.A., Kalu A.O. (2013). Ecology and Diversity of Phytoplankton in the Great Kwa River. Cross River State. Nigeria. International Journal of Fisheries and Aquatic Studies, 1(2): 1-7. Zalocar de Domitrovic Y., Devercelli M., Forastier M.E. (2014). Phytoplankton of the Chaco Pampean. Advances in Limnology, 65: 81-98. http://www.protist.i/