International Research Journal of Biological Sciences ________________ ______ ____ _________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 1 Composition and Distribution of Zooplankton Relationship to Environmental Factor in a Tropical River: (Bagoe, Côte d’Ivoire) N’da S.A.*, Etilé R.N., N’zi . K.G . , Berté S. and N’douba V. Hydrobiology Laboratory, UFR Biosciences, Félix Houphouët - Boigny University, Abidjan - Cocody, CÔTE D’IVOIRE, 22 BP 582 Abidjan 22, CÔTE D’IVOIRE , West Africa Available online at: www.isca.in , www.isca.me Received 10 th May 201 5 , revised 28 th Septmeber 201 5 , accepted 1 5 th Ocotber 201 5 Abstract The relation between zooplankton and environmental parameters were studied in Bagoe river in thirteen stations the dry (March - April 2012) and rainy (October - November 2012) seasons. Thirty - eight taxa of zooplankton were identified in Bagoe river, including Copepoda, Cladocera, Rotifera and others zooplankton. Rotifera was the richest group with 26 taxa belonging to 15 families and 18 genuses. Zooplankt on abundance varied significantly (p 0.01) according to station and season. Abundance seasonal variation was marked by dry season values (1 to 233 ind.L - 1 ) highest than those observed during wet season (0 to 6 ind.L - 1 ). Spatially, the highest zooplankton abundance was generally obtained in southern part of the river in both the dry and the wet (with the exception of station B11) seasons. Rotifera was the most abundant group with, in average 57 % of total abundance in the wet season and 91% over the dry se ason. During the dry season, the most abundance taxa of Rotifera, were Trichocerca sp. (mean: 24%), Polyharthra sp. (23%) and Lecane sp. (20%). During the wet season, Polyharthra sp ., was the most abundant Rotifera (70%). Zooplankton composition and abunda nce and its spatial and seasonal variations were linked to water temperature, Turbidity, conductivity, flow velocity, dissolved oxygen concentration, and dissolved solids rate. Keywords: Bagoe River, zooplankton composition and abundance, spatio - temporal variations. Introducti on Zooplankton is ecologically an important group of aquatic organisms that occupy a wide range of habitats. It is a important biotic component of an aquatic ecosystem. It influences they functionality and contributes to the energy flow, food chain, food web and cycling of organic matter 1 - 3 . Zooplankton actively grazing alga and consumes suspended matter and involving in the transfer of primary productivity to fish and other consumers 4 . Zoo plankton organisms offer also several advantages as indicators of environmental quality (pollution, water quality, and eutrophication) in aquatic ecosystem 5 , 6 . Therefore, zooplankton survey can help to manage aquatics ecosystems. In freshwaters ecosystems of Côte d’Ivoire, several studies on zooplankton have been achieved. Studies have mainly been performed in ponds 7 - 9 , lakes of hydroelectric dams 10 - 15 , agro - pastoral lakes 16 , Bandama stream basin 17 and, Agneby and Bia rivers 18,19 . Surveys on the zooplankton of the others freshwaters ecosystems of Côte d’Ivoire are scares or inexistent. Bagoe River is an aquatic ecosystem in the north of the Côte d’Ivoire and presenting 83 variety species of fish belonging to 43 genera, 19 families and 8 orders 20 . To date, zo oplankton community of this hydrosystem remains unknown. In the present study, our goal was to propose a focus on Bagoe River zooplankton, its specific composition and spatio - temporal variation in relation to environmental variables. Material and Methods S tudy area and data collection : The Bagoe River, tributary in the Niger stream, is situated in the North of Côte d’Ivoire between 9°15' and 10°50' N and 5°40 and 7°10' W ( f igure - 1). It is 350 km long and with a watershed of 33 430 km 2 ( 21) . The Bagoe river’s watershed undergoes the in fluence Soudano - guineen climate characterized by one rainy season (May - October) and one dry season (November - April). Zooplankton and environmental variables were collected during the dry (March - April 2012) and the rainy (Oc tober - November 2012) season in 13 sampling sites ( f igure - 1). The physical and chemical parameters (water temperature, dissolved solids rate (DSR), turbidity, dissolved oxygen concentration, pH, conductivity) were measured in surface, with a portable mult i - parameter probe, TURO T - 611. Water turbidity was measured using a Secchi disk. The zooplankton sampling was carried out using a cylindro - conical net of 64 µm in mesh opening size by filtration. Hundred liters of subsurface water were collected and ï¬ltere d through the plankton net of 64 µm mesh size. Samples were preserved with formalin at a final concentration of 5%. Zooplankton organism was identified using the following works 22 - 26 . The taxa were identified and counted under a dissecting microscope. Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 2 Figure - 1 Map of the Bagoe River in Côte d’Ivoire with sampling position sites Analytical procedure : The relationships between the zooplankton species and environmental parameters of the Bagoe were assessed by using a Canonical Correspondence Analysis (CCA). Kruskal - wallis test was use for zooplankton density comparison between stations and season . All st eps of this method were computed using Statistica 7.1 software. Results and Discussion Environmental parameters : Figure - 2 shows spatial variation of environmental variables of Bagoe River, during the rainy (October - November) and dry (March - April) seasons. Water temperature varied between 23.1 and 32.5 °C according to the station and the season. Highest temperatures were always obtained during the dry season. Turbidity, pH, conductivity and dissolved solids rate presented a spatio - temporal variation similar to that of water temperature ( f igure - 2 ). Dissolved oxygen concentration varied between 3.97 to 6.77 mg.L - 1 during rainy season versus 0.75 to 5.62 mg.L - 1 during dry season. Flow velocity varied between 0.1 to 0.6 m.s - 1 during the rainy season versus 0.02 to 0.3 m.s - 1 during the dry season. All environmental variables studied showed significant seasonal variations (p 0.001). Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 3 Figure - 2 Spatial variation of Environmental parameters measured during dry and rainy seasons in the Bagoe River ( DS = Dry S eason, RS = Rainy S eason ) Taxonomic composition spatio - temporal pattern : A total of Thirty - eight taxa of zooplankton were identified in Bagoe river including four groups: Copepoda, Cladocera, Rotifera and others zooplankton ( t able - 1 ). Copepods were represented by one family (Cyclopidae) and two genus plus unidentified copepod nauplii. Mesocyclops sp. and unidentified copepods nauplii presented highest occurrence �( 50%). Cladocera taxa belonging 20 40 60 80 100 120 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR Conductivity (µS.cm - 1 ) 0 0,1 0,2 0,3 0,4 0,5 0,6 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 SP SS Flow velocity (m.s - 1 ) 6,2 6,4 6,6 6,8 7 7,2 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR pH 0 1 2 3 4 5 6 7 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR Disolved Oxygen (mg.l - 1 ) 0 20 40 60 80 100 120 140 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR Turbi dity (cm) 10 20 30 40 50 60 70 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR DSR (mg.l - 1 ) 22 24 26 28 30 32 34 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 RS DR Temperature (°C) Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 4 to 4 families and 7 genuses. Daphnidae presented the highest diversity (3 taxa: Ceriodaphnia cornuta , Moina micrura and Moinodaphnia sp.) followed by Bosminidae (2 taxa: Bosmina longir ostris and Bosminopsis dietersi ). All cladocerans taxa have occurrences ≤ 30%. Rotifera dominate qualitatively the Bagoe zooplankton with 26 taxa (68% of total diversity) belonging to 15 families and 18 genuses. The most frequent taxa of the Rotifera group (occurrence ≥ 50%) were Trichocerca sp. (83%), Asplanchna sp. and Lecane sp. (58% each), Keratera tropica (50%) and Polyharthra sp. (50%). Bagoe river zooplankton species richness and diversity indices varied according to stations and seasons. During the dry season, zooplankton richness varied between 2 (Ba9) and 18 taxa (Ba3) with a total diversity of 35 taxa. During the rainy season, species richness varied between 0 (station Ba9) and 12 taxa (station Ba12) with a total diversity of 22 taxa. In Bagoe riv er, zooplankton richness was significantly more important during the dry season ( p 0.01 ). Similar seasonal pattern was observed for Shannon diversity indice, with values in dry season (Mean: 2.2 bit.ind - 1 ) higher than during the rainy season (Mean: 1.84 bit.ind - 1 ). Shannon’s diversity indice spatial variation was characterized by highest values at stations Ba1 and Ba3 to Ba7 (1.30 - 2 bit.ind - 1 ) and lowest value at station Ba9 (0 bit.ind - 1 ). Zooplankton structure and abundance spatio - temporal variation : Bagoe river zooplankton abundance varied significantly (p 0.01) according to stations and seasons. Abundance seasonal variation was marked by dry season values (1 to 233 ind.L - 1 ) highest than during rainy season (0 to 6 ind.L - 1 ) ( f igure - 3). Bagoe river’s zooplankton was dominated by Rotifera (mean: 57 - 91%), followed by Copepoda (8 - 27%) ( f igure - 4 A, B). Rotifera and Copepoda presented highest abundance at Ba3 to Ba6 and B11 ( ≈ 101 to 233 ind.L - 1 in the dry season) and at Ba11 ( ≈ 4 ind.L - 1 in the rainy seas on) ( f igure - 4 A, B). Figure - 3 Spatial and seasonal variations of the total zo oplankton abundance in the Bagoe river (DS: Dry S eason and RS: Rainy Season) During the dry season, 5 taxa of Rotifera collected accountetd for 89% of total ab undance: Trichocerca sp. (24%), Polyharthra sp. (23%), Lecane sp. (20%), Keratella tropica and Filinia opoliensis (8% each). Trichocerca sp. and Lecane sp. were more abundant at stations Ba4 (112 and 74 ind.L - 1 respectively) and Ba6 (42 and 64 ind.L - 1 resp ectively). Keratella tropica presented the most abundance at station Ba3 (31 ind.L - 1 ) while Filinia opoliensis highest abundance were observed at stations Ba3 - Ba5 (10 to 22 ind.L - 1 ) ( f igure - 4C). Copepoda were represented by 3 taxa during the dry season: na uplii stage (86%), Mesocyclops sp. (10%) and Thermocyclops sp. (4%). Highest abundances of these taxa were observed at stations Ba4 to Ba11 ( f igure - 4E). During the rainy season, Rotifera comprised mainly Trichocerca sp. (43%), Filinia opoliensis (17%), Lecane sp. (11%) and Polyharthra sp. (9%) ( f igure - 4D). Copepods were mainly constituted by Thermocyclops sp. (50%), nauplii stage (39%) and Mesocyclops sp. (11%) ( f igure - 4F). Environmental parameters influence on zooplankton: Canonical Correspondence An alysis (CCA) showed that the ï¬rst two axes expressed the variance percentage more than 60%, with ≈ 40% for the ï¬rst axis. During the dry season (DS), CCA showed a clear discrimination of 2 two zones ( f igure - 5A ). The ï¬rst group, positively correlated with t he axis II, comprised stations B1 to B4 and B6 located at the southern part of the Bagoe river’s watershed, and were opposed to stations B7 to B13 and B5, corresponding to the northern part of this river’s watershed. The first group of stations was posit ively correlated axis II and with dissolved oxygen and conductivity. Taxa associated to this zone were the copepods Thermocyclops decipiens , nauplii and the rotifers Filinia opoliensis , Filinia therminalis , Brachionus falcatus, Keratela tropica , Filinia sp ., Keratera sp. and Lecane sp.. The second group of stations was negatively correlated to the axis II and with pH, dissolved oxygen rate, temperature, conductivity, and flow velocity. Taxa mainly associated to this part were Mesocyclops sp., Moina micrur a , Trichocerca sp., Synchaeta sp., Ascomorpha sp., Asplanchna sp., Hexarthra sp., Filinia longiseta , Brachionus caudatus , Polyahrthra sp.. The analysis performed on the rainy season data doesn’t show clear spatial discrimination ( f igure - 5B ). However, let's observe that the main environmental factors controlling taxa distribution during the rainy season were dissolved oxygen, turbidity, pH and conductivity. D iscussion : Zooplankton community, is common to the traditional ones in tropical and sub - tropical fre shwater : Bia and Agneby rivers (Côte d’Ivoire) 19 , Lake Guier ( Senegal) 27 , Niger delta (Nigeria) 28 , Paraguay river (Paraguay) 29 , Paranapanema 30 and Jesumira rivers 31 (Brazil). 0 25 50 75 100 125 150 175 200 225 250 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 DS RS Abundance (ind.l - 1 ) Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 5 Table - 1 Distribution of zooplankton taxa collected in the Bag oe River; + : taxa presence Groups Families Taxa Stations Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 Copepod a Cyclopidae Mesocyclops sp. + + + + + + + " Thermocyclops decipiens + + + + + + + unidentified Nauplii + + + + + + + + + + + + Cladoceran Chydoridae Alona spp. + + + Bosminidae Bosmina longirostris + + " Bosminopsis dietersi + Sididae Diaphanosoma excisum + + + Daphnidae Ceriodaphnia cornuta + " Moina micrura + + + + " Moinodaphnia sp. + + Rotifera Gastropidae Ascomorpha sp. + Asplanchnidae Asplanchna sp. + + + + + + + Brachionidae Brachionus angularis + + " Brachionus calyciflorus + + " Brachionus caudatus + + + + + " Brachionus falcatus + + + + + + + " Keratella javana + + " Keratella tropica + + + + + + + + + " Keratella sp. + + " Platyias quadricornis + + Colurellidae Colurella sp. + Lepadellidae Lepadella sp. + + Conochilidae Conochilus sp. + + Epiphanidae Epiphanes sp. + + + Euclanidae Euchlanis sp. + Filiniidae Filinia longiseta + + + + " Filinia opoliensis + + + + + " Filinia terminalis + + + Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 6 Groups Families Taxa Stations Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 " Filinia spp. + + + Hexarthridae Hexarthra sp . + + + + Lecanidae Lecane sp. + + + + + + + + + Notommatidae Monommata sp. + Synchaetidae Polyarthra sp . + + + + + + " Synchaeta sp. + + Testudinellidae Testudinella sp. + + Trichocercidae Trichocerca sp. + + + + + + + + + + + + + Others unidentified Insect larvae + + + + Zooplankton " Ostracodes + + Total taxa 38 10 7 20 8 17 16 10 5 3 10 24 6 14 0 1 2 3 4 5 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 B13 Fopo Tricho Leca Poly Ktro others Abundance (ind.l - 1 ) (D) 0 1 2 3 4 5 6 7 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 B12 Ba13 Clado Cope Roti Others Abundance (ind.l - 1 ) (B) 0 50 100 150 200 250 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 B12 Ba13 Fopo Tricho Leca Poly Ktro others Abundance (ind.l - 1 ) (C) (A) 0 50 100 150 200 250 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 B12 Ba13 Clado Cope Roti Others Abundance (ind.l - 1 ) (A) Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 7 Figure - 4 Spatial variations of the total zooplankton abundance (A and B) and of the abundance of the main rotifera (C - D) and copepoda (E - F) taxa obtained during the dry (left) and the rainy (right) seasons in the Bagoé river (Clado: Cladoceran, Cope: C opepoda, Roti: Rotifera, Others : others zooplankton / rotifers , Ncop: Cop epods nauplii, Meso: Mesocyclops sp., Ther: Thermociclops decipiens , Fopo: Filinia opoliensis , Tricho: Trichocerca sp., Leca: Lecane sp., Poly : Polyathra sp., Ktro: Keratella tropica -1.0 1.0 -1.0 1.0 Meso sp. Tdec Naup Mmic Asco Aspl Bcau Bfal Ktrop Kera sp. Flong Fopo Fter Fil spp. Hexa Lecane s Poly Sync Tricho Water Te Disol Sol Rate Turbidity Flow vel pH Elect Cond Disolved Oxy Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 CCA Axe I (39.0%) CCA Axe II (22.20%) ( A ) 0 0,2 0,4 0,6 0,8 1 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 B12 Ba13 Ncop Meso Ther Abundance (ind.l - 1 ) (F) 0 10 20 30 40 Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 B12 Ba13 Ncop Meso Ther Abundance (ind.l - 1 ) (E) Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 8 Fig ure - 5 CCA of data from environmental factors and zooplankton species collected in the Bagoe river during the dry season (A) and the rainy season (B) Meso: Mesocyclops sp., Ther: Thermociclops decipiens , Fopo: Filinia opoliensis , Tricho: Trichocerca sp., Poly : Polyathra sp., Ktrop : Keratella tropica , Fter : Filinia terminalis , Bfal: Brachionus falcatus , Fil spp.: Filinia spp., Kera sp.: Keratella sp., Mmic: Moina micrura , Ncop : Copepods nauplii, Meso : Mesocyclops sp., Ther : Thermociclops decipiens , Fopo : Filinia opoliensis , Tricho : Trichocerca sp., Leca : Lecane sp., Poly : Polyathra sp., Ktrop : Keratella tropica , Sync: Synchateta sp., Asco: Ascomorpha sp, Aspl: Asplanchna sp., Hexa: Hexarthra sp., Flong: Filinia longiseta , Bcaud: Brachionus caudatus , Dexc: Diaphanosoma excisum , Epip: Epiphanes sp Total zooplankton richness (38 taxa) recorded in Bagoe River is relatively low compared to other tropical and subtropical rivers as Bia and Agneby river s in Côte d’Ivoire (68 taxa) 19 , Orogodo river in Nigeria (79 taxa) 28 , Okhuo river in Nigeria (51 taxa) 32 , Ikpa river in Nigeria (53 taxa) 33 . In contrast, zooplankton richness in Bagoe river is relatively higher that those reported in Sombreiro river (Niger ia) 34 , Calabar river (Nigeria) 35 . The difference in richness in this study and studies cited below could be explained by several factors among which we can mention conditions of study environment, sampling duration, the catchment area of the aquatic ecosys tem, activities performed on the watershed, etc. . According to Ezekiel et al. (2011) 34 , reported by FAO (2006) 36 zooplankton distributions vary in space and in the time in relation to the aquatic systems functioning. This study showed that, in Bagoe River, the great diversity was observed among Rotifera, with 26 taxa (68% of total diversity). This observation was similar to those observed in others freshwaters hydrosystem as : Bia and Agnebi rivers (Côte d’Ivoire) 19 , Lake Guier ( Senegal) 28 , Ovia rive r (Benin city, Nigeria) 37 . In contrast, in Sombreiro River (Nigeria), copepoda and cladoceran presented high diversity (29% of total diversity each) 34 . Branchionidae presented high family diversity with 8 taxa. The most representative genus belonged to the Rotifera Phylum, were Brachionus (4 species), Filinia (4 species) and Keratella (3 species). Similar observation was also found in others tropical and subtropical freshwater ecosystems as Lake Guier 27 , Paraopeba River 38 , Cuiaba river 39 , the Jacuí Delta f loodplain 40 . According to Borges and Pedrozo 40 , these genuses are typically dominate in large floodplain rivers. Our results show that zooplankton of Bagoe River differ markedly between -1.0 1.0 -1.0 1.0 Meso sp. Tdec Naup Alona Dexc Mmic Asco Bcal Bfal Ktrop Kera sp. Epip Fopo Fter Fil spp. Lecane s Tricho Ostra Larves ins Water Te Disolved Oxy Turbidity Flow vel pH Elect Cond Disol Sol Rate Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Ba13 CCA Axe I (42.5 %) CCA Axe II (18.8 %) (B) Research Journal of Biological Sciences _ _____________________ ______________ _______ ____ ____________ ISSN 2278 - 3202 Vol. 4 ( 11 ), 1 - 11 , November (201 5 ) Int. Res. J. Biological Sci. International Science Congress Association 9 hydrological phases, with high diversity in the dry season (2 to 19 ta xa according sites, total diversity: 35) versus 1 to 13 taxa during the rainy season (total diversity: 22). This result was similar to those found in other tropical and subtropical rivers as Paraguay Rivers (Brazil) 29 . In Bagoe river, rotifers was the most abundant zooplankton group (57% to 91% of total abundance according season). This situation characterizes the tropical freshwater ecosystems as lakes and rivers 31,43 and may be explained by their parthenogenetic reproductive pattern, short generation time under favorable conditions 41,42,43 , adaptation to turbulence and high concentrations of suspended solids 44 . According to Lampert 45 , the dominance of smaller zooplankton species as rotifers in rivers may possibly be due to predation pressure from plank tivorous fishes that selectively prey on larger sized zooplankton. In Copepoda populations, the nauplii predominance found in this study is also observed in different tropical freshwater habitats 38,36 . According to Edmondson 46 reported by Neves et al. 39 , high densities of copepod immature stages are generally attributed to their continuous reproduction in tropical regions. This can also be explained by intense predation on adult stages by invertebrates and vertebrates 47 . The Highest abundances values of z ooplankton obtained during the dry season confirms the results of previous studies in Orinoco River (Venezuela) 48 , in Parana River (Argentina) 44 , in Paraguay River (Paraguay) 29 . However, this result contrasts with several other studies where highest abunda nces were obtained during the rainy season 28,33,49 . Zooplankton abundance seasonal variation could be explained by several factors as changes in runoff, current velocity, turbidity 41,44 , water environmental characteristics, predation, quality and quantity of food and competition 50 . Conclusion This study provides for the first time the composition of the zooplankton population of the Bagoe river. In addition, it provides information on the spatial and seasonal variation of the zooplankton composition and abundance in relation with environmental factors. A total of Thirty - eight taxa were identified in Bagoe River divided including Copepoda, Cladocera, Rotifera and others zooplankton. Rotifera was the richest group with 26 taxa (68% of total diversity) belon ging in 15 families and 18 genuses. Rotifera was also the most abundant group, with 57% to 91% of the total population abundance according season . Our survey also showed that abundance and the diversity of zooplankton in Bagoe River vary significantly acco rding seasons, with the most important values in the dry season. Acknowledgments The authors would like to express their deepest gratitude to staff of Hydrobiology laboratory of the Felix Houphouët - Boigny University (Côte d’Ivoire) for their help in data collection and for the redaction and the revisions of the manuscript. The authors also would like to thank reviewer for their comments, observation and suggestions on the manuscript. References 1. Sinha B. and Islam M.R., Seasonal variation in zooplankt on population of two lentic bodies and Assam State Zoo cum Botanical garden, Guwahati, Assam , Ecol. Environ. Conser. , 8 , 273 - 278 ( 2002 ) 2. 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