Etude Préliminaire de la Diversité et de la Structure du Peuplement Zooplanctonique de la Rivière Okpara au Bénin

La présente étude a pour objectif d’évaluer la diversité et la structure du zooplancton dans la rivière Okpara. Le zooplancton de la rivière a été identifié et la structure de sa communauté évaluée dans neuf stations sélectionnées grâce à la richesse taxonomique, à la densité, à l’indice de diversité de Shannon et l’indice d’équitabilité de Piélou. Un total de 68 espèces de zooplancton a été enregistré, réparties en trois groupes : comprenant des rotifères (57), des copépodes (07) et des cladocères (04). Le nombre d’espèces a été plus élevé dans le cours inférieur, que dans le cours supérieur et dans le cours moyen. Les rotifères représentent les groupes les plus abondant avec 78,25 % de l'abondance totale, suivis des copépodes (20,7 %) et des cladocères (1,3 %). Keratella valga et Tropocyclops confinis sont les espèces les plus abondantes. Mytilina sp., Asplanchna girodi, Anuraeopsis sp., Lepadella sp., Asplanchna brightwellii, Asplanchna navicular, Trichocerca sp. et Trichocerca tenuior ont été enregistrés sur toutes les stations. Les valeurs moyennes d’abondance et de densité moyenne du zooplancton sont plus élevées en saison sèche qu’en saison pluvieuse. Les valeurs de la richesse, de l’indice de diversité de Shannon et d’équitabilité de Piélou sont plus élevées en saison pluvieuse qu’en saison sèche. La forte présence de certaines espèces indicatrices de pollution telles que Asplanchna brightwelli, Brachionus angularis, Brachionus falcatus pourrait indiquer la présence de polluants organiques dans la rivière. Il est donc nécessaire de mettre en place des mesures de surveillance efficaces afin de protéger la rivière de toute intrusion anthropogénique.   The objective of this study was to assess the diversity and structure of zooplankton in the Okpara River. The river's zooplankton were identified and their community structure assessed at nine selected stations using taxonomic richness, density, the Shannon diversity index and the Pielou equitability index. A total of 68 species of zooplankton were recorded, divided into three groups: rotifers (57), copepods (07) and cladocerans (04). The number of species was higher in the lower reaches than in the upper and middle reaches. Rotifers were the most abundant group with 78.25% of the total abundance, followed by copepods (20.7%) and cladocerans (1.3%). Keratella valga and Tropocyclops confinis were the most abundant species. Mytilina sp., Asplanchna girodi, Anuraeopsis sp., Lepadella sp., Asplanchna brightwellii, Asplanchna navicular, Trichocerca sp. and Trichocerca tenuior were recorded at all stations. The values for abundance and density of zooplankton are higher in the dry season than in the rainy season. The values for species richness, Shannon diversity index and Pielou equitability are higher in the wet season than in the dry season. The strong presence of certain pollution indicator species such as Asplanchna brightwelli, Brachionus angularis and Brachionus falcatus could indicate the presence of organic pollutants in the river. Effective monitoring measures therefore need to be put in place to protect the river from anthropogenic intrusion

Physicochemical Drivers of Zooplankton Seasonal Variability in a West African Lagoon (Nokoué Lagoon, Benin)

International audienceThis study aimed to investigate the seasonal variation of zooplankton diversity and abundance in the Nokoué Lagoon in southern Benin. Through extensive sampling, a total of 109 zooplanktonic taxa were identified and quantified. The average zooplankton abundance was found to be 60 individuals per liter, with copepods and rotifers being the most dominant groups, comprising 68.1% and 29.1% of the total abundance, respectively. The key factor identified as driving the structure of the zooplanktonic assemblages was salinity, which showed significant seasonal variation. The results revealed that during the high water period, when the lagoon was filled with fresh water, rotifers were dominant, zooplanktonic diversity was highest, and abundances were quite high. Conversely, during the low water period, when the lagoon was characterized by brackish water, diversity was minimal, and abundance decreased slightly. The study also found that some areas of the lagoon showed high abundances independent of salinity levels, suggesting that other factors such as riverine inputs or the presence of acadjas (home-made brush parks used as fish traps) may also have notable effects on the zooplankton community. Overall, the findings of this study provide valuable insights into the functioning of one of the most biologically productive lagoons in West Africa