Zooplankton distribution and community structure as a function of environmental variables in the Niger River and its tributaries in Niger

The Niger River and its main tributaries (Sirba, Tapoa and Mekrou) in Niger were sampled during high and low water flow with the objectives of (1) comparing the taxonomic structure, diversity and abundance of the zooplankton community (rotifers, copepods, crustaceans) along the river and (2) determining which environmental or biological factors influence the distribution of the zooplankton communities most strongly. The impact of the tributaries at high-flow was also evaluated. Rotifers (32 taxa) were the most abundant and diverse followed by Cladocerans (13 taxa) and Copepods (three taxa). The mean number of taxa (16.5 and 17.6, respectively), the Shannon-Weaver diversity index (2.7 and 2.8, respectively) and the evenness (0.7 and 0.7, respectively) did not vary significantly between the high- or low-flow periods, or along the length of the Niger River and its tributaries. The mean zooplankton abundance was highest during low-flow periods (40 x 10(3) +/- 45 x 10(3) ind. m(-3)) but showed no specific spatial pattern. During high-flow sampling, the mean zooplankton abundance (3 x 10(3) +/- 2 x 10(3) ind. m(-3)) increased progressively downstream. RDA analysis showed that rotifers are associated with high conductivity and NO2- concentrations, copepods with chlorophyll a, while cladocerans were abundant when conductivity, chlorophyll a and NO2- concentrations were low.