Species Composition and Seasonal Variation of Zooplankton in Kainji Lake, Niger State

This study examines the species composition and seasonal variation of zooplankton communities in Kainji Lake, Niger State, located in Nigeria's North Central geopolitical zone. Zooplankton are integral components of aquatic food webs and serve as sensitive indicators of water quality and ecological change in reservoir ecosystems. A descriptive survey was conducted over two seasons (wet and dry) with monthly water samples collected from five stations across the lake using a 55 micrometre mesh plankton net. Zooplankton were preserved in four percent formalin and identified to genus or species level under a compound microscope. Physical and chemical parameters were measured at each station. A total of 42 zooplankton taxa were recorded, comprising 18 Rotifera, 14 Cladocera, 7 Copepoda, and 3 Ostracoda. Rotifera dominated numerically in both seasons, while Copepoda biomass exceeded that of other groups. Species richness and total abundance were significantly higher during the wet season, attributed to increased nutrient input from runoff and greater primary productivity. The Copepod Mesocyclops thermocyclopoides was the most abundant taxon across all stations. Multivariate analysis showed temperature and conductivity as the strongest predictors of community composition variation among stations. The study provides current baseline data on Kainji Lake zooplankton and highlights the value of plankton monitoring as part of a long-term reservoir health assessment programme. Keywords: zooplankton, Kainji Lake, species composition, seasonal variation, Niger State.