Zooplankton Community Dynamics and Their Relationship to Eutrophication in Uwandiwe Reservoir, Ekiti State, South West Nigeria

This study examines zooplankton community dynamics and their quantitative relationship to eutrophication indicators in Uwandiwe Reservoir, Ekiti State, South West Nigeria. Eutrophication of drinking water reservoirs is a growing concern in South West Nigeria, and zooplankton communities offer time-integrated biological responses to nutrient enrichment that can guide reservoir management. Monthly water samples were collected from four stations from a clean inflow station to the dam wall over 18 months. Zooplankton were enumerated and identified to species level. Total phosphorus, total nitrogen, chlorophyll-a, Secchi Zoologyh, and dissolved oxygen were measured concurrently. Multivariate statistical analyses including canonical correspondence analysis and redundancy analysis related zooplankton species composition to eutrophication indicators. Generalised additive models were used to characterise non-linear relationships between zooplankton community change and chlorophyll-a gradients. A total of 51 zooplankton species were identified across the study period. Community composition shifted progressively from Copepod-Cladocera dominance at low-nutrient stations to Rotifer dominance near the dam wall as total phosphorus increased. Keratella cochlearis abundance showed a significant positive relationship with chlorophyll-a above 25 micrograms per litre, proposing this species as a practical eutrophication indicator for Uwandiwe. Shannon diversity declined significantly with increasing phosphorus loading. The study provides the first multi-year zooplankton dynamics dataset for Ekiti State reservoirs and proposes a biologically anchored eutrophication management trigger at chlorophyll-a = 25 micrograms per litre for reservoir management decision-making. Keywords: zooplankton, eutrophication, reservoir, Ekiti State, bioindicators.