Seasonal Fluctuations in Dissolved Oxygen in Jebba Reservoir, Kwara State

Dissolved oxygen is among the most critical parameters determining aquatic ecosystem health, and its depletion in stratified reservoirs is a recurrent and damaging phenomenon affecting fisheries and water supply. This study examines seasonal fluctuations in dissolved oxygen in Jebba Reservoir, Kwara State, North Central Nigeria, with emphasis on the mechanisms of thermal stratification, hypolimnetic anoxia, and turnover events. Dissolved oxygen profiles are measured biweekly at three stations from the surface to the deepest point using a calibrated optical dissolved oxygen logger for twelve months during 2023 to 2024. Temperature, conductivity, and turbidity are co-registered to characterise stratification dynamics using Brunt-Vaisala frequency calculations. Chlorophyll-a and phytoplankton biomass are monitored to relate primary productivity to oxygen consumption in the metalimnion. The study also samples sediment oxygen demand to quantify the contribution of benthic decomposition to hypolimnetic oxygen depletion. Nutrient loading from the Asa River, the primary inflow, is estimated from monthly water samples to assess eutrophication contribution to seasonal oxygen depletion. Findings are expected to demonstrate strong thermal stratification from March to October, with complete hypolimnetic anoxia from May to September at depths below 12 metres. Reservoir destratification through mechanical mixing is recommended as a management intervention for the Kainji-Jebba Hydroelectric Authority. Keywords: dissolved oxygen, Jebba Reservoir, thermal stratification, hypolimnetic anoxia, reservoir limnology