Phytoplankton Community Composition and Primary Productivity in Asejire Reservoir, Osun State, South West Nigeria

This study examines phytoplankton community composition and primary productivity in Asejire Reservoir, Osun State, located in Nigeria's South West geopolitical zone. Asejire Reservoir is a major drinking water source for Ibadan and surrounding communities, and the dynamics of phytoplankton communities are directly relevant to water treatment efficiency and public health risk assessment. Surface water samples were collected monthly over six months at four stations. Phytoplankton were concentrated by sedimentation and identified to genus level using standard taxonomic keys. Chlorophyll-a was determined spectrophotometrically as a proxy for primary productivity. Physicochemical parameters were measured at each station. A total of 46 phytoplankton genera from five divisions were identified. Cyanobacteria dominated in terms of abundance (52% of total counts), followed by Chlorophyta (28%) and Bacillariophyta (16%). Microcystis aeruginosa was the dominant Cyanobacterium at all stations, raising water quality concerns due to its potential for toxin production. Chlorophyll-a concentrations ranged from 18 to 64 micrograms per litre, indicating eutrophic conditions at two stations. Temporal variation showed peak Cyanobacterial blooms in the late dry season when water temperatures were highest. Total nitrogen and total phosphorus showed the strongest correlations with Cyanobacterial abundance. The study recommends regular cyanotoxin monitoring of Asejire Reservoir water, assessment of nutrient input sources, and catchment management interventions to reduce phosphorus loading. Keywords: phytoplankton, primary productivity, Asejire Reservoir, cyanobacteria, Osun State.