Occurrence and Distribution of Cyanobacterial Blooms in Usman Dam, Abuja

Cyanobacterial blooms in freshwater reservoirs pose serious risks to drinking water quality, aquatic biodiversity, and public health through the production of potent hepatotoxins and neurotoxins. This study examined the occurrence, spatial distribution, and physicochemical drivers of cyanobacterial blooms in Usman Dam, Federal Capital Territory, Abuja. Water samples were collected monthly from six stations over a twelve-month period from January to December 2024. Phytoplankton identification and enumeration were conducted under a compound microscope, and cyanobacterial cell density was quantified. Microcystin concentration was measured using enzyme-linked immunosorbent assay. Physicochemical parameters including temperature, pH, conductivity, transparency, nitrate, and total phosphorus were analysed concurrently. Cyanobacterial blooms were recorded during ten of the twelve sampling months, with peak densities during the dry season months of January to March. Microcystis aeruginosa and Anabaena flos-aquae were the dominant bloom-forming species. Total phosphorus and pH were the strongest predictors of bloom intensity as determined by multiple linear regression. Microcystin concentrations exceeded WHO guideline values for recreational water at two stations during bloom events. Bloom events coincided spatially with the main reservoir inlet receiving agricultural return flows. Monthly bloom duration increased progressively between January 2024 and December 2024. The study recommends phosphorus source reduction strategies, including agricultural buffer strips and effluent phosphorus stripping, to mitigate cyanobacterial bloom frequency and protect the public water supply. Keywords: cyanobacteria, Usman Dam, microcystin, bloom, Abuja.