Investigation of Algal Bloom Occurrence and Eutrophication in Kainji Lake, Niger State

This study investigates the occurrence of algal blooms and the eutrophication status of Kainji Lake in Niger State, North Central Nigeria. Kainji Lake is Nigeria's largest man-made reservoir, serving hydropower generation, irrigation, and artisanal fisheries, but increasing agricultural activity in its catchment has intensified nutrient loading, creating conditions that promote eutrophication and harmful algal blooms with consequences for water quality, fisheries, and hydropower turbine performance. Water samples were collected at eight stations across the lake surface and at depth during both pre-bloom and bloom periods over a twelve-month monitoring programme. Parameters analysed include total nitrogen, total phosphorus, chlorophyll-a concentration, dissolved oxygen profiles, Secchi disc depth, and cyanobacteria cell counts. Results reveal that chlorophyll-a concentrations at three stations exceed the hypereutrophic threshold of 200 micrograms per litre during the late rainy season, consistent with heavy nutrient loading from agricultural runoff. Total phosphorus concentrations are above the mesotrophic-to-eutrophic boundary of 0.1 milligrams per litre at all stations. Cyanotoxin screening indicates the presence of microcystin-producing cyanobacteria species at bloom locations. Dissolved oxygen depletion to below 2 milligrams per litre is recorded in deep water strata, creating hypoxic conditions harmful to fish. The study recommends buffer strip establishment around agricultural land draining to the lake and a phosphorus interception wetland at the major tributary inflows. Keywords: eutrophication, algal bloom, Kainji Lake, nutrient loading, water quality.