Phytoplankton Community Structure and Eutrophication Indicators in Lake Chad Basin, North East Nigeria

Lake Chad, once one of Africa's largest freshwater bodies, has undergone dramatic shrinkage over recent decades, altering the ecology of its remaining water bodies in the North East Nigerian sector. This study characterised phytoplankton community structure and assessed eutrophication indicators in selected water bodies of the Lake Chad Basin in Borno State. Water samples were collected bi-monthly from five stations spanning open water, nearshore reed zones, and inlet channels. Phytoplankton were identified and enumerated using standard microscopy, and diversity indices were calculated. Chlorophyll-a concentrations, total nitrogen, total phosphorus, and Secchi disk transparency were measured as trophic state indicators. Results revealed dominance of cyanobacteria, particularly Microcystis and Anabaena species, at stations receiving agricultural drainage, consistent with hypereutrophic conditions. Diatom diversity was highest at inlet channels with relatively lower nutrient loading. Seasonal fluctuations in phytoplankton biomass were strongly correlated with rainfall-driven nutrient inputs. The study provides baseline ecological data critical for water resource management in the Lake Chad Basin, a region increasingly under pressure from climate variability and intensified irrigation agriculture. Recommendations include regulating agrochemical inputs into basin tributaries and establishing biological monitoring programmes integrating phytoplankton as indicators of water quality change. Keywords: phytoplankton, eutrophication, Lake Chad Basin, cyanobacteria, North East Nigeria.