Community Structure of Epilithic Algae in the Benue River, Benue State

Epilithic algae, which colonise hard substrates in flowing waters, are widely used as ecological indicators of water quality and habitat conditions in tropical river systems. This study examined the community structure, diversity, and environmental drivers of epilithic algae in the Benue River, Benue State, North Central Nigeria. Algal samples were scraped from natural rock surfaces and pebbles at four stations representing upstream, midstream, and downstream reaches during wet and dry seasons. Specimens were mounted on microscopy slides and identified to genus level using standard keys. Cell density was quantified by direct counting under high magnification. Physicochemical parameters including current velocity, pH, conductivity, nutrient concentrations, and turbidity were simultaneously recorded. A total of 52 epilithic algal genera were documented, dominated by diatoms (Bacillariophyta), followed by green algae (Chlorophyta) and cyanobacteria. Diatom genera Navicula, Nitzschia, and Cyclotella were most abundant. Species richness was highest at the upstream reference station and declined progressively toward the polluted urban downstream station. Eutrophication-tolerant cyanobacteria increased significantly at stations receiving agricultural and urban runoff. Trophic diatom index values classified upstream reaches as oligotrophic and downstream reaches as eutrophic. Seasonal analysis showed lower diversity during the wet season, attributed to substrate instability from high flow velocities. The epilithic algal community accurately reflected the nutrient enrichment gradient, supporting its use as a complementary biomonitoring tool in Nigerian rivers. Keywords: epilithic algae, diatoms, biomonitoring, Benue River, water quality.