Diatom Assemblage Dynamics as Indicators of Water Quality Change in Lake Geriyo, Adamawa State

Diatoms are sensitive biological indicators of water quality change, and their assemblage dynamics in Lake Geriyo, an important water supply reservoir in Adamawa State, North East Nigeria, can provide empirical evidence of environmental deterioration that physicochemical monitoring alone may miss. This study analysed the temporal dynamics of diatom assemblages in Lake Geriyo over a 12-month period in relation to measured water quality parameters, with the aim of establishing diatom-based water quality indices applicable to the lake. Monthly water and surficial sediment samples were collected from 5 stations distributed across the lake. Live and recent diatom assemblages were prepared using acid cleaning and mounted in Naphrax medium. Diatom taxa were identified and counted (minimum 400 valves per sample), and species-based water quality indices (Trophic Diatom Index and Specific Pollution sensitivity index) were calculated. Water quality parameters including total nitrogen, total phosphorus, dissolved oxygen, biochemical oxygen demand, and conductivity were measured concurrently. Eighty-three diatom taxa from 31 genera were recorded. Assemblage composition changed significantly between wet and dry seasons (PERMANOVA, F = 6.74, p < 0.01). Nutrient-tolerant taxa (Cyclotella meneghiniana, Aulacoseira granulata) increased in relative abundance at the two inlet stations during the wet season, coinciding with highest total nitrogen and phosphorus values. Trophic Diatom Index values indicated eutrophic to hypertrophic conditions at inlet stations. The study recommends diatom-based indices as complementary tools in the Adamawa State Environmental Protection Agency's routine Lake Geriyo monitoring programme. Keywords: diatoms, bioindicators, water quality, Lake Geriyo, Adamawa State