Hydraulic–oxygenation and nutrient–ionic axes structure fish assemblages in a tropical river: a pre-impoundment baseline from the Lower Gurara River, Nigeria

This study establishes a comprehensive pre-impoundment ecological baseline for the Lower Gurara River, quantifying how physicochemical gradients structure fish assemblages across spatial and seasonal scales in advance of hydropower regulation. The assemblage comprised 3,831 individuals representing 15 species and 11 families, reflecting a typical West African tropical river community. Multivariate analyses identified two dominant environmental axes: a “hydraulic–oxygenation axis” associated with coarse substrates, high turbulence, and elevated dissolved oxygen in upstream reaches, and a “nutrient–ionic axis” that intensified progressively downstream with increasing conductivity, sulphate, and phosphate. These gradients corresponded closely with observed community patterns. Rheophilic and benthic foraging taxa were most abundant at well-oxygenated upstream stations (S1–S2), whereas generalist and disturbance-tolerant species dominated depositional, nutrient-enriched downstream habitats (S3–S4). Species richness remained constant across stations, but evenness declined and dominance increased downstream, indicating an altered assemblage structure rather than compositional turnover. Seasonal patterns showed strong wet-season increases in abundance driven by flood-pulse connectivity, while α-diversity metrics remained comparatively stable, consistent with hydrology-mediated dynamics in tropical rivers. Interannual increases in total abundance from 2021 to 2022 reflected regional hydroclimatic variability, particularly sensitivity of low flows and flood extent. Collectively, these results demonstrate that linked hydraulic–oxygenation and nutrient–ionic gradients offer a robust, mechanistic framework for interpreting fish assemblage responses. The baseline also identifies key sentinel stations, S2 for aeration sensitivity and S4 for enrichment, and thermal change, and supports the development of threshold-based monitoring dashboards to guide adaptive management following impoundment. This dataset provides a defensible reference condition crucial for safeguarding ecological integrity during and after hydropower development on the Lower Gurara River.