Arthropod diversity of Nunia river (West Bengal, India): Impacted by ecological and water quality parameters with water velocity serving as a secondary driver in a pollution-dominated system

Urban tropical rivers often experience interacting gradients of pollution and flow, yet the relative influence of water velocity on ecological responses remains poorly understood. This study assessed how water-quality degradation and velocity variation structure aquatic arthropod assemblages in the Nunia River, a small tributary of the Damodar in eastern India. Six sites spanning upstream rural reaches, a midstream urban-industrial corridor, and a downstream confluence were sampled bi-monthly over two years. Physicochemical variables (pH, TDS, EC, DO, BOD, COD, nitrate, phosphate), surface velocity, and aquatic arthropods were quantified; diversity indices and non-parametric and Kruskal-Wallis tests, together with Spearman correlations, were used to explore spatial and seasonal patterns. Water quality parameters showed a pronounced longitudinal gradient. Upstream sites maintained low organic loads, moderate nutrients, and stable DO, whereas midstream sites exhibited severe organic and nutrient enrichment, with critically low DO and elevated COD, BOD, nitrate, and phosphate. COD at the most polluted midstream site reached 74-77 mg/L in summer, while DO dropped to as low as 0.34 mg/L at Site 4. Nitrate peaked at 69.5 mg/L during winter, and phosphate exceeded 6 mg/L in the industrial stretch. Downstream, partial chemical recovery occurred, but nutrient levels remained high. Velocity varied seasonally, with the highest flows in the monsoon and reduced flows in summer and winter; however, velocity showed weak, non-significant relationships with water-quality variables, indicating that pollution inputs rather than hydrology determined chemical conditions. In contrast, arthropod assemblages responded strongly to pollution gradients: diverse communities with crustaceans and hemipterans occurred upstream, while midstream sites were dominated by tolerant Diptera and exhibited low diversity and evenness. Shannon diversity declined from 3.53 at the upstream control site to 0.81 at the most polluted midstream site. Downstream assemblages indicated partial recovery. Overall, water velocity acted as a secondary disturbance factor, whereas spatially concentrated anthropogenic pollution emerged as the primary driver of ecological degradation in the Nunia River.