Urban Pollution Signatures and Environmental Filtering of Benthic Macroinvertebrates along Abiotic Gradients in a Semi-Arid River in Botswana

Urbanization is a major driver of freshwater ecosystem degradation, yet its ecological effects on semi-arid African rivers remain poorly documented. This study investigated the influence of urban pollution signatures on physicochemical conditions and benthic macroinvertebrate assemblages along the Segoditshane River, Botswana. Physicochemical parameters and macroinvertebrates were sampled at three stations representing upstream, midstream, and downstream sections of the river. Community structure was assessed using diversity indices, a modified South African Scoring System (SASS5), functional feeding group analysis, and Canonical Correspondence Analysis (CCA). Significant spatial variation was observed in several physicochemical variables, including pH, dissolved oxygen, electrical conductivity, flow velocity, depth, ammonia, and phosphate concentrations (p < 0.05). Dissolved oxygen declined from 7.58 ± 0.09 mg L⁻¹ upstream to approximately 5 mg L⁻¹ downstream, whereas electrical conductivity increased significantly downstream, indicating increasing urban influence. A total of 1,238 macroinvertebrates representing 16 families and five insect orders were collected. Macroinvertebrate abundance decreased markedly from 718 individuals upstream to only 84 individuals downstream. Community composition shifted from dominance by pollution-tolerant taxa, particularly Chironomidae and Culicidae, to assemblages containing relatively sensitive taxa such as Baetidae and Simuliidae. Taxonomic richness, Shannon diversity, Simpson diversity, evenness, and ASPT values were highest at the midstream station, suggesting superior ecological condition relative to the other sites. Canonical Correspondence Analysis revealed that dissolved oxygen, pH, ammonia, conductivity, nutrients, flow velocity, and depth were the principal environmental drivers of community structure, with the first two canonical axes explaining 97.82% of the species-environment relationship. The results demonstrate that urban-derived physicochemical gradients act as strong environmental filters, restructuring macroinvertebrate assemblages and altering ecological condition along the river continuum. The study highlights the utility of benthic macroinvertebrates as indicators of river health and provides important baseline information for the management and conservation of urban rivers in semi-arid African environments.