The assemblage patterns of chironomid traits and ecological preferences in selected urban river systems, South Africa

Urbanisation significantly alters riverine ecosystems, disrupting their ecological functions, biodiversity and the services they provide. While the diversity and composition of macroinvertebrates have been extensively studied, the trait-based responses of chironomid assemblages to urban pollution in South African rivers remain poorly understood. This study presents the first attempt to investigate the distribution of chironomid traits and ecological preferences in relation to urban pollution stressors. Chironomids were sampled seasonally from fifteen sites classified based on land use and physicochemical variables into least-impacted sites (LIS sites), diffuse stormwater runoff impacted sites (DIS sites), sites immediately downstream of wastewater treatment works (WWTW) effluent discharge points (PSP sites), sites receiving impact from a combination of agricultural activities, stormwater runoff and effluent from WWTWs (CIS sites), and sites designated as system recovery sites (REV). Our results showed that chironomids with predatory lifestyle, small body size, cuticular respiration, and free-living behaviour were prevalent at least impacted and recovery sites. Conversely, species exhibiting very large body size, rigid tube construction, tracheal gills, high fecundity (over 1000 eggs per mass), and preference for detritus were dominant in highly impacted sites (DIS, PSP and CIS), indicating their tolerance to pollution. This study provides the first evidence linking chironomid functional traits to environmental water quality of Afrotropical urban streams, offering a novel framework for assessing ecological health in these systems.