Patterns of chironomid body-size distribution in an effluent-impacted river in the Eastern Cape,South Africa

Body size is an important determinant of assemblage structure in rivers and streams impacted by elevated concentrations of pollutants such as salts and metals. In the present study, because of the larger surface-area-tovolume ratio of small-bodied chironomid species compared with large-bodied species, it was hypothesised that the relative abundance of the small-bodied species would decrease at the impacted sites by elevated concentrations of total dissolved solids compared with that at the less-impacted control site. The aim of this study was to analyse and compare patterns of chironomid final instar body-size classes at impacted sites with those at the control site. Chironomid larvae were sampled seasonally from August 2009 to September 2012 from one control site and three impacted sites (Sites 2, 3 and 4) in the Swartkops River, Eastern Cape. Site 2 was impacted by diffuse pollution sources, whereas Sites 3 and 4 were impacted by wastewater effluent discharges in addition to diffuse pollution sources. Small-bodied species dominated the assemblage at the control site and declined significantly at the impacted sites, suggesting that chironomid body size responds predictably to deteriorating water quality in the Swartkops River.