Chironomid distribution along a pollution gradient in Ethiopian rivers, and their potential for biological water quality monitoring

Abstract

Water pollution resulting from the rapidly intensifying use of freshwater resources in Ethiopia is increasing the need for continuous follow-up and monitoring of the country's aquatic ecosystems, in order to maintain their biological diversity and water quality. Chironomids (non-biting dipteran midges) are often used for biological water quality assessment because their larvae tend to respond rapidly and sensitively to changes in the aquatic environment. Larval chironomid assemblages from 37 sites in 20 Ethiopian rivers were analysed in 2010 to assess the relationship between species composition and water quality. Cluster analysis of local physical and chemical variables was used to classify sites into three different water quality classes: 15 sites were classified as not or slightly polluted, 16 as moderately polluted and six as heavily polluted. We recovered 21 chironomid taxa, representing three subfamilies, with Chironominae (13 taxa) making the largest contribution, followed by Tanypodinae and Orthocladinae (four taxa each). Polypedilum nubifer, P. wittei, Polypedilum type Wabe, Cryptochironomus sp. and Conchapelopia sp. were the dominant species in unpolluted or slightly polluted sites. Chironomus alluaudi and C. imicola were indicators of heavily polluted sites, typified by low (<2 mg l−1) concentrations of dissolved oxygen. Heavily polluted sites, such as the Modjo and Sebata rivers, also held a lower number of species and genera than less-impacted sites. Larval chironomid assemblages proved useful as indicators of river water quality in Ethiopia when taxa are identified at the species and/or genus level.