Indicative value of different taxonomic levels of Chironomidae for assessing the water quality

Abstract

Most biological indices of pollution treat Chironomidae (Diptera) as a bad quality indicator, but under certain natural circumstances (e.g., frequent disturbs, dominance of fine sediments) this group forms almost the entire freshwater community. Metrics are needed to evaluate pollution under such circumstances, thus we tested the efficiency of Chironomidae larvae at morpho-species, subfamily and family levels, to detect a variety of human impacts on rivers. We used 75 samples (25 sampling stations, each with three replicates) and environmental data from mountain rivers (7 impacted and 18 non-polluted) from NW Argentina. Thirty (30) morpho-species in four subfamilies occurred in the studied sites. ROC (Receiving Operating Characteristic) curves were used to assess the efficiency of these taxa as indicators of water quality, and to propose thresholds separating both environmental groups (polluted/non-polluted). The number of morpho-species per subfamily or family proved to be a powerful discriminator between polluted and non-polluted sites. Various metrics are developed here based on richness and density of Chironomidae at different taxonomical levels. Non-polluted sites are predicted to present more than 6 morpho-species of Chironomidae, or at least 3 of Orthocladiinae, or 3 Chironominae, or 1 Tanypodinae. On the contrary, the presence of Chironomus gr. decorus indicated pollution.