Diversity and structure of Chironomidae (Diptera) communities along a gradient of heavy metal contamination in a subarctic watercourse

Community structure of chironomid larvae was investigated along a gradient of heavy metal pollution in the subarctic lakes of the Pasvik watercourse, bordering Northern Norway and Russia. Sixty taxa were present in the four investigated lakes. Species richness was lower in Lake Kuetsjarvi, nearest the main source of pollution, relative to the more distant lakes, and Simpson's diversity index was similarly highest in the less polluted site with higher maximum depth. The number of orthoclad chironomid species was considerably higher at the least contaminated sites, whereas the relative abundance of Orthocladiinae was highest at the most polluted site. Chironomid density increased with increasing distance from the main source of heavy metal pollution, especially in the subfamilies Tanypodinae and Chironominae, and total chironomid biomass was significantly higher at the least polluted site. Canonical correspondence analysis revealed spatial patterns consistent with species-specific tolerance to the heavy metal concentrations. Protanypus morio, Monodiamesa bathyphila, Chironomus spp., Polypedilum scalaenum gr. and Sergentia coracina seemed to be tolerant to heavy metal contamination, occurring most commonly at the most polluted sites. In contrast, Heterotrissocladius subpilosus, Psectrocladius semicirculatus/sp. , Chironomus anthracinus, Pagastiella orophila, Cladotanytarsus iucundus, and Tanytarsus spp. appeared to be more sensitive taxa, being common at the least polluted sites, but rare or absent at the two sites close to the pollution sources. Densities of Procladius ( Holotanypus) spp., Tanytarsus spp. and Chironomus anthracinus were negatively correlated with heavy metal concentrations and these taxa appeared to be the best indicators of heavy metal contamination.