Great taxonomic and functional diversity of larval chironomids make them potentially very useful group for testing the relationships between degradation of freshwaters and diversity and stability of freshwater faunal assemblages. The main aim of the study was to evaluate influences of moderate degradation on taxonomic composition of larval chironomid assemblages in lowland streams, on their biological and functional diversity as well as on assemblage temporal stability. Larval chironomids were collected once a month from March through to October at seven sampling sites located on three, third-order lowland streams Orzysza, Święcek and Konopka (38.6 km, 28.5 km and 20.5 km in length, respectively) in Mazurskie Lakeland (north-eastern Poland). Sampling sites differed in terms of type of catchment utilization and distance from potential pollution sources. Totally 10 060 individuals of 92 chironomid taxa were found in the streams – 48 were identified to the species level, the rest of them were identified as higher taxa. The most diverse chironomid fauna were found in stream Orzysza – 72 taxa, while both in streams Święcek and Konopka 52 taxa were found. The most important variable determining taxonomic composition of larval chironomids in studied streams was seasonality. Percentages of common, numerous and frequent species sampled at the stations diverse in terms of degradation did not differ significantly. An only exception was Chironomus sp. gr. plumosus – percentages of its larvae at degraded stations were significantly higher. Species richness, rarity, taxonomic diversity, taxonomic distinctness and temporal stability of chironomids were significantly lower in degraded habitats, while taxonomic composition of assemblages sampled at sites divergent in terms of degradation did not differ. Ecologically specialized species had been represented in higher percentages in more natural sites, which means, that assemblages were also more diverse functionally therein. It clearly shows increased ecological uniformity of degraded habitats when compared with more natural ones.
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