Paweł KOPERSKI Department of Hydrobiology, Warsaw University, Banacha 2, 02-097 Warsaw, Poland e-mail: p.t.koperski@uw.edu.pl REDUCED DIVERSITY AND STABILITY OF CHIRONOMID ASSEMBLAGES CHIRONOMIDAE, DIPTERA AS THE EFFECT OF MODERATE STREAM DEGRADATION ABSTRACT: Great taxonomic and func- tional diversity of larval chironomids make them potentially very useful group for testing the rela- tionships between degradation of freshwaters and diversity and stability of freshwater faunal assem- blages. The main aim of the study was to evaluate influences of moderate degradation on taxonom- ic composition of larval chironomid assemblages in lowland streams, on their biological and func- tional diversity as well as on assemblage tempo- ral 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, respec- tively) in Mazurskie Lakeland (north-eastern Po- land). 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 Ko- nopka 52 taxa were found. The most important variable determining taxonomic composition of larval chironomids in studied streams was sea- sonality. 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. plu- mosus – percentages of its larvae at degraded sta- tions were significantly higher. Species richness, rarity, taxonomic diversity, taxonomic distinct- ness 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 natu- ral sites, which means, that assemblages were also more diverse functionally therein. It clearly shows increased ecological uniformity of degraded hab- itats when compared with more natural ones. KEY WORDS: biological diversity, Chiron- omidae, degradation, stability, stream 1. INTRODUCTION The larval forms of family Chironomidae are the most widely distributed and frequent- ly the most abundant of all aquatic insects. They have been found to display a wide range of tolerances, with some species being ubiq- uitous and the others exploiting extremes of numerous ecological variables (C r a n s t on 1995). F r a n q u e t (1999) concludes that chi- ronomids seem to have a much wider range of feeding behaviour and lifestyles than other aquatic insect larvae. Chironomids can pro- vide a great deal of information about water POLISH JOURNAL OF ECOLOGY (Pol. J. Ecol.) 57 1 125–138 2009 Regular research paper journal 17.indb 125 journal 17.indb 125 2009-03-14 09:49:11 2009-03-14 09:49:11