Influence of environmental factors on the distribution of caddisfly (Trichoptera) communities in medium-sized lowland streams in Latvia

The understanding of community structuring factors is a fundamental issue in community ecology. The objectives of the study were (1) to identify the distribution of the caddisfly community along the stream continuum; (2) to ascertain hydrochemical, hydrological, substrate, and catchment factors responsible for the distribution of communities; and (3) to summarize the key variance types determining the variance in caddisfly communities. Standard methods were used to investigate hydrological and hydrochemical parameters of nine medium-sized streams in 2003. Sampling and processing of samples of macroinvertebrates followed AQEM methods. A total of 28 taxonomic units of caddisflies were analysed. The upper reaches of streams revealed a relatively low abundance of caddisflies while the middle reaches had high abundances. The abundance in the lower reaches varied in a wide range. TWINSPAN separated the caddisfly species into two distinct groups. One group represented lithal habitat and rheophilous species. The other represented stream reaches with fine mineral sediments rich in organic matter of different size. Hydrochemical and physical variables explained the majority of variance in the preferences of caddisflies to bottom type and other environ- mental parameters, while all studied variables together explained up to 58% of the data variance. Thus, caddisflies could be used to study the influence of local and regional factors on stream ecosystems. The results of the present study could be also applied in practice to solve problems related to the evaluation of the ecological quality of running waters using benthic macro- invertebrates.