Unravelling the determinants of stream midge biodiversity in a boreal drainage basin JANI HEINO* AND LAURI PAASIVIRTA † *Finnish Environment Institute, Research Programme for Biodiversity, University of Oulu, Oulu, Finland † Salo, Finland SUMMARY 1. Few extensive lotic studies have examined patterns in the biodiversity of non-biting midges (Diptera: Chironomidae) along major environmental gradients. Our aim was to fill this gap by describing patterns in species diversity, assemblage composition and distributions of midges across a boreal drainage basin. 2. We found that the diversity of midges, as measured by rarefied species richness, Fisher’s a and Pielou’s evenness, responded positively to stream size in regression analysis. By contrast, species density was most strongly correlated to a gradient in suspended solids and phosphorus in stream water, as well as macrophyte cover. Spatial variables were not significantly correlated with species diversity. 3. Midge assemblage composition was best explained by a model incorporating five composite environmental gradients in canonical correspondence analysis. The environ- mental gradients were stream size, macrophyte cover, alkalinity, nitrogen and suspended solids. Spatial variables did not overcome the effects of environmental gradients on assemblage composition. 4. Cluster analysis divided the 27 study sites into four groups with relatively similar midge assemblages. These groups were statistically significant in multi-response permutation procedure, and 15 of the 49 midge taxa recorded varied significantly among the groups in indicator value analysis. Discriminant function analysis showed that stream size, macrophyte cover and habitat structure predicted 66.7% of sites into correct groups. 5. The information provided by the present analyses may be of considerable importance in conservation planning at the drainage basin level. The fact that species diversity and assemblage composition varied primarily along the stream size gradient suggests that sites belonging to the different size classes (first to fifth order) are needed to conserve the biodiversity of midges. The other environmental gradients should also be considered in conservation planning, because they explained significant amounts of variability in midge assemblage composition. Keywords: assemblage–environment relationships, Chironomidae, lotic assemblages, non-biting midges, species diversity Introduction Efficient conservation, management and assessment of ecological systems requires information about populations, species and assemblages of various taxonomic groups. In many cases, such information is very limited, and biodiversity studies are often based on taxonomically and ecologically well-known, charismatic and publicly-valued taxonomic groups (Pimm & Gittleman, 1992; Oliver, Beattie & York, 1998). By contrast, many ecological studies ignore taxonomically more demanding and less conspicuous Correspondence: Jani Heino, Finnish Environment Institute, Research Programme for Biodiversity, PO Box 413, University of Oulu, FI-90014 Oulu, Finland. E-mail: jani.heino@ymparisto.fi Freshwater Biology (2008) 53, 884–896 doi:10.1111/j.1365-2427.2007.01946.x 884 Ó 2007 The Authors, Journal compilation Ó 2007 Blackwell Publishing Ltd