The response of chironomid assemblages (Diptera: Chironomidae) to hydraulic conditions: a case study in a gravel-bed river

The response of chironomid assemblages to hydraulic conditions was analyzed within a gravel-bed river reach in central Europe. Multiple linear regression was used to quantify changes in the taxonomic composition along the hydraulic gradient. The results revealed that 47 % of the variability in the taxonomic compositional dissimilarity between pairs of samples could be explained by hydraulic conditions. Values of assemblage dissimilarity naturally increased with the increasing hydraulic difference between pairs of samples, however, the taxonomic composition of the assemblages changed faster at the beginning of the hydraulic gradient. According to the model, the assemblages of two streambed patches with the same hydraulic conditions differed in about 30 % of taxa, while the most hydraulically distant patches in about 65 % of taxa. To identify the taxa responsible for the shift in the chironomid assemblage, presence/absence and abundance of frequent taxa, as well as the proportions and number of taxa of chironomid subfamilies and the proportions of functional feeding strategies were modelled in relation to hydraulic conditions using Generalized Linear Models (GLM). The presence/absence of 15 taxa and abundance of 22 taxa (of the 29 taxa tested) showed a significant response to hydraulic conditions. Of these taxa, all Orthocladiinae showed a preference for hydraulically rough, while all Chironominae for hydraulically smooth conditions. The proportions and number of taxa of these subfamilies closely followed the responses of their individual taxa, when increased (in Orthocladiinae) and decreased (in Chironominae) along the hydraulic gradient. Tanypodinae showed a weak unimodal response. The proportions of feeding strategies were significantly related to hydraulic conditions too. However, their responses were not in absolute agreement with the expected distribution of food resources they probably rather indicated feeding potential of the assemblages than gave exact information about actually utilized food resources.