HYDRAULIC MICRODISTRIBUTION OF CASED CADDIS LARVAE IN AN AUSTRIAN MOUNTAIN BROOK

Water velocity acting at the upper edge of the case opening was measur ed at the locations of 1074 cased caddis larvae along a cross section of the Seebach near Lunz Biological Station (Lower Austria). In additi on, Froude number, boundary Reynolds number, bottom shear stress and t hickness of the viscous sublayer were measured at the center of contag ious sampling squares of 30 x 30 cm over the same cross section. Measu rements were made at monthly intervals from 2 November 1993 to 24 Octo ber 1994 using an impeller-meter (Ott C2; propeller diameter = 30 mm) and a set of FST hemispheres. A total of 16 species was recorded with Drusus biguttatus PICTET, Allogamus auricollis PICTET and Potamophylax cingulatus STEPHENS being most abundant. Maximum values of current ve locity and hydraulic stress parameters (e.g. Froude numbers up to 0.90 ) were recorded for D. biguttatus, P. cingulatus and Glossosoma confor mis NEBOISS, whereas lowest values of hydraulic stress parameters were observed in Sericostoma flavicorne SCHNEIDER. In addition, up to 40% of the population in Drusinae species and P. cingulatus was over-repre sented in high stress areas at the streambed, whilst up to 36% of the population in S. flavicorne and Allogamus spp. were over-represented i n low stress areas.