Hydraulic microdistribution of cased caddis larvae in an Austrian mountain brook

Water velocity acting at the upper edge of the case opening was measured at the locations of 1074 cased caddis larvae along a cross section of the Seebach near Lunz Biological Station (Lower Austria). In addition, Froude number, boundary Reynolds number, bottom shear stress and thickness of the viscous sublayer were measured at the center of contagious sampling squares of 30 x 30 cm over the same cross section. Measurements were made at monthly intervals from 2 November 1993 to 24 October 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 velocity and hydraulic stress parameters (e.g. Froude numbers up to 0.90) were recorded for D. biguttatus, P. cingulatus and Glossosoma conformis 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-represented in high stress areas at the streambed, whilst up to 36% of the population in S. flavicorne and Allogamus spp. were over-represented in low stress areas.