The Influence of Hydraulic Stress on Microdistribution Patterns of Zoobenthos in a Sandstone Brook (Weidlingbach, Lower Austria)

In the Weidlingbach, a low‐order forest stream in the Wienerwald area near Vienna (Austria), hydraulic stress was measured at 112 locations from which benthic fauna (=33318 specimens of macro‐invertebrates) was collected between March and September 1995. For measuring hydraulic stress a set of FST hemispheres was used. The most abundant taxa studied (= 90.7% of the total) were Gammarus fossarum Koch (Amphipoda), Leuctra spp. (Plecoptera), the ephemeropterans Baetis rhodani (Pictet), B. vernus (Curtis), Habroleptoides confusa Sartori and Jacob, Ecdyonurus starmachi Sowa and the caddisfly Hydropsyche instabilis (Curtis). Five mayfly, four stonefly and six caddisfly taxa significantly (p < 0.05) preferred the lotic subsection of the research area, whereas Ephemera danica Müller significantly (p < 0.001) preferred the lentic subsection. In the species studied in detail, medians of their FST hemisphere ranges varied from 8 (e.g. Brachyptera risi (Morton)) to 2 (e.g. Paraleptophlebia submarginata (Stephens)). A comparison of microdistribution patterns at low and high discharge revealed no significant shift in FST hemisphere numbers and, hence, in exposure to hydraulic stress in Hydropsyche instabilis and Rhyacophila fasciata Hagen; both species were most abundant in high stress areas, independent of discharge. On the other hand, most other species studied in detail exhibited a shift to lower hydraulic stress at high discharge, whereas Electrogena ujhelyii (Sowa) was evenly distributed (p > 0.05) over the whole range of hydraulic stress, independent of discharge.