| Abstract: | From May 1996 to February 1998, the density of debris dams was surveyed in monthly intervals along a 750-m-long, third order section of the Weidlingbach, a fourth order sandstone brook in the Wienerwald. The mean density over the whole study period was one woody debris accumulation per 35 m stream length. The longevity of these structures ranged from one month to over one year, depending on the hydrology of the stream. More than 50% of the accumulations outlasted moderate floods (0.3–0.4 m3 s−1); of these, 52% were tightly fixed to the bank vegetation. However, only 12% of the destroyed dams were anchored in any way. In addition, two dams were sampled for macroinvertebrate colonisation every other month from June 1996 to June 1997. Dry weights of the leaf and wood fraction were determined and macroinvertebrates identified. Within dams, organismic densities were lowest in temporarily dry levels at the top of debris accumulations (mean density =171 specimens dm−3) and highest in permanently submerged levels near the water surface as well as in patches with high leaf litter concentrations (mean density = 1240 specimens dm−3). Within the topmost 5 cm sediment layer mean macroinvertebrate density was 67.7 specimens dm−3. Within debris dams, taxa composition was not significantly different between the lowermost level 1 and the sediment surface and among levels within dams; however, significant differences existed between the topmost level 4 and all other dam levels. In level 4, we observed significantly more terrestrial Diptera taxa and Coleoptera and significantly fewer Plecoptera and Gammaridae. |
