Vertical distribution of benthic macroinvertebrates in riffles versus deep runs with differing contents of fine sediments (Weidlingbach, Austria)

Due to the erosiveness of their sediments, sandstone streams transport high loads of fine particles. The catchment of the Weidlingbach, a 12 km long tributary of the Danube northwest of Vienna, is dominated by calcareous sandstone, marl and slate. Mean sediment grain size ranges from 29.3 to 31.0 mm, and mean pore volume is approximately 20%. The third order study site is divided into a wide, shallow riffle section with high accumulations of fine sediments in the hyporheic zone and a narrow, deep run section with a lower percentage of fine sediments. Invertebrates were sampled to a sediment depth of 60 cm every second month using the freeze-coring-technique with electropositioning. Hyporheic invertebrate densities were significantly higher in sediments of the run than in those of the riffle section. Generally, hyporheic abundances decreased with increasing depth; however, the decline was more distinct within the riffle than the run section. Furthermore, we observed a strong negative correlation between the percentage of fine sediments (<2 mm) and hyporheic invertebrate densities.