Benthic invertebrate and fish communities as indicators of morphological integrity in the Enz River (south-west Germany)

Between 1994 and 1995 we sampled invertebrate and fish communities of the lower Enz River, a second order tributary of the Rhine River (total length, 108 km, mean annual discharge, 16.6–23.9 m3 s-1), in the vicinity of man-made weirs, their associated fishways, and more natural river sections. We focused on the ecological consequences of these man-made alterations to river morphology and flow conditions on the presence/absence and relative densities of invertebrate feeding groups, including the effects on fish and benthos habitat preference types. We further attempted to identify species useful as indicators of morphological integrity, particularly of physical disturbance of lotic habitats typical for the Enz River. We found pronounced differences in species composition and abundance, and in the contribution of feeding groups and habitat preference types, between disturbed and more natural sites. For example, the benthic community characteristics of two bypass fishways were quite similar to each other, but differed from those found in the respective adjacent river sections, suggesting a local ‘rhithralisation’ effect of fishways that mimic artificial streams. Among the 109 invertebrate and the 28 fish taxa identified, both the presence/absence and the abundance of some species or functional groups were useful indicators of the morphological integrity of typical riverine habitats. The mayfly Oligoneuriella rhenana, for example, occurred almost exclusively in riffle sections, and its density correlated positively with turbulent, high velocity currents and the presence of larger rocks.