Response of aquatic organisms to river rehabilitation – can we simply rebuild nature?

Within the last years the economic, ecological and social meaning of rivers moved increasingly into the foreground, making river rehabilitation an issue of increasing importance. It is generally believed that organisms will rapidly re-colonize formerly degraded river stretches following restoration efforts. However, the biological effectiveness of rehabilitation measures is still unclear. In our study we performed simultaneous investigations of the response of fish, benthic invertebrate and aquatic macrophyte communities to river rehabilitation at 25 sites in Germany. The use of highly standardized methods allowed for the first profound examination of this topic. Rehabilitated stretches were 1.5 km long on average and monetary costs amounted to 600.000 € per measure. Although habitat diversity increased significantly following rehabilitation, a positive organismal response was barely detectable. Assessment results of fish communities showed an improve in only one third of all cases, while benthic invertebrates and aquatic macrophytes showed almost no positive trend. Our results reveal that even several years after the implementation of the rehabilitation measures, biodiversity still remains on a low level. This points out the necessity to consider about impeding factors that counteract successful re-colonization events, e.g. missing source populations or high background levels of chemical load.