Nahrungskettenmanipulation im Altrheinarm Kehl, einem eutrophen Stadtgewässer : Wie therapiert man eutrophierte Seen?

An attempt was made to improve water quality via food chain manipulation in an eutrophic backwater of the Rhine River located in Kehl (Baden-Wurttemberg). The main nutrient load results from groundwater input (infiltration of water from the Rhine) and the release of phosphorus from sediments (which cannot be decreased at present). Though phytoplankton provides enough food for herbivorous zooplankton, Daphnia could develop only minor amounts of biomass. Large zooplankton (> 900 mm) was found in the stomachs of plankton eating fish. These fish, mainly perch, pump-kinseed and roach were caught during three years of manual removal. Stocking with pike was unsuccessfully attempted in 2000. Removal of fish using gill nets (26 to 100 mm) and electric shocks did not result in an increase of Secchi depth in the first years, but the biomass of Daphnia did increase measurably. Daphnia development improved after both the l+-generation - using a net mesh of 12 to 15 mm - and the spawning fish were removed in the spring of 2002. For the first time, single individuals greater than 1 mm in length were found and water clarity increased significantly. Fish removal shall be continued using small-meshed nets.