Biomanipulation as a useful water quality management tool in deep stratifying reservoirs

In six deep, soft-water reservoirs, ranging from oligotrophic to eutrophic, fishery management has been guided by the use of biomanipulation to improve water quality and opportunities for recreational fishing. As evidenced by the establishment of larger-bodied daphnids, a low level of zooplanktivory could be maintained in the newly filled Grosse Dhünn and refilled Brucher and Lingese Reservoirs only by regular stocking of piscivores (Sander lucioperca, Esox lucius, Salmo trutta lacustris, Onchorhynchus mykiss) in combination with size and bag limitations for the recreational fisheries. However, in the mesotrophic Pre-Reservoir Grosse Dhünn, Bever Reservoir and the newly filled slightly eutrophic Wupper Reservoir it took between 8 and 10 years before the predator-resistant zooplankton community responded to management with a switch to larger daphnids. Except for oligotrophic Grosse Dhünn Reservoir where angling is prohibited, the expected enhancement of piscivore biomass through stocking not least was prevented by anglers. However, growth of perch (Perca fluviatilis) benefited from the changed fishery management relying upon stocking piscivores only allowing them to reach the size of piscivory. The appearance of larger daphnids in Pre-Reservoir Grosse Dhünn and Wupper Reservoir resulted in the biomass of the Daphnia spring peak to increase and occur earlier causing prolonged clear water conditions. Although the larger daphnids increased transparency, total summer mean chlorophyll concentrations in the euphotic zone only decreased in the slightly eutrophic reservoirs due to reduced phosphorus availability following unchanged external phosphorus loadings. Reduced phosphorus availability in these reservoirs caused a real oligotrophication. Although the edible seston fraction was controlled “top down” the results did not support the trophic cascade hypotheses because total phytoplankton remained controlled “bottom up”, admittedly triggered by “top down” forces. In general, the results support the importance of indirect (non-lethal) effects as the driving forces for the successful biomanipulations, particularly in slightly eutrophic reservoirs. Not least, the delayed zooplankton response provides an indication that the underlying change of internal feedbacks was not driven by external forces to stabilize the reservoirs trophic state in these deep stratifying reservoirs.