BIOMANIPULATION, WILL IT WORK FOR YOUR LAKE - A SIMPLE TEST FOR THE ASSESSMENT OF CHANCES FOR CLEAR WATER, FOLLOWING DRASTIC FISH-STOCK REDUCTION IN SHALLOW, EUTROPHIC LAKES

From the results of biomanipulation case studies in the Netherlands, a simple test was derived for the assessment of chances for clear water, following fish stock reduction. The test is directed primarily to lake managers of shallow, eutrophic lakes in order to enhance the performance of new biomanipulation projects. A prerequisite for success is that at least 75% of the planktivorous and benthivorous fish are removed from the lake. The chance for a clear-water period after fish reduction is determined by the opportunities for large Daphnia species and by the impact of benthivorous fish and wind on sediment resuspension. A bloom of filamentous Cyanobacteria (> 100000 ind./ml) and large numbers of mysid shrimp (Neomysis spp. > 100 ind./m2) in early spring may prevent the development of Daphnia and consequently a decrease in algal biomass. Submerged macrophytes play a key role in stabilizing the clear-water state. Long-term stability is expected at total phosphorus concentrations < 0.10 mg/l. By proceeding through questionnaires the lake manager can assess the chances for success of biomanipulation.