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Abstract

Lake restoration may be supported by means of "biomantipulation", i.e., the reduction of the stock of planktivorous fish, in order to increase the biomass of herbivorous zooplankton. Thus, zooplankton grazing is the key factor to be known for a successful management. The paper attempts to provide an estimate how much zooplankton is required to significantly reduce the seston concentration. Particle concentration will be reduced when the grazing rate exceeds the growth rate of the particles. In many lakes zooplankton builds up high biomasses at times wehn fish predation is low. Clear-water phases appear wehn the zooplankton biomass reaches "critical" concentrations between 1.5 and 3.0 mg dr.w./m2. The connection between the seasonal cycle of Secchi transparency and the grazing activity is demonstrated by a particular example (Schöhsee). Grazing rates measured in situ may be as high as 200%/day in mesotrophic lakes but are much lower in eutrophic system. The amount of carbon eliminated by a certain biomass of zooplankton is estimated from the metabolic requirements of the grazers. The relationship can be used to estimate the zooplankton biomass needed to balance a given primary production of nannoplankton.