A whole-lake biomanipulation experiment (lasting since 1981) in Bautzen reservoir (Saxony) has led to a high ratio of piscivorous to planktivorous fish and thereby to extended periods (including winter) characterised by high biomasses of DAPHNIA GALEATA. One of the most important requirement for the success of biomanipulation in improving the water quality is to establish the maximum possible grazing pressure of daphnids. The aim of this study was to analyse the filtration and ingestion rates of D. GALEATA under the hypertrophic conditions of the long-term biomani-pulation experiment. Furthermore, the structure-forming impact of grazing on phytoplankton as well as the feedback on DAPHNIA feeding behaviour was under investigation. The control mechanisms of the feeding behaviour were determined by a combination of all-year in-situ experiments, microscopic obser-vations of filtering daphnids and laboratory experiments from 1994 to 1996.

From in-situ experi-ments it could be concluded that D. GALEATA during periods of mass development (in May and June) filtered the total water body of the Bautzen reservoir up to 1.4 times per day. Contrary to this effect, which indicates a successful biomanipulation, periods characterised by very low feeding rates were predominant during the remaining part of the year. The water temperature limited the filtration and ingestion rate of D. GALEATA only below a threshold of 10°C. An inhibition of the ingestion rate of D. GALEATA (up to 40 %) was caused by dissolved photosynthesis products released by naturally occurring phyto-plankton. Furthermore, low ingestion rates during the summer period resulted from a dominance of hardly or non-ingestible phytoplankton species or from very high levels of any particles. If freely suspended cells or small colonies of the cyanobacterium PSEUDANABAENA CATENATA occurred, which are of appro-priate size for food uptake, a very strong inhibition of the feeding rates was determined. Other results show that D. GALEATA of equal body length may change the upper size limit of particles to be ingested over a wide range during the annual succession (between 6 and 52 µm). This change is achieved by adapting the width of the mouth in order to ensure an optimum use of the actual particle spectrum in spite of fast altera-tions of food conditions which are typical under hypertrophic conditions. Only under conditions of low inhibition of the ingestion and filtration rate, the grazing impact by D. GALEATA controls the biomass of the edible autotrophic fraction and temporarily influences the primary production. The response of the phytoplankton community to such a high grazing pressure was a shift towards a dominance of phytoplankton species which are hardly or non-ingestible or non-digestible by D. GALEATA. Therefore, biomanipulation in a hypertrophic lake seems to be effective only if it can be combined with a reduction of the phosphorus load.