In the framework of a joint 3-year biomanipulation programme carried out in different lake types, factors influencing the development of the zooplankton community in a stratified eutrophic lake was assessed. ARIMA intervention analysis was used to estimate long term effects and the effect of reduced planktivorous fish predation. A new type of in situ enclosure experiments was used to test if gape-limited predation by the invertebrate predator CHAOBORUS FLAVICANS could compensate size-selective predation by fish and whether the lack of large herbivores in Plussee could be attributed to strong fish predation. In 2 x 2 factorial experiments carried out in 2 different seasons of 3 successive years the differential impact of invertebrate predation and competition by large-sized laboratory-grown DAPHNIA on the plankton was tested.

The intervention analysis showed that the calanoid EUDIAPTOMUS and the rotifer ASPLANCHNA increased within the last decade whereas the cladocerans DIAPHANOSOMA and CERIODAPHNIA decreased in number. The reduced fish predation during the biomanipulation period did not result in an increase of DAPHNIA number as predicted by the Size Efficiency Hypothesis. However the number of the invertebrate predators CHAOBORUS and LEPTODORA increased as expected according to the Cascading Trophic Interactions Hypothesis. Despite the fact that DIAPHANOSOMA was a preferred prey of CHAOBORUS in the enclosure experiments, they increased in number in the course of the biomanipulation using temporal windows.

In the enclosure experiments CHAOBORUS was important for the structure and size distribution of the plankton. The large sized laboratory grown DAPHNIA survived in the enclosures. They were reduced by CHAOBORUS while the biggest naturally occurring daphnid D. LONGISPINA was least influenced among the cladocerans. The strongest impact of CHAOBORUS was found on small sized cladocerans like DIAPHANOSOMA and CERIODAPHNIA. The competition effect of the large DAPHNIA was low. The impact of CHAOBORUS on the zooplankton cascaded down to the phytoplankton when the predator prey pair ASPLANCHNA and CERATIUM was present. In the absence of CHAOBORUS invertebrate predation could be taken over by CYCLOPS.

It is generally believed that the impact of fish and invertebrates on zooplankton differs in that fish causes a shift to smaller body sizes whereas invertebrates do the opposite. This was not the case for the different cladoceran taxa in Plussee. The influence on size distribution and life history parameters depended on how the preferred prey sizes of the predators related to the possible body size range of the prey.