IS THE MIDSUMMER DECLINE OF DAPHNIA REALLY INDUCED BY AGE-0 FISH PREDATION - COMPARISON OF FISH CONSUMPTION AND DAPHNIA MORTALITY AND LIFE-HISTORY PARAMETERS IN A BIOMANIPULATED RESERVOIR

We analysed the temporal pattern of a Daphnia galeata population and t he development of the age-0 fish community in a long-term biomanipulat ed lake (Bautzen reservoir, Germany) during spring and early summer of two successive years. In Bautzen reservoir, the age-0 fish-Daphnia in teraction is a key process within the food web due to the low abundanc e of adult zooplanktivorous fish. Daily consumption of daphnids by age -0 fish, as estimated by a bioenergetics model, was compared to daily mortality rates of daphnids. In addition, we estimated life history pa rameters of Daphnia that may indicate predation impact by fish. A mids ummer decline of daphnids occurred only in July of 1996, whereas in 19 95 the Daphnia biomass remained >4 mg wet weight l(-1) for the entire summer. The percentage of total Daphnia mortality due to fish predatio n before the onset of the midsummer decline was similar to 2% day(-1). Temporal patterns of individual size, clutch size and size at maturit y also indicated that the 1996 midsummer decline of daphnids was not t he exclusive consequence of age-0 fish predation. Instead, low reprodu ctive capacity of daphnids also contributed significantly to the decli ne. Consequently, year-to-year variation of the Daphnia dynamics may b e determined by a fine-tuned 'timing' between the period of reduced fe cundity and the time of the strongest predation impact by age-0 fish.