Is the midsummer decline of Daphnia really induced by age-0 fish predation? Comparison of fish consumption and MDaphnia mortality and life history parameters in a biomanipulated reservoir

We analysed the temporal pattern of a Daphnia galeata population and the development of the age-0 Ash community in a long-term biomanipulated lake (Bautzen reservoir, Germany) during spring and early summer of two successive years. In Bautzen reservoir, the age-0 Rsh-Daphnia interaction is a key process within the food web due to the low abundance 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 parameters of Daphnia that may indicate predation impact by fish. A midsummer decline of daphnids occurred only in July of 1996, whereas in 1995 the Daphnia biomass remained >4 mg wet weight 1 −1 for the entire summer. The percentage of total Daphnia mortality due to fish predation before the onset of the midsummer decline was −2% day −1 . Temporal patterns of individual size, clutch size and size at maturity also indicated that the 1996 midsummer decline of daphnids was not the exclusive consequence of age-0 fish predation. Instead, low reproductive capacity of daphnids also contributed significantly to the decline. Consequently, year-to-year variation of the Daphnia dynamics may be determined by a fine-tuned ‘timing’ between the period of reduced fecundity and the time of the strongest predation impact by age-0 fish.