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JOURNAL OF PLANKTON RESEARCH | VOLUME 20 | NUMBER 9 | PAGES 1797-1811 | 1998
© Oxford University Press
research-article |
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
Institut fuür Hydrobiologie, Technische Universitaät Dresden D-01062 Dresden, Germany
1To whom correspondence should be addressed at: Institute of Freshwater Ecology and Inland Fisheries, Department of Biology and Ecology of Fishes, POB 850119, D-12561 Berlin, Germany
Received on February 26, 1998; accepted on May 15, 1998 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 11 for the entire summer. The percentage of total Daphnia mortality due to fish predation before the onset of the midsummer decline was 2% day1. 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.
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