Initiation of the midsummer decline of Daphnia as related to predation, non-consumptive mortality and recruitment:: a balance

The midsummer decline (MSD) of daphnids has been attributed to a range of factors related to either resource availability or predation. However, the relative magnitude and importance of each of these factors remained unknown. Therefore, we quantified simultaneously, but independently, both consumptive mortality due to predation and non-consumptive mortality of Daphnia galeata due to senescence, disease, food limitation as well as limiting physical or chemical factors between May and July 1998 in Bautzen Reservoir, Germany. These losses were balanced by the population gains by recruitment. The predation pressure exerted by age-0 fish, by older zooplanktivorous fish and by carnivorous zooplankton did not completely account for the observed mortality pattern of D. galeata that caused the MSD. Non-consumptive mortality in the field was calculated from recruitment patterns estimated from field samples and from age-specific survival probabilities of Daphnia offspring according to life-table experiments. The results suggest a high non-consumptive mortality during the clear-water phase which is comparable to the magnitude of consumptive losses and may give a possible explanation for the remaining part of Daphnia mortality. By integrating all data concerning the initial abundance in early May as well as population gains and losses of daphnids, a balance sheet was drawn up for four years of investigation (1995-1998). The differences between the cumulated positive and negative book values corresponded well to the actually observed Daphnia abundance in Bautzen Reservoir at the transition period to an MSD of every year. Generally, the hypothesis is supported that an MSD should be expected if the period with bottom-up limitation during the clear-water phase overlaps strongly with top-down effects whereby in detail the following combination of conditions should be fulfilled: (i) a high biomass of age-0 percids in May/June, (ii) an early start of Daphnia consumption by carnivorous zooplankton, (iii) a quick increase in Daphnia density leading to the dominance of a peak cohort during the clear-water phase and (iv) a long lasting clear-water period characterized by reduced longevity and low recruitment of Daphnia.