Adult, not juvenile mortality as a major reason for the midsummer decline of a Daphnia population

We analysed dynamics and structure of a Daphnia galeata population prior to and during a midsummer decline of this species in Bautzen reservoir (Saxon y, Germany). Patterns of juvenile and adult mortality were determined by co mbining field data with laboratory estimates of juvenile growth. After esta blishing high densities, fecundity and recruitment of D. galeata markedly d ecreased, whereas size at maturity was high. Immediately before the populat ion decline, adult mortality increased and remained high even after the dec line, whereas juvenile mortality was low during the whole investigation per iod, and virtually absent after the decline. We conclude that the successio n of events leading to a midsummer decline of Daphnia is as follows. (i) A quick increase in Daphnia abundance leads to the formation of a strong 'pea k cohort' of about the same age. (ii) During the clear-water phase, food co nditions deteriorate, fecundity declines and hence, recruitment is low. Juv enile mortality during this period is low, but present. (iii) Adult mortali ty increases when the 'peak-cohort' reaches its mean life-span, which is re duced due to interactions between age-specific and starvation-induced morta lity. At this point, Daphnia population dynamics can no longer be explained without the onset of size-selective predation. Hence, the timing between e nhanced mortality due to senescence on the one hand and predation on the ot her hand, both directed towards adult daphnids, may be decisive for the ini tiation of a midsummer decline of Daphnia.