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 (Saxony, Germany). Patterns of juvenile and adult mortality were determined by combining field data with laboratory estimates of juvenile growth. After establishing high densities, fecundity and recruitment of D. galeata markedly decreased, whereas size at maturity was high. Immediately before the population decline, adult mortality increased and remained high even after the decline, whereas juvenile mortality was low during the whole investigation period, and virtually absent after the decline. We conclude that the succession 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 'peak cohort' of about the same age. (ii) During the clear-water phase, food conditions deteriorate, fecundity declines and hence, recruitment is low. Juvenile mortality during this period is low, but present. (iii) Adult mortality increases when the 'peak-cohort' reaches its mean life-span, which is reduced due to interactions between age-specific and starvation-induced mortality. At this point, Daphnia population dynamics can no longer be explained without the onset of size-selective predation. Hence, the timing between enhanced mortality due to senescence on the one hand and predation on the other hand, both directed towards adult daphnids, may be decisive for the initiation of a midsummer decline of Daphnia.