The combined effect of bottom-up and top-down factors on life history and reproduction of Daphnia in the field: is a strategic dilemma underlying population declines?

In a detailed field study covering three years, population dynamics, life history shifts and reproductive patterns of a population of Daphnia galeata were investigated in relation to food availability and dynamics of young of the year fish, the main vertebrate predators. In all years an increase of Daphnia abundance in spring was associated with declining food conditions (clear water stage). The size at maturity (SAM) during this period was high and even increased, brood size declined, while egg volume increased. These patterns may be explained as response to food limitation. A decline of Daphnia abundance in every year was associated with increasing food conditions and the end of the clear water stage. Egg volumes as well as the size of egg-carrying daphnids also decreased, while brood size remained at low values. In two years in which these changes were particularly pronounced, the available fish data suggest that both the biomass (determining predation pressure), as well as the size of the fish (determining size selection) are important for the observed dynamics. No decrease of SAM occurred as long as fish were smaller than 25 mm TL, when they are still gape-limited and not able to feed on the largest size-classes of Daphnia. Although fish biomass, which should correspond to kairomone level, would have suggested a reduction of SAM as induced defence, probably the selection of small (egg-carrying) daphnids by small fish, besides severe food limitation, prevented this response to become effective. The sudden decline of SAM at the end of the clear water stage may not be explained by gradual phenotypic responses or indirect demographic effects, but must be due to an alternation of generations. SAM may be further reduced by direct and indirect effects of predation, interacting with increasing food levels. The Daphnia population is most vulnerable to predation at the time when the new generation takes over. Consequently, predation impact depends on timing of both age-0 fish consumption and feeding selectivity.