Direct and indirect effects of predation on the genetic structure of a Daphnia population

The effects of fish predation on zooplankton communities are well documented, but relatively little is known about how predation structures the genetic composition of individual populations. This study illustrates how a perturbation in the timing and strength of predation by rainbow trout directly and indirectly altered the genetic composition of a Daphnia pulicaria population in a Minnesota (USA) lake. Trout were stocked in autumn in the first 2 years of the study and in spring in the second 2 years. In autumn stocking years, predation was highest over winter and in spring but relatively low during summer stratification. In contrast, in spring stocking years, predation was low over winter and high during summer stratification. In all years, the Daphnia population became genetically differentiated with respect to depth, as summer stratification became more pronounced. In addition, allozyme analyses of trout stomach contents revealed selective predation on Daphnia in the metalimnion. In spring stocking years, when trout were abundant during the summer, this directly caused a shift in the dominant clone type from a metalimnetic to a hypolimnetic specialist. The fisheries manipulation indirectly affected the genetic composition of the Daphnia population by altering the importance of recruitment from diapausing embryos in the sediment (the egg bank). In autumn stocking years, when the over-wintering population was small, genotype frequencies in early summer indicated the recent emergence of sexually derived individuals from the egg bank. Conversely, in spring stocking years when over-wintering populations of Daphnia were large, no emergence events were detected.