The significance of nonviable eggs for Daphnia population dynamics

Egg mortality was studied in populations of Daphnia galeata, Daphnia cucullata, and the hybrid between these species. In Tjeukemeer, a shallow eutrophic lake in the Netherlands, egg mortality in daphnids manifested itself as an apparent increase in the frequency of eggs in the early developmental stages. Close examination of these eggs revealed that many of them stopped their development. Egg mortality was observed during a 1-month period in autumn 1990 and for about 2 months in spring 1991. The percentage of females carrying nondeveloping eggs averaged 20% (SD = 11%) in the 1990 period and 35% (SD = 20%) in spring 1991, resulting in an average reduction of the birth rate of 0.043 (SD = 0.027) and 0.040 (SD = 0.030). The maximum incidence of nondeveloping eggs was as high as 70%. Because it is difficult to recognize nondeveloping eggs, little is known about the cause of this arrested development. Our results suggest that food quantity was not an important factor and that egg mortality was caused by nutritional deficiency of the lake seston in certain periods of the year. Egg mortality can have serious consequences for the population development of daphnids, and it might be a more common phenomenon than is supposed. An analysis of zooplankton population dynamics is essential to properly assess the viability of eggs, since ignoring egg mortality will cause both birth and death rates to be overestimated.