Resting eggs of lake-Daphania II. In situ observations on the hatching of eggs and their contribution to population and community structure

SUMMARY. 1. To investigate the contribution of hatchlings from lake- Daphnia resting eggs (ephippia) to population and community structure, hatching was monitored in situ from April to November 1986 in two north German lakes. Hatching traps were placed on the sediments and the incidence, genetic and species composition of ephippial hatchlings determined twice-weekly.
2. Hatching began in April after the thawing of ice. and continued for 3–4 weeks until the lakes stratified. There was no obvious relationship between the onset and duration of hatching and the environmental variables recorded, namely Secchi depth, surface temperature, temperature and oxygen concentration at the sediment-water interface.
3. Among the hatchlings were D. galeata, D. hyalina and their interspecific hybrid.
4. In the lake which contained overwintering animals the number of ephippial hatchlings was approximately one third of the total number of juveniles present in the lake.
5. In the second lake, no adults were recorded during winter and the population was probably founded by ephippial hatchlings alone. There was good agreement, in the short term, between the proportion of each species represented among hatchlings and the subsequent species composition in the lake.
6. The hatchlings were genetically diverse, and alleles were representative of those alleles present in contemporary populations.