Effects of extreme floods on the Daphnia ephippial egg bank in a long narrow reservoir

Ephippial egg banks are important reservoirs of dormant stages that allow the recovery of Daphnia populations after unfavourable periods. Although the contribution of hatchlings from ephippia to a population in the water column is probably of minor importance in permanent water bodies with a year-round Daphnia persistence, this may differ after major disturbances. In 2006, two 500-year floods hit the long and narrow Vranov Reservoir (Czech Republic), in which we had investigated zooplankton densities and ephippia distribution in the sediment in preceding years. In this study, we evaluated the impact of those extraordinary floods on the population of the Daphnia longispina complex, and particularly on the local dormant egg bank. We considered two alternative hypotheses related to the egg bank: that either the substantial input of new material with the flood completely buried and therefore reset the existing egg bank, or that the sediment including ephippia was redistributed in the reservoir after the flood and dormant eggs could be exposed to hatching stimuli. A year after the floods, we did not observe any sediment layer that would be devoid of ephippia. However, we observed a significant increase in the proportion of empty ephippia and a decrease in the proportion of those containing eggs. We attribute these changes to disturbance of the sediment due to the floods, which caused redistribution of ephippia stored in the sediment and those detached from the reservoir shoreline. Dormant eggs inside ephippia deposited to shallow parts of reservoir after the floods could therefore receive and respond to hatching stimuli. Hatching from ephippia may have contributed to Daphnia recovery after the spring flood; however, a significant proportion of the Daphnia population probably survived the summer flood protected in the epilimnetic refuge of the thermally stratified environment.