1. Cyclical parthenogens, organisms switching between sexual and asexual mode of reproduction, are common in aquatic habitats. In permanent waterbodies with year-round persistence of populations, the relevance of sexually produced resting eggs for population dynamics and structure is still largely unexplored. 2. The aim of this study was to investigate the importance of recruitment from resting eggs to Daphnia spp. in a deep dimictic reservoir with overwintering populations (Saidenbach Reservoir, Germany). We linked field studies on Daphnia population dynamics, hatching rates and diapausing egg abundances in the sediment with allozyme analysis of the genetic variability of both the pelagic population and hatchlings from resting eggs. 3. We found higher hatching rates at intermediate depths (10-18 m), although highest diapausing egg abundances were found in the deeper parts of the reservoir. From the extensive field data, we calculated a population budget for the main basin of the reservoir, which showed that hatched daphnids accounted for <1% of the total abundance and explained only a tiny fraction of population growth. 4. The genetic composition (MultiLocus Genotypes, MLG) of the pelagic population was relatively stable throughout the stratified period with up to five among 39 detected MLGs attaining frequencies >5% per date. From 11 MLGs differentiated among hatched daphnids, three were recovered in the pelagic population in perceptible, albeit low frequencies (1-4%). 5. We conclude that the contribution of hatched animals to the spring population dynamics is negligible in situations with an overwintering population present and will only become relevant if densities are extremely low. In this type of system, the significance of the resting egg bank is presumably mainly related to the maintenance of genetic diversity.

Summary 1. Winter conditions shape plankton dynamics and community composition in temperate regions, but their effect on dynamics and genetic composition of cyclical parthenogens like Daphnia is largely unclear. 2. For 5 years, we studied the dynamics, hatching from resting eggs and genetic structure of a D. galeata × longispina hybrid complex in a dimictic, temperate reservoir. Our main hypothesis was that higher spring densities and an earlier population peak will be observed after warmer winters, with a lower genetic diversity because of a lower contribution of resting eggs to population growth. 3. The study period could clearly be categorised into cold‐winter years (n = 3) and warm‐winter years (n = 2). Daphnia densities at the end of spring overturn were ∼10‐fold lower after cold winters than after warm ones, but no pattern emerged concerning the timing and the height of the population peak in early summer. 4. Hatching intensity from resting eggs was higher and contributed up to 8.5% to Daphnia abundance in a cold‐winter year compared to a negligible contribution in a warm‐winter year. Consistent with this finding, new multilocus genotypes (MLGs) adding to the overwintering stock after the end of spring overturn and presumably originating from resting eggs increased genetic diversity and attained high frequencies within the population only after a cold winter. New MLGs were recorded also after warm winters, but they never gained dominance and no shift in genetic diversity was observed. However, genetic diversity was not generally reduced after warm winters. 5. Our results confirm earlier findings that winter conditions have only a limited effect on the main growth phase and the peak of Daphnia during late spring and early summer. However, winter conditions determine the contribution of resting eggs to the population development, which may profoundly alter the genetic composition of the population compared to the previous season.

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1. Our aim was to analyse the impact of zooplankton dynamics on the relative importance of two mechanisms contributing to the loss of phosphorus (P) from the epilimnion of stratified lakes: net population incorporation into zooplankton biomass and P sedimentation. 2. We established enclosures without Daphnia (control), with a growing Daphnia population (treatment D) and with a high, stable Daphnia population (treatment D+). The P incorporated in zooplankton biomass and sedimented was measured at short intervals over a period of 17 days. 3. In both Daphnia treatments, sedimentation increased and the P content of sedimented matter was higher than in the control and highest in D+. The P loss by sedimentation between day 3 and 17 was generally high (>25%, >1.8% day )1 ) with particularly high values in D+ (60%, 4.3% day )1 ). Phosphorus sedimentation was higher in the zooplankton treatments, although the contribution of exuviae and dead Daphnia was minor. Faecal material was probably a major component of sedimentation. 4. By contrast, the amount of P in zooplankton (mainly Daphnia) biomass increased in D but remained constant in D+. Phosphorus loss owing to net population incorporation was generally low and ranged up to 6.5% (0.5% day )1 ) in treatment D. A positive relationship between Daphnia dry mass and P sedimentation, as well as P incorporation, was found. 5. Sedimentation is evidently an important cause of P loss from the epilimnion where Daphnia is abundant. By contrast, P incorporation into Daphnia biomass may only become an important loss factor when the population is growing.