Lake origin determines Daphnia population growth under winter conditions

In large oligotrophic lakes, growth rates of zooplankton populations decline to low or negative values in winter as a result of low food concentration and water temperature. Daphnia, a key species in the aquatic food web, has two strategies to overwinter: either by sexual reproduction resulting in diapause or as asexual clones in the open water. We investigated how asexually overwintering Daphnia clones, originating from different taxa and lakes with different trophic state, survive under winter conditions. We performed a laboratory experiment exposing D. hyalina and D. hyalina×galeata clones to low water temperature (5°C) and either no or low food (<0.1 mg C L−1) conditions. We used clones from three pre-alpine lakes in Switzerland with contrasting trophic state: ultra-oligotrophic Lake Brienz, mesotrophic Lake Constance and eutrophic Greifensee. Our results show that Daphnia clones can withstand starvation for an average of 6 weeks under low food concentration and for almost 2 weeks under complete absence of food. Besides strong clonal variation, we found a significant effect of food concentration and its interaction with lake origin on population growth rate. Our study indicates that adaptation to local winter conditions is a key factor in defining the clonal, but not necessarily the taxonomic composition of a Daphnia population in unproductive lakes