Swim or rest during the winter - what is best for an alpine daphnid?

Surviving the winter is a special challenge for herbivorous aquatic inverte- brates in the alpine region due to the long period of ice cover and cessation of primary production. Since there can still be open water under the ice in lakes and ponds, aqua- tic invertebrates can be active if they have enough stored resources. Daphnids usually survive the difficult winter period by producing and depositing resting eggs (ephippia) at the end of the summer. Some daphnids in alpine lakes and ponds, however, have a mixed strategy by both producing ephippia and living active in the water during the winter. We ask why these two strategies coexist stably: Do individuals both produce ephippia and stay active during the winter or do they only have resources for one of the strategies? We studied this phenomenon by sampling of Daphnia umbra twice per month from June to September and monthly the rest of the year in an alpine pond at Finse, Norway, 1207 m a. s. l. The species reproduced parthenogenetically in August and early September, with sexual reproduction of ephippia starting in late August. Highest ephippia densities were found in October, with about 10 % females carrying such eggs. The remaining females did not carry eggs but stored large reserves of lipids. During the winter, population size decreased, as did accumulated lipids. In spring, very few winter-active daphnids had survived until the ice melted and active winter survival seemed not to be a successful strategy during the year of study. Accumulating lipid reserves is assumed to be a special adaptation to postpone reproduction until spring which is unusual in daphnids. It is in contrast to their food dependent reproduction tak- ing place during the summer, and it appears paradoxal that those specially adapted active winter survivors almost disappeared just before ice break-up. Most likely due to year to year variation in the conditions under the ice, the relative success of the two strategies varies. Parasitism, predators and available volume of open water under the ice might be the main factors. Ephippia producing females seem to have less lipids than those without ephippia, indicating that the two strategies compete for the animals resources and that the former might have problems with surviving the winter as active animals.