Costs of Daphnia diel vertical migrations under forecasted increase of lake temperature

The aim of our work was to experimentally test the hypothesis that changes in the thermal stratification in lakes of the temperate zone, expected as a result of progressive global warming, will cause an increase in fitness costs in a planktonic cladoceran Daphnia, associated with diel vertical migrations. In a programmable thermostatic chamber, Daphnia magna have been cultured from the first juvenile instar till the first reproduction, in thermal regimes corresponding to the conditions experienced by non-migratory (remaining in epilimnion day and night) and migrating (spending nights in epilimnion and days in hypolimnion) animals. We exposed Daphnia to thermal regimes: corresponding to the temperatures typically reigning stratified lakes nowadays (20 °C in epilimnion and 8 °C in hypolimnion), and expected at the end of this century (28 °C and 12 °C, respectively). The analysis of life-history parameters confirmed the assumption of an increase in metabolic costs resulting from vertical migration of Daphnia along with the expected increase in temperature differences between epilimnion and hypolimnion, which is a daily anti-predator refuge for migrating animals. This may consequently lead to significant changes in the composition of lake zooplankton communities, i.e., to an increase in the share of non-migrating species with small body sizes.