Ecophysiological responses to warming events by two sympatric zooplankton species

Under current climate change scenarios, numerous lakes are predictedto experience increases in maximum temperatures and in the timing,frequency and duration of heat waves. To investigate the effectsof the magnitude and timing of thermal stress on zooplanktoncommunities we compared the thermal responses and seasonal phenologiesof two sympatric species, Epischura lacustris and Daphnia catawba.Thermal responses were measured in the laboratory as differencesin demography of field animals at temperatures characteristicof temperate lakes in spring, summer and autumn (15–30°C).Epischura lacustris had a significantly lower temperature tolerancethan D. catawba, which appears to explain differences in theirseasonal phenologies. Divergence in thermal tolerances and seasonalphenologies of these sympatric species strongly suggests thatthe response of the zooplankton community in this lake to warmingevents will differ greatly with the timing and severity of thethermal stress. Similar effects are to be expected in otherlakes containing these species and other zooplankton communities.