Ecophysiological responses to warming events by two sympatric zooplankton species

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