CONSEQUENCES OF FALL WARMING FOR ZOOPLANKTON OVERWINTERING SUCCESS

Increased temperatures in fall as predicted by recent climate models w ill have important consequences for the overwintering strategies and s easonal phenologies of organisms in temperate lakes. We compare fall t hermal responses of species representatives of the copepod Epischura l acustris and the cladoceran Daphnia catawba. Daily temperatures and we ekly population densities were measured in a lake where these species co-occur, and demographic variables for each species (survival, reprod uction, egg hatching, and production of resting stages) were assessed over a temperature range of 15 to 30 degrees C in the laboratory. This study provides three types of evidence that the success of these spec ies could be altered by fall warming events (temperatures > 15 degrees C). First, resting eggs of E. lacustris could be stimulated to hatch from the ''seed bank'' in fall. If so, they would be unable to reach m aturity before winter, and the cohort could be lost. Second, sexually reproducing D. catawba populations in fall could be induced to switch from sexual to asexual reproduction. This switch could delay productio n and reduce the number of ephippial resting eggs. Third, on the basis of thermal tolerances, E. lacustris is representative of a species wh ose geographic range is likely to be altered by warming at any time. O ver the long term, the disruption of overwintering strategies by warmi ng in fall could be a mechanism that alters the geographic ranges of s pecies.