Inter-lake comparisons indicate that fish predation, rather than high temperature, is the major driver of summer decline in Daphnia and other changes among cladoceran zooplankton in subtropical Florida lakes

The mid-summer disappearance of Daphnia and the small size of cladocerans in subtropical lakes have been explained as phenomena caused by high water temperature. Recent experimental and observational studies suggest that instead, fish predation could be the cause. In the present study, we evaluated effects of temperature and fish predation by comparing summer occurrence of Daphnia and seasonality of other cladocerans in subtropical lakes where there was (a) vegetation and a high abundance and diversity of planktivorous fish; versus (b) no vegetation, turbid water, and primarily benthivorous and piscivorous fish. Daphnia disappeared from mid-summer plankton samples at all locations with vegetation, yet it persisted year-round at the turbid vegetation-free location despite identical water temperatures (all lakes were shallow and isothermal). Body size of cladocerans, in general, was twofold lower at vegetated locations, and biomass was threefold lower. Smaller species dominated at the locations with vegetation, especially in summer. Our results demonstrate that in subtropical lakes Daphnia is able to tolerate high summer water temperatures, and suggest that fish predation may be a more important regulator than temperature for cladoceran size, biomass and taxonomic structure.