The response of Daphnia to changes in trophic status and weather patterns: a case study from Lake Constance

Relationships between changes in trophic status and weather patterns and biomass of Daphnia in Lake Constance were analysed by comparing three long-term records conducted at approximately 30-year intervals. These describe the zooplankton of Lake Constance in its oligotrophic (1920-1924), mesotrophic (1952-1962), and finally meso/eutrophic (1979-1995) state. Biomass increased approximately 30-fold from the 1920s to the 1950/60s and 6-fold from the 1950/60s to the 1980/90s. The former increase was evident throughout the season, the latter was restricted to the second half of the year. Daphnid biomass has not yet shown any response to the re-oligotrophication process of Lake Constance during the 1980/90s. Within the period 1979-1995, biomass in May was significantly correlated with the North Atlantic oscillation, whereas the annual average showed no response. The magnitude of inter-annual variation in spring biomass due to climate variability was as large as the long-term response to eutrophication. Because of the key role of daphnids in freshwater ecosystems, even a temporally restricted response to climate forcing might have important consequences for the ecosystem. (C) 1998 International Council for the Exploration of the Sea.