Meteorological forcing of plankton dynamics in a large and deep continental European lake

Abstract

The timing of various plankton successional events in Lake Constance was tightly coupled to a largescale meteorological phenomenon, the North Atlantic Oscil- lation (NAO). A causal chain of meteorological, hydrological, and ecological processes connected the NAO as well as winter and early spring meteorological conditions to planktonic events in summer leading to a remarkable memory of climatic effects lasting over almost half a year. The response of Daphnia to meteorological forcing was most probably a direct effect of altered water temperatures on daphnid growth and was not mediated by changes in phytoplankton concentrations. High spring water temperatures during high-NAO years enabled high population growth rates, resulting in a high daphnid biomass as early as May. Hence, a critical Daphnia biomass to suppress phytoplankton was reached earlier in high-NAO years yielding an early and longerlasting clear-water phase. Finally, an earlier summer decline of Daphnia produced in a negative relation- ship between Daphnia biomass in July and the NAO. Meteorological forcing of the seasonal plankton dynamics in Lake Constance included simple temporal shifts of processes and successional events, but also complex changes in the relative importance of different mechanisms. Since Daphnia plays an important role in plankton succession, a thorough understanding of the regulation of its popu- lation dynamics provides the key for predictions of the response of freshwater planktonic food webs to global climate change.