Climate-driven changes in spring plankton dynamics and the sensitivity of shallow polymictic lakes to the North Atlantic Oscillation

Climate is increasingly recognized as a major factor driving long-term changes in plankton communities in both marine and limnetic ecosystems. In a shallow and polymictic lake, marked changes in the dynamics of phytoplankton and in the timing of distinctive successional events in spring were observed during two recent decades. We studied whether and how these changes were linked to a series of uncommonly warm winter and spring seasons, focusing on the predictive power of a macroscale atmospheric circulation pattern, the North Atlantic Oscillation (NAO). In the warm period 1988-1998, phytoplankton developed about I month earlier than in the cool period 1979-1987, and high total phytoplankton biomass was recorded in early spring. These changes were significantly related to the NAO, which explained about 35% of the variance, and can be attributed to a shortening or lack of ice cover periods in winter Zooplankton (Keratella, Bosmina, Daphnia) developed about 2 weeks earlier in the warm period, consequently leading to an early establishment of the clearwater phase. The shift in the timing of the daphnid peak and of the clearwater phase required an additional warming trend in late April and early May and was net significantly correlated with the winter situation The memory effect of the NAO was restricted to a rather short postwinter period and was overtaken by the prevailing weather in April. Our results suggest that assessments of potential impacts of climate warming an biological processes in freshwater ecosystems demand basic knowledge about the relationships between global climate indices, local meteorological conditions, and the thermal response of various lake types.