The influence of year-to-year variations in winter weather on the dynamicsof Daphnia and Eudiaptomus in Esthwaite Water, Cumbria

1. Micro-crustacea of the genus Daphnia and Eudiaptomus have evolved differ ent physiological mechanisms for coping with lift: in a rapidly changing en vironment. In this paper, we analyse some of the physical and biological fa ctors influencing the winter abundance of the two species in a small lake ( Esthwaite Water in Cumbria). 2, The results demonstrate that much of the year-to-year variation in their relative abundance can be related to long-term changes in the weather. The highest numbers of Daphnia were typically found in cold, calm winters when small flagellates were relatively abundant. In contrast, the highest numbe rs of Eudiaptomus were found in mild, windy winters when the phytoplankton community was dominated by colonial diatoms. 3, Year-to-year variations in the winter abundance of Eudiaptomus had no ef fect on their subsequent development but the numbers of overwintering Daphn ia had a significant effect on the size of their first spring 'cohort'. The most important factor influencing the overwintering performance of the two species was the water temperature. 4. Winter air temperatures over much of Europe are influenced by the atmosp heric pressure variation known as the North Atlantic Oscillation (NAO). Win ter water temperatures in Esthwaite Water were strongly correlated with thi s empirical index and there was a significant positive correlation between the NAO and the number of overwintering Eudiaptomus.