The impact of regional-scale changes in the weather on the long-term dynamics of Eudiaptomus and Daphnia in Esthwaite Water, Cumbria

1. Year-to-year changes in the weather have a profound effect on the seasonal dynamics of zooplankton in lakes. Here, I analyse some zooplankton data from Esthwaite Water in Cumbria and demonstrate that much of the recorded inter-annual variation can be related to regional-scale changes in the weather. 2. The first data set analysed shows the effect of changes in the water temperature on the winter abundance of the calanoid copepod Eudiaptomus gracilis. The highest numbers of Eudiaptomus were recorded when the winters were mild and the lowest when the winters were cold. 3. Winter temperatures in northern and western Europe are now known to be influenced by the atmospheric feature known as the North Atlantic Oscillation (NAO). Positive values of the NAO are associated with mild winters and westerly winds and there was a significant positive correlation between the winter abundance of Eudiaptomus in Esthwaite Water and this empirical index of change. 4. The second data set analysed shows the effect of wind-induced mixing on the summer abundance of Daphnia. The highest numbers of Daphnia were recorded in years when the early summer thermocline was deep and the lowest number in years when the thermocline was shallow. 5. One of the most important factors influencing the depth of the early summer thermocline in the English lakes is the position of the north-wall of the Gulf Stream. Southerly movements of the Gulf Stream are typically associated with higher winds, whilst northerly movements are associated with stable conditions. In Esthwaite Water, a significant positive correlation was recorded between the abundance of Daphnia and the depth of the early summer thermocline and significant negative correlations between the same variables and the position of the Gulf Stream. 6. A detailed analysis of the seasonal variations recorded in one calm and one windy year suggest that the main factor responsible for these correlations was the entrainment of nutrients which then stimulated the growth of edible algae. Daphnia numbers were low in 1968 (a 'north' Gulf Stream year which was relatively calm) and high in 1972 (a 'south' Gulf Stream year with intense wind-mixing).