The effect of nutrient enrichment and changes in the weather on the abundance of Daphnia in Esthwaite Water, Cumbria

1. A simple counting technique has been used to produce a long-term (19561991) record of the seasonal variations in the abundance of Daphnia in Esthwaite Water, a productive lake in the English Lake District. For most of this period, there was a progressive increase in the winter concentration of phosphorus and a gradual increase in the temperature of the lake. 2. The seasonal variations in the abundance of Daphnia followed the same pattern throughout the period with a strong cohort (C1) produced in early summer and a weaker cohort (C2) in late summer. 3. The main factors regulating the strength of the C1 cohort were the trophic status of the lake and the physical stability of the water column. The gradual decline in the strength of this cohort is attributed to the combined effects of enrichment and warming on the growth of the edible algae that follow the Asterionella decline. The timing of the Asterionella decline has advanced in recent years, but there has been no comparable change in the timing of the Daphnia maximum. 4. The only factors to have a significant effect on the strength of the C2 cohort were the stability of the water column and the depth of the thermocline. The Daphnia were never abundant when the lake was strongly stratified since these conditions favoured the growth of the cyanobacteria Microcystis and Aphanizomenon. 5. The ultimate factor regulating the strength of the C2 cohort was the year-to-year variations in the weather type that dominates the U. K. during the summer. Daphnia numbers were low when the synoptic situation was dominated by high pressure i. e. when there was a marked increase in the frequency of calm, anticyclonic days. 6. The results are discussed in relation to the climatic changes projected for the area at the end of the 21st century. The greatest uncertainties are those connected with mesoscale changes in the circulation of the atmosphere and the consequent shift in the trajectory of storms across the Atlantic.