1. Thirty-six years of winter meteorological and limnological measurements from four lakes in the English Lake District are analysed and related to variations in the North Atlantic Oscillation (NAO). Winter weather conditions were strongly influenced by the NAO with mild, wet winters being associated with strongly positive values of the NAO index (NAOI). 2. Lake surface and bottom temperatures were strongly positively correlated with the NAOI, with the highest correlations being recorded in the shallower lakes. 3. Variations in the NAOI also had a significant effect on the winter concentration of nitrate. In all the lakes, there was a significant negative correlation between the NAOI and the detrended winter concentration of nitrate. The key driving variable was the local air temperature, which appeared to limit the quantity of nitrate reaching the lake by increasing the amount assimilated in the surrounding catchment in mild winters. 4. Dissolved reactive phosphorus (DRP) concentrations were not significantly correlated with the NAOI in the two larger basins but significant positive correlations were recorded in the two smaller lakes. The key driving variable was the local rainfall with higher DRP concentrations being recorded after heavy rain in the lakes with a short retention time. 5. The NAOI-related changes in rainfall also influenced the phytoplankton. In wet winters the concentration of chlorophyll in the two smaller lakes with the shortest retention time was lower and the spring growth of Asterionella formosa was delayed in the smallest lake. 6. These differential responses demonstrate how the large-scale effects associated with the NAO can be 'filtered' by the physical characteristics of a particular site.

1. The factors influencing the seasonal and inter-annual variations in the numbers of Daphnia hyalina in Esthwaite Water between 1956 and 1972 are analysed. Esthwaite Water has always been eutrophic, but the phosphorus and nitrogen loadings to the lake increased significantly in the mid 1960s.2. Qualitatively, the phytoplankton and zooplankton populations in the lake changed relatively little during the period of study. Quantitatively, however, eutrophic species of algae became more abundant and the numbers of Eudiaptomits declined as the numbers of Daphnia increased.3. The seasonal dynamics of the Daphnia was governed partly by the seasonal temperature cycle, and partly by the periodicity of edible algae. The birth rate of the Daphnia was constrained by temperature from January to April and from October to December. At other times their rate of increase was governed by the relative abundance of edible and inedible algae.4. Edible and inedible species of algae tended to appear in a recurring annual sequence. Diatoms such as Asterionella were abundant in the spring, the early summer phytoplankton was dominated by edible flagellates, but inedible algae such as Aphanizomenon and Microcystis become dominant later in the year.5. Daphnia could only reproduce in late summer when there were periodic regrowths of edible algae. Such regrowths were most likely to occur when there had been some entrainment of deep nutrients by episodic wind mixing. Calm weather encouraged the growth of blue-green algae that effectively 'blocked' the development of the Daphnia for the remainder of the summer.6. The factors that controlled the seasonal dynamics of the Daphnia also influenced the average number recorded in a particular year. The average number of Daphnia increased in the early sixties when Cryptomonas was abundant and decreased in ihe late sixties when blooms of Aphanizomenon appeared in mid summer. Detailed analyses showed that a similar increase in the numbers of Aphanizometion had occurred in the late fifties. The critical factor throughout was the prolonged period of calm resulting in stable stratification. 7, This 'weather' effect was highlighted by comparing de-trended timeseries of Daphnia and Aphanizometion numbers with a simple measure of thermocline stability. De-trending removed the superimposed effects of progressive enrichment and revealed a 10-year cycle of thermocline stability that matched the temperature cycle recently reported in Windermere, These cycles are related to the movement of weather systems in the Atlantic so could change if the pattern of atmospheric circulation is altered by global warming.8. The possible effects of climate change on Daphnia dynamics are discussed in relation to the findings in Esthwaite Water.

1. The larger lakes of the English Lake District have been the subject of intensive scientific study for more than 60 years. Year-to-year variations in the weather have recently been shown to have a major effect on their physical characteristics. The area is mild but very wet and the dynamics of the lakes are strongly influenced by the movement of weather systems across the Atlantic. 2. Here, we combine the results of long-term measurements and the projections from a Regional Climate Model (RCM) to assess the potential impact of climate change on the surface temperature and residence times of the lakes. 3. The RCM outputs used were produced by the U.K. Hadley Centre and are based on the IPCC 'A2' scenario for the emission of greenhouse gases. These suggest that winters in the area will be very much milder and wetter by the 2050s and that there will be a pronounced reduction in the summer rainfall.4. An analysis of the meteorological data acquired between 1940 and 2000 shows that there have been progressive increases in the winter air temperature and in the rainfall which are correlated with the long-term change in the North Atlantic Oscillation. The trends reported during the summer were less pronounced and were correlated with the increased frequency of anticyclonic days and a decrease in the frequency of westerly days in the British Isles. 5. A simple model of the year-to-year variations in surface temperatures showed that the highest winter temperatures were recorded in the deeper lakes and the highest summer temperatures in the lakes with the shallowest thermoclines. When this model was used to predict the surface temperatures of the lakes in the 2050s, the greatest winter increase (+1.08°C) was observed in the shallowest lake and the greatest summer increase (+2.18°C) in the lake with the shallowest thermocline. 6. The model used to estimate the seasonal variation in the residence time of the lakes showed that the most pronounced variations were recorded in lakes with a short residence time. Average winter residence times ranged from a minimum of 10 days to a maximum of 436 days and average summer values from a minimum of 23 days to a maximum of 215 days. When this model was used to predict the residence time of the lakes in the 2050s, the greatest winter decrease ()20%) was observed in the smallest lake and the greatest summer increase (+92%) in the lake with the shortest residence time. 1647 7. The results are discussed in relation to trends reported elsewhere in Europe and the impact of changes in the atmospheric circulation on the dynamics of the lakes. The most serious limnological effects were those projected for the summer and included a general increase in the stability of the lakes and a decrease in the flushing rate of the lakes with short residence times.

Perch Perca fluviatilis recruitment (based on the catch per unit effort, CPUE, of 2 or 3 year male perch) varied greatly from 1941 to 1995 in Windermere, U.K., with year class strengths strongly synchronous between the two basins of this temperate, mesotrophic lake. Statistically significant modified Ricker (Saila‐Lorda) stock‐recruitment curves could be fitted to data from three of four sampling sites, while the presence of widespread disease and late summer water temperature were also important explanatory variables. Multiple‐regression analyses revealed no influences of pike Esox lucius recruitment, zooplankton abundance, or the climatic influences (i.e. North Atlantic Oscillation and displacement of the summer position of the Gulf Stream) on perch recruitment over and above associated influences from temperature. Fitted models realistically explained up to c. 60% of the observed variation in perch recruitment.

IntroductionIn a recent report, the Inter-governmental Panel on Climate Change (IPCC 1996) suggested that average winter temperatures in the UK could be 2-3°C higher than they are today by the year 2040. The planktonic micro-crustacea found in lakes can tolerate quite high summer temperatures, but small increases in the winter temperature can have a significant effect on their seasonal dynamics. One of the most important factors influencing the winter survival of these short-lived animals is the rate at which they acquire and store energy. In winter, their food requirement is controlled by the temperature of the water but the growth rate of their phytoplankton food is limited by light (Talling 1971). Mild winters could therefore favour the survival of species that require less food or those that are less susceptible to periods of starvation. The overwintering performance of the micro-crustacea can also be influenced by qualitative as well as quantitative changes in the phytoplankton community. Such qualitative changes can be influenced by a number of factors (Reynolds 1984) but one of the most important is the frequency and intensity of wind-induced mixing. For example, the large diatoms that dominate the winter phytoplankton grow best in a well-mixed water column but the flagellates that serve as an important source of food for many filter feeders require more stable mixing conditions.In this paper, we analyse some of the physical and biological factors that influence the winter abundance of the cladoceran Daphnia hyalina var lacustris Sars and the copepod Eudiaptomus gracilis Sars. These two species are functionally very different and have evolved different physiological strategies for coping with life in a rapidly changing environment. Daphnia hyalina is particularly common in large lakes and feeds by filtering small species of phytoplankton from the water with an array of rapidly beating limbs. Eudiaptomus gracilis is found in a wider range of habitats and feeds by actively grasping large particles and passively filtering small particles by low frequency movements of its feeding appendages. In this paper, we analyse some of the physical and biological factors 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 numbers 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 effect on their subsequent development but the numbers of overwintering Daphnia 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 ai...