Assessing the long term changes in the water quality of the sensitive waters of the Cumbrian lakes

1. The results of biological, chemical and physical analyses undertaken on samples collected monthly during 2007 from the North and South Basin of Windermere, Grasmere, Derwent Water and Bassenthwaite Lake are presented and interpreted in this report. This work continues long-term measurements, of many years duration, although the reduction in sampling frequency from fortnightly to monthly has meant that some comparisons with historic data are difficult. 2. In many of the lakes, surface water temperature was higher than usual in April and May during a period of unseasonably hot weather, and cooler than usual during summer as a result of particularly cool and wet weather. As a result of the different volumes and depths of the lakes, temperature at depth was substantially different among lakes. Temperature at depth in the North Basin of Windermere was about 7ºC all summer while in Bassenthwaite Lake it reached 16.1ºC due to much weaker and intermittent stratification. 3. Oxygen depletion at depth was most marked in the productive and strongly stratified Grasmere with several months of anoxic water at the sediment surface. The weakly stratified Bassenthwaite Lake showed little oxygen depletion as did the relatively unproductive North Basin of Windermere. 4. Alkalinity varied seasonally with the concentration building up during the summer. Based on annual means, and using the categories of the EU Water Framework Directive, Derwent Water, Grasmere and Bassenthwaite Lake would be classified as having low alkalinity (less than 10 g CaCO3 m-3, although Bassenthwaite Lake is very close to the upper boundary) while the two basins of Windermere would be classified as having moderate alkalinity (between 10 and 50 g CaCO3 m-3). There was variation in pH across the lakes because of varying alkalinity. Superimposed on this were episodes of high pH, particularly in the South Basin of Windermere, resulting from high rates of phytoplankton carbon uptake during photosynthesis. 5. Phosphorus is the main nutrient controlling biological productivity in these lakes. Concentrations of total phosphorus are relatively conservative but tended to be lowest in summer and varied over almost a 2-fold range across the five lake basins. Soluble reactive phosphorus, the main form available to phytoplankton, was at or below detection in all lake basins in the summer but varied in the winter according to lake productivity, being highest in productive lakes such as the South Basin of Windermere and lowest in less productive lakes such as Derwent Water. 6. There was a tendency for declining water clarity, quantified by Secchi depth, in all three basins in the Windermere catchment but little change in the two other lakes. The decrease could be linked to greater input of material from the land but there are no data to test this possibility. Secchi depth was very variable seasonally but generally lowest in Bassenthwaite, probably because of the large amount of suspended solids in this lake. 7. All five lakes produced two main maxima of phytoplankton, quantified as the concentration of chlorophyll a, one in the spring and the other in mid- to late-summer. Concentrations were generally lowest in Derwent Water and highest in one of the four other basins depending on the time of year. The spring peak was typically dominated by diatoms such as Asterionella formosa or Aulacoseira subarctica accompanied by a range of different species such as Chlorella sp., Rhodomonas sp. and Chrysochromulina parva. The summer peak was very diverse within a lake and variable among the lakes. 8. There was good evidence for coherence in year-to-year variation among lakes which indicates the importance of the weather in influencing lake function and characteristics, particularly surface temperature. However, temperature at depth, oxygen depletion at depth, secchi depth, chlorophyll a, concentration of total and soluble reactive phosphorus were correlated for at least one pair of lakes. 9. Long term trends were apparent in many of the data. The North Basin of Windermere is showing a worrying decline in water quality, particularly the increasing mean total phosphorus, declining Secchi depth and declining mean and minimum oxygen concentration at depth. The South Basin of Windermere has evidence for declining Secchi depth, but maximum soluble reactive phosphorus is also declining so there are mixed signals as regards water quality. Grasmere is relatively stable, although the maximum Secchi depth has declined. Derwent Water is also stable but there is a trend of increasing maximum concentration of chlorophyll a which is concerning and is presumably linked to the tendency for the phosphorus concentration to increase. Bassenthwaite Lake is the only lake showing signs of improvement: the maximum and mean concentration of total phosphorus has declined as has the maximum concentration of soluble reactive phosphorus, but to date the effect of on other aspects of the ecology of the lake have been marginal. Hopefully, if the current trends on Bassenthwaite continue the lake may be poised for tangible future improvements in water quality. 10. The likely status of the five lakes in terms of the Water Framework Directive were assessed. The North Basin of Windermere is in moderate ecological status for total phosphorus and chlorophyll a. The South Basin is close to the moderate: poor boundary for total phosphorus and chlorophyll a. Grasmere is in the middle of moderate ecological status for both total phosphorus and chlorophyll a. Derwent Water is the only lake reported on here at good ecological status, and it is at good status for both total phosphorus and chlorophyll a. In Bassenthwaite Lake, the total phosphorus has improved into good ecological status for the first time for many years and is at moderate ecological status for chlorophyll a. Accordingly, four out of these five lakes are likely to require a programme of measures to increase their ecological status. 11. The long-term monitoring, started in 1945 in some lake basins, is an essential resource to diagnose change and its likely cause. Consistency of monitoring is essential in order that these valuable datasets are not compromised. In addition to its continuation we suggest three specific pieces of research: - (i) a comprehensive literature review on Windermere to support the new Windermere Restoration Programme; - (ii) an assessment of the nutrient sources to Grasmere, the causes of the failure of the lake to improve and practical suggestions on ways to manage the lake to bring it to good ecological status; and - (iii) a baseline study to identify the current distribution of C. helmsii in Derwent Water so that any future changes in abundance or spread of this species can be assessed