A survey of the status of the lakes of the English Lake District: the Lakes Tour 2015

1. This report presents information resulting from a survey of the limnology of the 20 major lakes and tarns in the English Lake District based on samples taken in January, April, July and October 2015. This ‘Lakes Tour’ supplements similar tours in 1984, 1991, 1995, 2000, 2005 and 2010. 2. On each sampling occasion, depth-profiles were collected of water temperature and oxygen concentration and Secchi depth was measured. An integrated water sample was analysed for pH, alkalinity, major anions and cations, heavy metals, micro-organic pollutants, plant nutrients, phytoplankton chlorophyll a and species composition and zooplankton abundance and species composition. Some of the field work and chemical analyses were carried out collaboratively between staff from CEH and the Environment Agency. 3. The lakes had a range in tendency to stratify in summer with the weakest stratification in large, relatively shallow and exposed lakes such as Bassenthwaite Lake. During summer stratification, oxygen-depletion at depth was only found in the more productive lakes. 4. Water clarity, assessed by Secchi disc, varied between about 12 m in clear unproductive lakes such as Wastwater in January, to 2 m in the more productive lakes during summer such as Esthwaite Water in October. 5. Major ion composition varied with geology and altitude. Lakes on the Silurian slates (those in the Windermere and Coniston Water catchments) tended to have anions dominated by alkalinity (bicarbonate) and cations dominated by calcium whereas the other lakes tended to have anions dominated by chloride and cations dominated by sodium. 6. Availability of phosphorus is the main factor that affects lake productivity. Concentrations of total phosphorus were lowest in Wastwater and Crummock Water and highest in Blelham Tarn and Elterwater. Nitrate was the dominant form of nitrogen. Nitrate concentrations tended to be lowest in July because of biological uptake and seasonal fluctuations were most marked in the productive lakes. Silica, an essential nutrient for diatoms, showed a similar seasonal pattern to nitrate but the depletion was more marked in April because the spring bloom is typically dominated by diatoms. In unproductive lakes, with low biological demand, such as Wastwater and Ennerdale Water, concentrations of silica did not vary seasonally. 7. The concentration of chlorophyll a was used as a measure of phytoplankton abundance. Comparisons across lakes showed low concentrations all the year in the unproductive lakes and seasonally high concentrations in the more productive lakes. Blelham Tarn had the highest annual mean concentration of chlorophyll a at 23 mg m 3, but the highest concentration recorded was 40 mg m 3 at Elterwater in July. The lowest annual annual mean concentration was 0.8 mg m 3 at Wastwater. 8. The species composition varied seasonally in all the lakes, even unproductive ones with limited seasonal changes in nutrient concentrations, underlying the sensitivity of phytoplankton to environmental conditions. Overall, diatoms dominated in January and particularly, April, but in July and October a range of different groups dominated depending on the lake. Cyanobacteria were generally more common in productive lakes. 9. Zooplankton abundance was very variable and greatest in the productive lakes and seasonally, abundance tended to be greatest in July and October. Fifteen genera of zooplankton were recorded in total. The unproductive lakes tended to be dominated by Eudiaptomus gracilis and this species dominated most of the lakes in January. Daphnia spp. were often important in the summer in the more productive lakes. At the genus level, diversity increased in the summer and autumn. Abundance of Ceriodaphnia, Diaphanosoma and Mesocyclops were an important part of the zooplankton community in some lakes in summer. 10. The known status of fish populations, although not undertaken in the project, was summarised. Nineteen species have been recorded in these lakes, but of these seven are probably introduced. Some lakes have very few fish-data and require more research. 11. Heavy metals were measured for the second time. Although many samples were below the limit of detection, copper concentrations were elevated in Coniston Water, lead was elevated in Bassenthwaite Lake, Haweswater and Thirlmere and zinc was elevated in Bassenthwaite Lake, Buttermere, Crummock Water and Haweswater. 12. Micro-organic pollutants were measured for the second time and most samples were below current detection limits. Of the sixty-nine compounds analysed, five gave values above the detection limit but only three exceeded the limit more than twice. Of these, Diazinon, an organophosphorus insecticide, was detected on all four sampling occasions in Blelham Tarn, Buttermere, Crummock Water, Ullswater and the North Basin of Windermere; this merits further investigation. 13. The current state of each lake was summarised in terms of key limnological variables, trophic state and ecological status under the current definitions of the Water Framework Directive (WFD), although note that this does not conform completely to WFD guidelines and Rydal Water and Loughrigg Tarn do not fall within the WFD remit. 14. Compared to 2010 there was an increase in the number of lakes at high or good status of from 10 to 14 for total phosphorus and from 11 to 14 for chlorophyll a. Of these, only Wastwater, Buttermere, and Brothers Water were at high status for both variables. Based on chlorophyll a, Loweswater, Loughrigg Tarn, Blelham Tarn, Esthwaite Water, Elterwater and the South Basin of Windermere were only at Moderate ecological status. 15. Long-term change from 1984 to 2015 (1991 to 2015 for some variables) were analysed. There have been changes in the concentration of major ions in many sites. This has largely been caused by reduction in sulphate deposition from acid rain, causing widespread increases in alkalinity and pH and reductions in concentration of calcium, magnesium, sodium and potassium because of reduced cation-exchange in the soil. Reducing concentrations of sodium and chloride are probably also linked to reductions in stormy weather since the mid 1990s and hence reduced input of sea-salt in rain. On average, in comparison to the 2010 Lakes Tour, there has been a reduction in concentration of TP and phytoplankton chlorophyll a and Secchi depth has remained stable. While the magnitude of change is small, it is encouragingly in the right direction. 16. The lakes in the English Lake District are extremely valuable scientifically as they are highly diverse. This was illustrated by showing the link between catchment elevation (as a proxy for land use and soil type) and a range of water chemistry variables and the relationship between phytoplankton chlorophyll a and total phosphorus which shows that the productivity of these lakes is controlled by phosphorus. The magnitude of the seasonal changes in silica and nitrate is positively linked to lake productivity. Secchi depth is negatively correlated with phytoplankton, but in January Secchi depth is less for a given chlorophyll a concentration, probably because of attenuation by dissolved organic carbon and particulate material brought in to the lakes by winter rains. Minimum oxygen concentration at depth is also negatively related to phytoplankton chlorophyll a. 17. It is suggested that more work is needed at lakes which have failed Good ecological status in order to obtain a better understanding of the reasons for this and possible remediation actions. The fish populations in many lakes need to be studied in more detail. 18. The joint-manning of the Lakes Tour by CEH and the EA worked well again and could be a model for future work