The state of Esthwaite Water in 2010

1. Esthwaite Water is one of the most nutrient-enriched lakes in the English Lake District, but the enrichment has mainly occurred in the last 50 years and in particular since the establishment of the wastewater treatment works (WwTW) at Hawkshead in 1973 and a fish farm on the lake in 1981. The fish cages were removed from the lake in November 2009 and recent upgrades have been made to the WwTW. This report describes the conditions and water quality at Esthwaite Water in 2010, places them into the context of recent conditions and assesses evidence for any change. 2. The seasonal temperature and stratification cycle was typical with surface water reaching nearly 20 °C and the bottom temperature only 10 °C. Stratification lasted for about 180 days from the beginning of April to mid-October. 3. The average alkalinity was 0.43 equiv m-3 placing Esthwaite Water in the medium alkalinity category of the Water Framework Directive. 4. Nutrients showed typical seasonal patterns. Total phosphorus was relatively conservative with peaks during time of high phytoplankton biomass and an average concentration of 21.4 mg m-3. Soluble reactive phosphorus had peak concentrations of about 12 mg m-3 at the start of the year but fell rapidly in March to the limit of detection, 0.6 mg m-3, and the concentration remained low for most of the summer and only increased on the breakdown of stratification in autumn. Silica concentrations also fell rapidly in spring as is was removed by the growing spring diatoms. Nitrate was the dominant form of nitrogen and fell more slowly than phosphorus and silica and reached minima of 30 mg m-3 that could indicate a short-period of nitrogen limitation in an otherwise phosphorus-limited lake. 5. The phytoplankton produced a spring bloom of about 16 mg m-3 comprising mainly diatom and an extensive summer bloom of cyanobacteria that reached 35 mg m-3 that did not decline until the beginning of November. The annual average concentration was 15.6 mg m-3. 6. The phytoplankton had a major effect on the light climate with Secchi depth minim of 1.5 m in August. The annual average light attenuation coefficient of about 0.87 m-1 would allow macrophyte colonisation down to between 2 and 4.3 m depending on species. 7. Nine species of crustacean zooplankton were recorded with an early summer peak population in May dominated by Daphnia hyalina/galeata. Later in the year, smaller bodied Bosmina longirostris and Ceriodaphnia quadrangula produced a population peak in mid September. 8. Statistical comparisons of the monthly-average and annual-average values of various water quality parameters in 2010 and the previous ten years showed encouraging changes. Statistically significant reduction in concentration in 2010 compared to the previous ten years were found for: (i) total phosphorus in ten months and as an annual average; (ii) soluble reactive phosphorus in 4 months and as an annual average; (iii) nitrate in eight months and as an annual average. This led to statistically significant reductions in the concentration of chlorophyll a in eight months and as annnual average and increases in Secchi depth in five months and as an annual average. The minimum concentrations of oxygen at depth were essentially unchanged and the density of zooplankton was lower in some months. 9. The current ecological status of Esthwaite Water under the Water Framework Directive is ‘Moderate’ for both total phosphorus and chlorophyll a. In previous years, the ecological status was close to the Moderate: Poor boundary for both measures. This underlines the necessity of the programme of measures that are currently underway on the lake. 10. While these results are extremely encouraging, weather patterns can lead to periods of improvement and worsening in water quality so continued monitoring is essential. Furthermore, some of these improvements started to be evident in 2009 (concentrations of total phosphorus, soluble reactive phosphorus and nitrate) and so in order to be able, confidently, to link these to management changes, it is very to obtain more information on waste-water handling at the Hawkshead WwTW and the stocking densities and feeding regime of the fish farm on the lake