Lake Ecosystem Group, Centre for Ecology & Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster LA1 4AP March 2011 The state of Esthwaite Water in 2010 A report to Natural England prepared by: S.C. Maberly, M.M. De Ville, J. Kelly & S.J. Thackeray Project Leader: S.C. Maberly Report Date: March 2011 Report to: Natural England CEH Project: NEC04038 CEH Report Ref: LA/C04038/2 This is an unpublished report and should not be cited without permission, which should be sought through the project leader in the first instance. INTELLECTUAL PROPERTY RIGHTS CONFIDENTIALITY STATEMENT ‘In accordance with our normal practice, this report is for the use only of the party to whom it is addressed, and no responsibility is accepted to any third party for the whole or any part of its contents. Neither the whole nor any part of this report or any reference thereto may be included in any published document, circular or statement, nor published or referred to in any way without our written approval of the form and context in which it may appear’. Executive Summary 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. Table of Contents Page number 1. Introduction ......................................................................................................................... 1 2. Methods .............................................................................................................................. 3 3. Meteorological conditions at Esthwaite Water in 2010 ........................................................ 5 4. Physical conditions in Esthwaite Water in 2010 .................................................................. 6 5. Water quality in Esthwaite Water in 2010 ............................................................................ 9 6. Esthwaite Water in 2010 compared with the previous decade ........................................... 22 7. Summary & Conclusions ................................................................................................... 30 8. Acknowledgements ........................................................................................................... 32 9. References ....................................................................................................................... 33 1. Introduction Esthwaite Water (54° 22’ N, 2° 56’ W) is situated in a side valley that drains into the South Basin of Windermere in the English Lake District, Cumbria UK. It was formed at the end of the last glaciation, around 12,000 years ago. Esthwaite Water is a relatively small lake compared to many in the English Lake District: it has an area of 1 km2 and a volume of 6.4 Mm3. Its catchment is 17 km2 and is lower lying than most of the other major English Lake District catchments. The average discharge recorded between 1976 and 2009 of 26.9 Mm3 y-1 (equivalent to 0.85 m3 s-1) gives an average retention time of 87 days. Esthwaite Water is one of the most nutrient-enriched lakes in the English Lake District. Palaeolimnological records show that there was little evidence for a change in nutrient status between about 800 and 1850 AD, but a gradual and accelerating increase from then until 1970 and a more dramatic increase after that time. The rapid recent increase in eutrophication coincided with two man-made changes to the lake. The first was the establishment, in 1973, of a Wastewater Treatment Works (WwTW) that serves the village of Hawkshead and discharges just upstream of where the main inflow, Black Beck, enters the lake in the north. The second was the introduction, in 1981, of a fish farm at the southern end of the lake. Upgrade of the WwTW to reduce phosphorus inputs by including tertiary phosphorus-removal in 1986 appeared to have relatively little effect on the lake, possibly because at that stage P-inputs from the fish farm were dominating the P-load. Further details about the physical, chemical and biological features of Esthwaite Water are described in Maberly et al. (2011). Recently, two changes have been implemented to reduce further nutrient-loading to the lake: the removal of the fish cages on the lake in 2009 and upgrading of the waste water handling and treatment at the Hawkshead WwTW which began in 2010. 1 The purpose of this report is to describe the limnology of Esthwaite Water in 2010, the first year after the implementation of these remedial measures, to assess whether or not the first signs of a change can be discerned. Lakes usually respond relatively slowly to changes in the supply of nutrients because of internal supplies, for example, internal loads from the sediments, that take several years to dissipate. Nevertheless, it is important to assess whether a positive change can be detected especially in the face of other large scale changes, such as climate change, that can also negatively affect a lake like Esthwaite Water (Elliott 2011). This report will describe the conditions at Esthwaite Water in 2010, place them in the context of recent conditions and assess evidence for any change. 2 2. Methods The data reported here derive from two types of measurement. The first is based on traditional sampling at the deepest point in the northern bay of the lake, undertaken every two weeks. The second is automatically collected high resolution data. Fortnight limnological data This is a continuation of a long term monitoring series that began