The response of Windermere to external stress factors: analysis of long-term trends

Maberly, S. C.; Thackeray, S. J.; Jones, I. D.; Winfield, I. J.. 2008 The response of Windermere to external stress factors: analysis of long-term trends. NERC/Centre for Ecology & Hydrology, 45pp. (CEH Report Ref: LA/C03468/2) (Unpublished)

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1. The motivation for this report was the documented recent deterioration in the water quality in the two basins of Windermere since around 2000. The aim was to analyse the long-term records to assess the likely causes of this deterioration. 2. Records were analysed between 1950 and 2007, where available, including data on lake physics (annual mean water temperature at the surface, water temperature at depth, duration of stratification), nutrient availability (mean winter concentration of total phosphorus, soluble reactive phosphorus and nitrate); responding chemical variables (minimum concentration of nitrate, minimum concentration of silica, maximum pH and minimum oxygen concentration at depth); phytoplankton chlorophyll a (annual mean, spring maximum and summer maximum); zooplankton grazing (spring mean and summer mean abundance); and the fish predation on zooplankton (hydroacoustic data, perch recruitment and Arctic charr abundance, the latter only available in the North Basin). 3. Changes in the period upto around 1991 are largely the result of increasing nutrient load to the lake. This led to increasing winter concentrations of soluble reactive phosphorus, nitrate and total phosphorus (latter significant in South Basin only). This caused increased concentrations of phytoplankton chlorophyll a as an annual mean and as spring and summer maxima in both basins. This in turn caused a greater depletion of silica in the late spring, increased pH in the summer (significant in North Basin only) and reduction in the minimum oxygen concentration at depth. 4. Since around 2000 there has been a significant reduction in water quality in both basins: summer phytoplankton chlorophyll a has increased, as has maximum pH and the minimum concentration of oxygen (only significant in the North Basin). Summer Secchi depth has declined in both basins. 5. Annual phosphorus load from the two wastewater treatment works (WwTWs) that discharge directly to the lake has fallen by about 50% in the period 1993-2007 following tertiary treatment compared to 1978-1991 before treatment. At the Ambleside WwTW the load has fallen from 2.2 to 1.09 Mg P y-1 and at Windermere Tower Wood WwTW it has fallen from 5.85 to 2.82 Mg P y-1. 6. It is difficult to estimate catchment loads of phosphorus (contributions from storm overflows are not quantified for example), nevertheless the data suggest that at the moment the two WwTWs contribute about 37% of the total phosphorus and 66% of the soluble reactive phosphorus load to the lake. However, there is no indication of a recent return to the higher phosphorus loads of the past and so the two WwTWs are unlikely to be the cause of the recent deterioration in water quality. 7. Variables such as water temperature varied very coherently year-to-year across the two basins, suggesting that they are controlled by lake-scale regional factors such as the weather. This is also true of variables such as minimum concentration of nitrate and perch recruitment. In contrast, summer phytoplankton and summer zooplankton were the least coherent year-to-year across the two basins, suggesting that they are controlled more by local in-lake processes than by large-scale changes. 8. An analysis of interactions among the variables across the whole time period, detrended to focus on year-to-year rather than long-term variation highlighted the expected links between increased phytoplankton chlorophyll a and other symptoms of eutrophication such as high pH, low nitrate concentration, low oxygen concentration at depth and low water transparency. There are hints, particularly in the South Basin, that there is a linkage between phytoplankton, zooplankton and fish grazing pressure. 9. An analysis of the timing of the various trends showed that the increase in surface water temperature occurred around 1989. The effect of the tertiary treatment can be seen in the trends in TP. In the South Basin, winter TP increased up until 1992 and then, after a delay, declined. In the North Basin, the steep rate of increase stalled after 1992 and has shown a subsequent very slight decline. The minimum oxygen concentration started to increase in 1980 and 1983 in the South and North Basins respectively, showed signs of increase around the time of the tertiary treatment but started to decline again in 2001. In the South Basin summer chlorophyll a started to increase around 1971, peaked in 1992 and then declined. In the North Basin summer chlorophyll a did not start to increase until 1986, peaked in 1992 but after around 1998 it started to increase again. 10. An analysis of seasonal patterns shows that following the implementation of the tertiary treatment plants there has been an increase in summer phytoplankton chlorophyll a in both basins. This appears to be linked to a marked reduction in the density of zooplankton in May, June and July in both basins. One consequence of this is reduced Secchi depth and Secchi depth normalised to unit chlorophyll a at around this time of year in both basins. The lack of any trend in the winter months suggests that the previously observed reduced Secchi depth is more likely to be cause by internal processes within the lake than by increased load of particulate or dissolved organic material from the catchment. 11. Together, the results suggest that the lake has switched from a strong control from nutrient availability before 2000 to a system where some form of trophic control is playing an important role. The very low zooplankton numbers in the summer appear to be allowing a greater phytoplankton crop to be produced per unit phosphorus than before. The precise cause of the reduced zooplankton numbers is unclear, but it is quite likely that it is linked to the documented large increase in roach abundance. This in turn may be an indirect symptom of climate change with increasing water temperature allowing roach to become established and possibly start to outcompete the native fish. 12. Clearly more analysis of the long-term data is needed to confirm or refute these ideas. A modelling study would also help to test them further. This study has important implications for lake management and the Water Framework Directive. Trophic changes, possibly brought about by climate change, may cause water quality to deteriorate even when phosphorus loads have been reduced. This might require phosphorus loads to the lake to be reduced even further in order to achieve good ecological status. 13. While Windermere is one of the world’s best studied lakes, and a focus of a new Restoration Programme led by the Environment Agency, the large body of published and unpublished information is not readily available to lake managers. We recommend that a major review of the ecology of Windermere should be undertaken.

Item Type: Publication - Report (UNSPECIFIED)
Programmes: CEH Programmes pre-2009 publications > Water > WA02 Quantifying processes that link water quality and quantity, biota and physical environment > WA02.3 Physico-chemical processes and effects on freshwater biot
CEH Sections: Parr
Funders/Sponsors: Environment Agency
NORA Subject Terms: Ecology and Environment
Related URLs:
Date made live: 19 Aug 2008 09:23

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