Major pollutants affecting the Lake District’s iconic waters are continuing to decline across the region, but a combination of factors continue to impact the health of lake ecosystems, say scientists. 

The UK Centre for Ecology & Hydrology (UKCEH) has presented results from its latest survey of the physical, chemical, and biological state of the 20 lake basins in the region. It says there are several different pollutant sources including sewage discharges, agricultural run-off and acid rain, as well as other threats like climate warming, invasive species, damage to habitats and recreational activities.

The UKCEH scientists found:

  • A continued reduction across all lakes in chemical pollution caused by acid rain over the past 30 years, primarily due to a fall in coal and oil use in the UK in response to stricter limits on emissions. 
  • A significant long-term decline in water clarity in more pristine lakes.
  • A general long-term downward trend in nitrogen, but no significant long-term change was observed in phosphorus concentrations across all lakes. However, more data are needed to better quantify the changes in phosphorus concentrations, particularly as there are concerns about water quality in some lakes. Sources of these two key nutrients include agricultural runoff and sewage discharges, while atmospheric pollution caused by industry and traffic are also sources of nitrogen.

The UKCEH scientists say the improvements show it is possible to tackle threats to the Lakes but add that some of the 20 lake basins in Cumbria are faring worse than others. The challenge is complex because the extent of each of the threats, and an ecosystem’s response to them, varies between lakes.

UKCEH’s Lakes Tour report looks at how the lakes’ condition is changing over time. This snapshot seasonal survey has been held around every five years since 1984, with the latest being carried in spring, summer and autumn 2021 and early 2022.

Separate, ongoing monitoring by UKCEH has found that surface water temperatures have increased by around 1 degree Celsius over the past 40 years in Windermere, Esthwaite Water and Blelham Tarn, which is exacerbating threats to their ecosystems. For example, in addition to the presence of nutrients, warm and dry weather conditions can also exacerbate the growth of harmful algal blooms.

UKCEH freshwater ecologist Dr Eleanor Mackay, who led the Lakes Tour survey, explains: “In many cases, nutrient pollution into the lakes is the only part of the system that we can manage, to counteract other factors such as climate change which we can’t control at the local level. This means there is a need to further reduce sewage pollution from wastewater plants, combined sewer overflows, septic tanks and agricultural run-off to counteract these threats.

“The long-term downward trend in concentrations of some atmospheric pollutants shows we can tackle threats to the Lakes when a concerted effort is made.”

The scientists add that the long-term reduction in acid rain has contributed to a continued decline in sulphur and nitrogen compounds in the Lakes over the past five years. Sulphur and nitrogen compounds affect the pH of natural waters and react with metals in soil, such as aluminium, which can then end up in lakes. In severe cases, these chemical changes can stress or result in the death of some fish and invertebrate species.

However, scientists believe that reductions in acid rain could also have resulted in declines in water clarity in more pristine lakes. Separate UKCEH research shows that reductions in acid rain cause organic matter in the soil to become increasingly soluble and leach into lakes and rivers, causing them to become browner. 

On average, water clarity ranges from just 2m at Blelham Tarn and Esthwaite Water, where there are higher concentrations of nutrients and more algal growth, to over 10m at Buttermere, Crummock Water and Wastwater. For Crummock Water, Ennerdale Water, and Wastwater, average water clarity has reduced by approximately 4m since 1984. 

The dissolved organic matter entering lakes also contains nutrients such as phosphorus and nitrogen that can, under some circumstances, stimulate the growth of algae and cyanobacteria, also known as blue-green algae.

Observations of algal blooms have featured in recent media stories on water quality in Windermere, and the precise combination of factors causing these blooms is yet to be determined. The decay of algal blooms removes oxygen from water, low oxygen concentrations can result in the death of fish and other organisms. Water with high concentrations of cyanobacteria can also be harmful for pets and people.

Some lakes including Windermere have experienced long-term reductions in concentrations of phosphorus, which is present in human and animal waste as well as in fertiliser. The highest concentrations of phosphorus in 2021/22 were found in Esthwaite Water, Blelham Tarn and Elterwater. These lakes also had the highest concentration of algae and experienced a loss of oxygen in deep waters during the summer.

The report authors suggest further work to investigate 

  • how the amount of phosphorus in a lake affects how much algal growth there is, 
  • the causes of reductions in the minimum oxygen concentrations and declining water clarity at more pristine lakes 
  • the sources of increasing nutrient concentrations at some sites. 

They also call for more research to address knowledge gaps around the fish populations in the lakes.

UKCEH lake ecologist Dr Stephen Thackeray, a co-author of the survey, says: “Factors such as location, altitude, surrounding land use and the mix of species present can greatly alter how each lake responds to the same environmental pressures. We need a good understanding of the factors influencing pollution at individual lakes to target interventions where they will be most effective.” 

The 2021/22 Lakes Tour survey was funded by United Utilities and Natural England.

A pdf of the report summary is attached at the bottom of this webpage.

Further information

  • Lakes monitored in the survey were: Bassenthwaite Lake, Blelham Tarn, Brothers Water, Buttermere, Coniston Water, Crummock Water, Derwent Water, Elterwater, Ennerdale Water, Esthwaite Water, Grasmere, Haweswater, Loughrigg Tarn, Loweswater, Rydal Water, Thirlmere, Ullswater, Wastwater and Windermere North and South Basins.
  • For the Lakes Tour 2021/22, UKCEH scientists measured seasonal water temperatures and transparency, concentrations of oxygen, nitrogen, phosphorus and ions, micro-organic pollutants, heavy metals, acidity and alkalinity, phytoplankton and zooplankton abundance and species composition each season over the course of a year. This is the eighth such survey by UKCEH and its predecessors since 1984. The data provide a robust and comprehensive picture of how lakes have responded to long-term environmental pressures, and change over the year, because scientists have used consistent methods of analysis.
  • In addition to the Lakes Tour survey, UKCEH carries out ongoing research at Windermere, Esthwaite Water, and Blelham Tarn, including monitoring water temperature, dissolved oxygen, nutrient concentrations, algal biomass and community composition of algae and zooplankton every two weeks This is part of its UK-SCAPE programme, supported by National Capability funding from the Natural Environment Research Council (NERC) 
  • Sulphur dioxide emissions in the UK reduced by 98 per cent 1970-2021. Nitrogen oxides fell 77 per cent over the same period, also partly because of the switch to less polluting vehicles.