Over 300,000 people visited Windermere on some days in 2020

Despite this, the study conducted by Save Windermere with analysis by Map Impact and data from the UK Space Agency did not blame tourists for the algal spikes, instead pinning it on 'inadequate investment' by United Utilities into their sewage infrastructure to deal with the number of people, and the Environment Agency, which regulates water companies. 

The project, conducted over a year, examined the concentration of chlorophyll-a in the lake using satellite data, which can serve as an indicator for algal growth and phosphorus concentrations. 

Blue-green algal blooms can be toxic to humans and wildlife if ingested, and deplete oxygen levels in the water. 

This is the end result of a year long study conducted by Save Windermere, Map Impact and the UK Space Agency (pictured Matt Staniek, founder of Save Windermere (Image: Save Windermere)

The study combined the satellite imagery with anonymised cellular data from one of the UK's largest mobile networks, to gauge the number of people within the Windermere catchment at any one time. 

Save Windermere said that the resident population of the Windermere catchment was around 35,000 during the Covid-19 lockdown, but the lifting of restrictions saw 300,000 people in the catchment on certain summer days in 2020. 

"The increased loading of people around the lake was directly correlated with the increased presence of algae in the lake," a spokesperson from Save Windermere said. "Even when numbers returned to ‘normal’ in 2022, we still saw over 200,000 people in the catchment on given days." 

Save Windermere said that in 2022 there was a more severe algal bloom 'due to higher water temperatures in the lake.' The campaigners said that the there has 'not been enough investment' to ensure sustainable development, meaning the lake is not adaptable to climate change. 

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The campaigners also slammed the water company for 'declining' to share data for the project that would have allowed them 'to determine the full contribution of United Utilities to the algal spikes to the lake.' 

The group ultimately wants a solution similar to Lake Annecy in France, where there are no sewage discharges into the water. They found 'no correlation' between agricultural input and algal spikes. 

An Environment Agency spokesperson said: “Windermere is affected by pollution from different source, including wastewater treatment works, storm overflows, septic tanks and road and agricultural run-off. Recent data shows a 30% reduction in phosphorus entering the lake since 2020, and the lake’s bathing waters were rated ‘excellent’ in 2023, but we know there is more to do.

“We are absolutely committed to improving the water quality in Lake Windermere. That is why we are working closely with the local community and a range of partners to reduce pollution from all sources, as well as reviewing wastewater treatment works permits within the Windermere catchment.”

A United Utilities spokesperson said: “Our wastewater treatment works are sized to treat the sewage from maximum population numbers at peak times, and use the highest treatment standards including phosphorus removal and UV treatment to kill bacteria.

“To contribute to the debate about a “discharge-free” solution with no treated or untreated UU discharges, no effluent from the 1,800 private septic tanks or from the additional 89 discharge points around the lake that are owned by others, we carried out a feasibility study that has already been shared with key local stakeholders and we are keen to engage with anyone who might want to investigate this further.”