Why does sewage turn a lake green? Windermere, phosphorus and algal blooms

Sewage can turn a lake green because it carries phosphorus, a plant nutrient, and in many freshwater lakes phosphorus is the limiting factor for plant growth. Add more of it and algae multiply into a bloom. This process is called eutrophication: nutrients accumulate in a body of water, increasing the growth of organisms that may deplete its oxygen. It happens when sewage, industrial wastewater, fertilizer runoff and other nutrient sources are released into the environment.

Windermere shows why people care. It is the largest lake in England by length, area and volume. Its water quality is mixed: monitoring found localised hotspots of elevated phosphorus, but most areas within good or acceptable ranges outside peak summer periods. Phosphorus levels are sometimes above ideal targets, though substantially lower than their 1990s peak.

The argument is about who contributes what, and what to do. The Save Windermere campaign pins United Utilities as the main source of phosphorus input, which can lead to large blue-green algal blooms. In March 2025 the then environment secretary pledged to work towards only rainwater entering the lake. On 21 October 2026 the BBC reported a £3.6bn proposal, backed by United Utilities and campaigners, built around a tunnel up to 12 miles long to carry sewage away from the lake; it is up to the government to decide whether it goes further.

Why does a bloom hurt? Algae die and rot, and under eutrophic conditions dissolved oxygen can drop greatly after dark, until fish suffocate. Some blooms are also toxic.