Blue-Green Algae: What Water Managers Need to Know This Summer

Despite its name, blue-green algae is not actually an algae but a group of naturally occurring cyanobacteria found in many lakes, ponds, reservoirs and slow-moving rivers throughout the UK. Under the right conditions—particularly warm weather, sunlight and elevated nutrient levels—these organisms can multiply rapidly to form visible blooms or surface scums.

Large blooms can reduce water quality, lower oxygen levels as they decay and produce toxins that may affect people, pets, livestock and wildlife. Fish are not normally harmed directly by the algae themselves, but severe blooms can contribute to oxygen depletion, particularly during hot weather or overnight when oxygen levels naturally fall.

While blooms cannot always be prevented, long-term management focuses on reducing the conditions that encourage them. Controlling nutrient inputs, improving water circulation where practical and managing excessive aquatic vegetation all contribute to healthier waterbodies. Regular monitoring during the warmer months allows potential blooms to be identified early so that appropriate action can be taken where necessary.