This summer, Britain and Ireland faced warnings about harmful algal blooms, with swimmers advised not to enter a new bathing spot on the River Thames near Ham and Kingston due to the presence of blue-green algae. Similarly, an Ironman event in Leeds had to cancel its swimming section because of a bloom in Waterloo Lake. These blooms are caused by cyanobacteria, a type of bacteria that can produce toxins harmful to humans and animals. Cyanobacteria thrive in warm, sunny, and still water, but their growth is often fueled by excess nutrients like phosphorus and nitrogen. These nutrients come from various sources, including agricultural runoff, wastewater, septic tanks, and urban drainage. While warm weather can speed up the growth of these blooms, the real issue is the buildup of nutrients in water systems over time. Cyanobacteria are naturally found in freshwater environments, but they become problematic when they multiply rapidly and form dense blooms. Some species produce toxins that can harm the liver, nervous system, or skin. Not all blooms are toxic, and toxin levels can vary widely, making it difficult to determine the safety of a water body just by looking at it. Blooms can appear as green paint-like streaks, foam, or clumps, and may be mistaken for harmless algae or duckweed. Climate change is increasing the risk of harmful algal blooms by warming water, extending growing seasons, and causing more frequent and intense rainfall. However, the impact of these changes depends on the existing nutrient levels in the water. Lakes with high nutrient levels are more likely to experience severe blooms. Lough Neagh, the largest lake in Britain and Ireland, has seen significant cyanobacterial blooms since 2023, partly due to nutrient runoff from agriculture and urban areas. Invasive zebra mussels, which have been filtering the lake’s water, may also contribute by making the water clearer and allowing more sunlight to reach the bottom, promoting cyanobacterial growth. To reduce the risk of harmful blooms, efforts should focus on reducing nutrient inputs into water systems. This includes better farm management, preventing soil erosion, upgrading wastewater systems, and restoring wetlands. While the effects of these changes may not be immediate, they are crucial for making water bodies more resilient to climate change. Advances in rapid testing for toxins could also improve monitoring and response times, helping to protect public health. For now, people should follow local advisories and avoid contact with water where blooms are suspected, especially to protect pets and children.