The lake looked green and murky, with foam along the shoreline and a faint smell of something unpleasant. A family swam in it anyway. A dog that drank from it died within hours. This scenario has played out with devastating regularity across American summer recreation sites in recent years — and the record heat of summer 2026, which is already driving emergency room visits from heat-related illness in Phoenix, Houston, and Chicago, is simultaneously creating ideal conditions for the worst harmful algal bloom season in years.
Harmful algal blooms (HABs) are rapid overgrowths of algae — primarily cyanobacteria, commonly called blue-green algae — in lakes, ponds, rivers, reservoirs, and coastal waters. They are called "harmful" because many cyanobacteria species produce toxins — collectively called cyanotoxins — that can cause serious illness in humans, pets, and wildlife. They are not always visually alarming: some blooms are invisible. But many produce the characteristic blue-green, pea-soup, or neon green discoloration of the water, surface scums or mats, and foaming along shorelines that should prompt immediate avoidance.
