Rising temperatures and stagnant water generally signal trouble for human life, but they make for a great environment for the bright, blue-green scum often found in lakes, rivers and reservoirs that flourishes and blooms in hot weather.

These scum blooms, known as harmful algal blooms, are natural parts of the ecosystem, but can also release toxins that sicken or even kill people and animals. They’re becoming more common as temperatures rise and water systems are starved and disrupted, threatening not only public and wildlife health, but the state’s water supply, as well as beloved recreation areas like Lake Merritt in Oakland.