Harmful algae blooms occur when algae and cyanobacteria grow out of control in bodies of water. This overgrowth can produce toxins that cause mild to severe symptoms in people and animals. The blooms can kill animals and plants in the water by severely depleting oxygen when they decompose.
Normally, algae are important organisms in bodies of water because they are the building blocks of the food chain and ecosystem. When they grow out of control, however, they can harm people, animals and the environment.