Algae are a type of plant that usually grows on the surface of still water, such as a small pond, when there is an ecological imbalance. In some cases, algae is able to grow at a rapid rate, potentially spreading across the entire surface.
An algal bloom can occur in saltwater or freshwater sources. The ecological imbalance is caused by the presence of too much organic material from dead leaves, animal waste or grass clippings. If the amount of organic material is too high, the water loses the ability to maintain adequate oxygen levels. The absence of oxygen and the proliferation of nutrients provided by organic material allows algae to thrive. If the lack of oxygen in a body of water is severe, fish and other aquatic animals can die, further promoting algae growth.