Algal Blooms

Algal blooms are anoverabundanceof algae that can severely affect the aquatic ecosystems in which they occur. Some marine species of dinoflagellates grow wildly at times, causing red tides that turn the surrounding sea a deep red color. The great numbers of microorganisms can rob the water of oxygen, causing many fish to suffocate.

The dinoflagellates also produce extremely poisonous chemicals that can kill a wide range of marine animals, as well as humans who eat shellfish containing the toxins. Red tides are natural events, although some scientists believe human interference contributes to their occurrence in certain regions.

Freshwater algae can also cause problems when they are overly abundant. Algal blooms can cause foul tastes in water stored in reservoirs that are used to provide drinking water to nearby communities.

Eutrophicationis a major problem that is associated with algal blooms in lakes. A direct result of human interference, eutrophication is caused by the addition of excess nutrients (runoffs of phosphate and nitrate from chemical fertilizers and sewage disposal) to the water that encourage algae to grow abundantly. As the algae die and sink to the bottom, most of the water's oxygen is consumed in breaking down the decaying plant matter. Fish and other animals that require large amounts of oxygen can no longer survive and are replaced by organisms with lower oxygen demands.