Algae are simple, single-celled, filamentous aquatic plants; they grow in colonies and are commonly found floating in ponds, lakes, and oceans. Populations of algae fluctuate with the availability of nutrients, and a sudden increase in nutrients often results in a profusion of algae known as algal bloom.
The growth of a particular algal species can be both sudden and massive. Algal cells can increase to very high densities in the water, often thousands of cells per milliliter, and the water itself can be colored brown, red, or green. Algal blooms occur in freshwater systems and in marine environments, and they usually disappear in a few days to a few weeks. These blooms consume oxygen, increase turbidity, and clog lakes and streams. Some algal species release water-soluble compounds that may be toxic to fish and shellfish, resulting in fish kills and poisoning episodes.
Algal groups are generally classified on the basis of the pigments that color their cells. The most common algal groups are blue-green algae, green algae, red algae, and brown algae. Algal blooms in freshwater lakes and ponds tend to be caused by blue-green and green algae. The excessive amounts of nutrients that cause these blooms are often the result of human activities. For example, nitrates and phosphates introduced into a lake from fertilizer runoff during a storm can cause rapid algal growth. Some common blue-green algae known to cause blooms as well as release nerve toxins are Microcystis, Nostoc, and Anabaena.
Red tides in coastal areas are a type of algal bloom. They are common in many parts of the world, including the New York Bight , the Gulf of California, and the Red Sea. The causes of algal blooms are not as well understood in marine environments as they are in freshwater systems. Although human activities may well have an effect on these events, weather conditions probably play a more important role: turbulent storms that follow long, hot, dry spells have often been associated with algal blooms at sea. Toxic red tides most often consist of genera from the dinoflagellate algal group such as Gonyaulax and Gymnodinium. The potency of the toxins has been estimated to be 10 to 50 times higher than cyanide or curare, and people who eat exposed shellfish may suffer from paralytic shellfish poisoning within 30 minutes of consumption. A fish kill of 500 million fish was reported from a red tide in Florida in 1947. A number of blue-green algal genera such as Oscillatoria and Trichodesmium have also been associated with red blooms, but they are not necessarily toxic in their effects. Some believe that the blooms caused by these genera gave the Red Sea its name.
The economic and health consequences of algal blooms can be sudden and severe, but the effects are generally not long lasting. There is little evidence that algal blooms have long-term effects on water quality or ecosystem structure.
[Usha Vedagiri and Douglas Smith ]
Lerman, M. Marine Biology: Environment, Diversity and Ecology. Menlo Park, CA: Benjamin/Cummings, 1986.
Culotta, E. "Red Menace in the World's Oceans." Science 257 (11 September 1992): 1476–77.
Mlot, C. "White Water Bounty: Enormous Ocean Blooms of White-Plated Phytoplankton Are Attracting the Interest of Scientists." Bioscience 39 (April 1989): 222–24.