Algal Bloom


Algae are simple, filamentous aquatic organisms that can be unicellular (single-celled) or multicellular (having more than one cell); they grow in colonies and are commonly found floating in ponds, lakes, and oceans. Populations of algae fluctuate with the availability of nutrients; 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 appear discolored to brown, red, or green. Algal blooms occur in freshwater systems and in marine environments, and they usually disappear in a few days or weeks. These blooms consume oxygen, increase turbidity (transparency of the water), and clog lakes and streams with solid debris. Some algal species release water-soluble compounds that may be toxic to fish and shellfish, resulting in fish kills and poisoning episodes.

Risk factors

Algal groups are generally classified on the basis of the pigments that color their cells. The most common algal groups are cyanobacteria (commonly referred to as blue-green algae), green algae, red algae, and brown algae. Algal blooms in freshwater lakes and ponds tend to be caused by cyanobacteria and green algae. The excessive amounts of nutrients that cause these rapid growing algal blooms are often the result of human activities, such as fertilizer runoff during a storm. This accumulation of organic or artificial products in an ecosystem, such as nitrates, salt, and phosphates, is called eutrophication. The hypertrophic response of algal bloom in an aquatic system is one effect of the introduction of these substances to an ecosystem.

A catfish appears on the shoreline in the algae-filled waters of North Toledo, Ohio.

A catfish appears on the shoreline in the algae-filled waters of North Toledo, Ohio. The U.S. Environmental Protection Agency called for stepped-up efforts Wednesday, March 7, 2018, to reduce nutrient pollution that contributes to algae blooms in Lake Erie but recommended no new federal regulations to accomplish the task.
(Andy Morrison/The Blade via AP File)
Effects on public health

Historically, algal bloom had been considered a natural phenomenon. In the early twenty-first century, however, the frequent occurrences of harmful algae have increased. The most common negative effect of algal bloom is hypoxia, where the depletion of oxygen in the water causes animal and vegetation die-off. Humans illnesses associated with algal bloom are primarily the result of seafood consumption of both shellfish and fish from affected areas. People who eat exposed shellfish may suffer from paralytic shellfish poisoning within thirty minutes of consumption. Approximately 2,000 cases of human poisonings are reported per year worldwide. Neurotoxic shellfish poisoning (NSP) is most commonly connected to red tides. While nonfatal in humans, NSP can cause gastrointestinal and neurological symptoms and can sometimes produce respiratory symptoms similar to asthma. Algal blooms can also occur in drinking water and are able to survive typical water purification procedures.

The economic and health consequences of algal blooms can be sudden and severe, but the effects are generally not long-lasting. In the past, there has been little evidence that algal blooms have long-term effects on water quality or ecosystem structure. However, some scientists argue that a factor in the development of algal blooms is climate change and that increased water temperatures may result in an increasing occurrence of algal blooms and elongate the lifespan of each bloom. An increase in the number and duration of HABs may have a dramatic impact on the seafood industry and the ecosystems in which they occur.

See also Climate change ; Water pollution ; Water quality .



Evangelista, Valtere. Algal Toxins: Nature, Occurrence, Effect, and Detection. Dordrecht, Netherlands: Springer, 2008.

Graham, Linda E., James M. Graham, and Lee Warren Wilcox. Algae. San Francisco: Benjamin Cummings, 2009.

Gualtieri, Paolo, and L. Barsanti. Algae: Anatomy, Biochemistry, and Biotechnology. Boca Raton, FL: Taylor & Francis, 2006.

Hagen, Kristian N. Algae: Nutrition, Pollution, Control, and Energy Sources. New York: Nova Science, 2008.

Lembi, Carole A., and J. Robert Waaland. Algae and Human Affairs. Cambridge: Cambridge University Press, 2007.

Washington State Department of Health, Office of Environmental Health Assessments. Toxic Blue-Green Algae Blooms. Olympia: Washington State Dept. of Health, Division of Environmental Health, Office of Environmental Health Assessments, 2007.


Van Dolah, F. “Marine Algal Toxins: Origins, Health Effects, and Their Increased Occurrence.” Environmental Health Perspectives, 108, Supp. 1 (2000): 133–141.


Centers for Disease Control and Prevention (CDC). “Harmful Algal Blooms (HABs).” (accessed May 29, 2018).

United States Geological Survey (USGS). “Toxic Substances Hydrology Program.” (accessed May 15, 2018).

University of California Museum of Paleontology. “Algae and Seaweeds: Cyanobacteria.” (accessed May 29, 2018).

Usha Vedagiri
Douglas Smith
Alyson Heimer, MA

  This information is not a tool for self-diagnosis or a substitute for professional care.