Algal Blooms in Fresh Water

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Algal Blooms in Fresh Water

Aquatic ecologists are concerned with blooms (very high cell densities) of algae in reservoirs, lakes, and streams because their occurrence can have ecological, aesthetic, and human health impacts. In waterbodies used for water supply, algal blooms can cause physical problems (e.g., clogging screens) or can cause taste and odor problems in waters used for drinking. Blooms involving toxin-producing species can pose serious threats to animals and humans.

Algae in Aquatic Ecosystems

The term "algae" is generally used to refer to a wide variety of different and dissimilar photosynthetic organisms, generally microscopic. Depending on the species, algae can inhabit fresh or salt water.

In modern taxonomic systems, algae are usually assigned to one of six divisions (equivalent to phyla; see box on page 22). The misnamed blue-green algae are often grouped with algae because of the chloroplasts contained within the cells. However, these organisms are actually photosynthetic bacteria assigned to the group cyanobacteria.

Fresh-water algae, also called phytoplankton, vary in shape and color, and are found in a large range of habitats, such as ponds, lakes, reservoirs, and streams. They are a natural and essential part of the ecosystem . In these habitats, the phytoplankton are the base of the aquatic food chain . Small fresh-water crustaceans and other small animals consume the phytoplankton and in turn are consumed by larger animals.

Bloom Occurrences and Impact

Under certain conditions, several species of true algae as well as the cyanobacteria are capable of causing various nuisance effects in fresh water, such as excessive accumulations of foams, scums, and discoloration of the water. When the numbers of algae in a lake or a river increase explosively, an algal "bloom" is the result. Lakes, ponds, and slow-moving rivers are most susceptible to blooms.

Algal blooms are natural occurrences, and may occur with regularity (e.g., every summer), depending on weather and water conditions. The likelihood of a bloom depends on local conditions and characteristics of the particular body of water. Blooms generally occur where there are high levels of nutrients present, together with the occurrence of warm, sunny, calm conditions. However, human activity often can trigger or accelerate algal blooms. Natural sources of nutrients such as phosphorus or nitrogen compounds can be supplemented by a variety of human activities. For example, in rural areas, agricultural runoff from fields can wash fertilizers into the water. In urban areas, nutrient sources can include treated wastewaters from septic systems and sewage treatment plants, and urban stormwater runoff that carries nonpoint-source pollutants such as lawn fertilizers.

An algal bloom contributes to the natural "aging" process of a lake, and in some lakes can provide important benefits by boosting primary productivity. But in other cases, recurrent or severe blooms can cause dissolved oxygen depletion as the large numbers of dead algae decay. In highly eutrophic (enriched) lakes, algal blooms may lead to anoxia and fish kills during the summer. In terms of human values, the odors and unattractive appearance of algal blooms can detract from the recreational value of reservoirs, lakes, and streams. Repeated blooms may cause property values of lakeside or riverside tracts to decline.

Toxic Blooms

Some algae produce toxic chemicals that pose a threat to fish, other aquatic organisms, wild and domestic animals, and humans. The toxins are released into the water when the algae die and decay.

The most common and visible nuisance algae in fresh water, and the species that are often toxic, are the cyanobacteria. A cyanobacterial bloom will form on the surface and can accumulate downwind, forming a thick scum that sometimes resembles paint floating on the water. Because these mats are blown close to shore, humans and wild and domestic animals can come into contact with the unsightly material.

Blooms of toxic species of algae and cyanobacteria can flood the water environment with the biotoxin they produce. When toxic, blooms can cause human illnesses such as gastroenteritis (if the toxin is ingested) and lung irritations (if the toxin becomes aerosolized and hence airborne). Other cyanobacterial toxins are less drastic, and cause skin irritation to people who swim through an algal bloom. Toxicity can sometimes cause severe illness and death to animals that consume the biotoxin-containing water.

Cyanobacterial toxins are known to affect bean photosynthesis when they are present in irrigation water. The toxins also can modify zooplankton communities, reduce growth of trout, and interfere with development of fish and amphibians. In some cases, toxins can be bioconcentrated by fresh-water clams.

Microcystins comprise the most common group of about fifty cyanobacterial toxins. Among these toxins are ones that, if ingested in sufficient quantity, can harm the liver (hepatotoxins) or nervous system (neurotoxins). Microcystins can persist in water because they are stable in both hot and cold water. Even boiling the water, which makes the water safe from harmful bacteria, will not destroy microcystins. As a result of this threat, the Canadian government implemented a recommended water-quality guideline of 1.5 μg per liter of microsystin-LR (the most common hepatotoxin), and other countries will likely follow suit. In Canada as well as the United States, there are few reports of injury and no reports of human deaths resulting from microcystins in drinking water, in large part because surface-water sources of drinking water (e.g., reservoirs, lakes, and rivers) must undergo filtration and chlorination at water utilities prior to being distributed to customers. (Cyanobacterial toxins can be removed from water only by activated charcoal filters and chlorination.)

Control Considerations

Repeated episodes of algal blooms can be an indication that a river or lake is being contaminated, or that other aspects of a lake's ecology are out of balance. While cyanobacterial blooms receive the most public and scientific attention, the excessive growth of other algae and other aquatic plants also can cause significant degradation of a lake or pond, particularly in waters receiving sewage or agricultural runoff. Aquatic biologists and other water-quality specialists often are called to identify the causes and recommend management steps to reduce or control the problem.

However, prevention of a problem is always better than trying to fix the problem after it happens. Controlling agricultural, urban, and stormwater runoff; properly maintaining septic systems; and properly managing residential applications of fertilizers are probably the most effective measures that can be taken to help prevent human-induced fresh-water algal blooms.

see also Algal Blooms, Harmful; Algal Blooms in the Ocean; Ecology, Fresh-Water; Human Health and Microbes; Nutrients in Lakes and Streams; Plankton; Pollution Sources: Point and Nonpoint; Wastewater Treatment and Management.

Brian D. Hoyle

K. Lee Lerner

and Elliot Richmond


Carmichael, Wayne W. "The Toxins of Cyanobacteria." Scientific American. (January 1994): 78-86.

Elder, G. H., Hunter, P. R., and Codd, G. A. "Hazardous Freshwater Cyanobacteria (Blue-Green Algae)." Lancet 341 (1993):15191520.

Falconer, Ian R. "An Overview of Problems Caused by Toxic Blue-green Algae (Cyanobacteria) in Drinking and Recreational Water." Environmental Toxicology. 14 (1999):512.

Oberemm, A. et al. "Effects of Cyanobacterial Toxins and Aqueous Crude Extracts of Cyanobacteria on the Development of Fish and Amphibians." Environmental Toxicology 14 (1999):7788.

Internet Resources

Blue-green Algae (Cyanobacteria) and their Toxins. Health Canada. <>.


Scientists use a system called taxonomy to organize all the biological organisms in the world. Organisms are put into various classification groups according to the distinguishing properties they share. These groups are (from highest to lowest) kingdom, phylum, class, order, family, genus, and species.

Although there are several different kingdom classifications in use, it is now generally accepted that all biological organisms can initially be placed into one of five kingdoms: monera, protists, fungi, plants, and animals.

Prokaryotes (Cells That Have No Distinct Nuclei)

  • Monera: Includes aquatic bacteria and blue-green algae, more properly called cyanobacteria. Monerans, though microscopic, are the most dominant organisms on Earth. They have existed for about 3.5 billion years.

Eukaryotes (Cells Have Distinct Nuclei)

  • Protista: Includes plant-like and animal-like primitive organisms, such as algae and protozoa. Organisms are generally unicellular.
  • Fungi: Includes a large group of parasitic and saprophytic species. Some are parasitic on animals, including humans (ringworm, or athlete's foot). Others are parasitic on plants and include rusts and mildews. Fungi are, along with the bacteria, important decomposers of dead organic matter.
  • Plants: Make their food by the process of photosynthesis.
  • Animals: Ingest their food and digest it internally in specialized body cavities.


Algal blooms can cover a large area. In 1991, a bloom affected an estimated 1,000-kilometer stretch of the Barwon and Darling Rivers in New South Wales, Australia.