What are blue-green algae (cyanobacteria)?

Cyanobacteria, formerly called "blue-green algae" are relatively simple, primitive life forms closely related to bacteria. Typically much larger than bacteria, they photosynthesize like algae. Depending upon the species, cyanobacteria can occur as single cells, filaments of cells, or colonies. Cyanobacteria contain a characteristic pigment which gives the group their blue-green coloration. When cyanobacteria blooms begin to die and disintegrate, this pigment may color the water a distinctive bluish color. Cyanobacteria are found throughout the world in terrestrial, freshwater and marine habitats, but blooms typically occur in freshwater. (see also less technical information about algae in general)

What is a cyanobacteria bloom?

Nutrient-rich bodies of water such as some lakes or ponds may support rapid growth of cyanobacteria. With the right conditions, a "clear" body of water can become very turbid with green, blue-green or reddish-brown colored algae within just a few days. High concentrations of an alga species in a water body form "blooms." Many species can regulate their buoyancy and float to the surface to form a thin "oily" looking film or a blue-green scum several inches thick. The film may be mistaken for a paint spill. Cyanobacteria cannot maintain this abnormally high population for long and will rapidly die and disappear after one to two weeks. If conditions remain favorable, another bloom can quickly replace the previous one. In fact, successive blooms may overlap so that it may appear as if one continuous bloom occurs for up to several months.

Why are they a problem?

Blue-green blooms can pose a human health concern. Although most blue-green blooms are not toxic, some blue-green algae produce nerve or liver toxins. Toxicity is hard to predict in part because a single species of algae can have toxic and non-toxic strains. Also a bloom that tests non-toxic one day can turn toxic the next day.

People may become ill after swimming or water skiing in lakes with toxic blue-green algae. Rarely, humans may experience stomach pains, vomiting, diarrhea, and skin rashes. Nerve and liver damage have been observed following long-term exposure such as drinking water with toxic blooms. Pets and wildlife have died after exposure to toxic blue-green algae in Washington lakes, but worldwide there are no confirmed deaths of humans from algal toxins.

Three genera of cyanobacteria account for the vast majority of blooms, including toxic blooms, world-wide: Anabaena, Aphanizomenon, and Microcystis - sometimes referred to as Annie, Fannie, and Mike. In Washington, Aphanizomenon does not seem to produce toxic blooms, although the other two genera have produced toxic blooms in Washington lakes. A bloom can consist of one or a mix of two or more genera of cyanobacteria.

Type of toxins

Cyanobacteria can produce several toxins, but two types of toxins are of particular concern and are tested for: Microcystins and Anatoxin-A. Microcystins are a group of hepatoxins (toxins that affect the liver). Microcystins are very stable and do not break down quickly in water. Microcystins are considered to be the most commonly-found cyanobacterial toxins in water and are the toxins most responsible for human and animal poisonings.

Anatoxin A is a potent neurotoxin (toxins that affect the nervous system) which can cause lethargy, muscle aches, confusion, memory impairment, and, at sufficiently high concentrations, death. It is believed that a fast-acting algal neurotoxin caused the deaths of two pet dogs at Anderson Lake in Jefferson County in the summer of 2006.

For additional information see Health Canada: Blue-Green Algae (Cyanobacteria) and their Toxins

Is a particular bloom toxic?

Not all cyanobacteria blooms are toxic. Even blooms caused by known toxin-producing species may not produce toxins or may produce toxins at undetectable levels. Scientists do not know what triggers toxin production by cyanobacteria. Just a few years ago, it was believed that only about 10 percent of all blooms produced toxins. Recent studies have shown that the probability that an individual bloom containing Anabaena, Microcystis, and/or Aphanizomenon will be toxic is greater than previously thought (45-75%). Since cyanobacteria toxins can be lethal to pets in relatively small amounts, caution should always be taken when a bloom occurs.

Signs that a cyanobacteria bloom is toxic may be large numbers of dead fish, waterfowl, or other animals within or around a body of water. Terrestrial animals found dead may have algae around the mouth area or on the feet and legs, indicating possible ingestion of and contact with a toxic bloom.

Symptoms from sub-lethal poisonings differ with the kind of animal, nature of toxin, and quantity of toxin consumed. Any sudden, unexplained animal illness or death occurring near a water body containing a bloom should be suspect. There have been no confirmed deaths in humans due to consumption of bloom toxins. However, some blooms have caused "outbreaks" of dermatitis (a form of "swimmers itch") and attacks of gastroenteritis in groups of swimmers. These causative agents appear not to be the same toxins as the potentially lethal toxins.

Only laboratory tests can confirm whether a bloom is toxic or non-toxic. If you live in Washington state and you want to report a cyanobacteria bloom and have it tested, please see this site Washington State Toxic Algae, go to the "Report a bloom" tab and follow the instructions.

Where can blooms be found?

Cyanobacteria blooms have been documented throughout Washington State. Blooms can occur in any nutrient-rich standing water such as lakes, ponds, roadside ditches, sewage lagoons, or overflows and embayments of rivers. Chemical and physical factors needed for a bloom to form or produce toxin are complex and not fully understood. However, certain requirements are known which include elevated nutrients such as nitrogen and phosphorus, pH 6-9, and temperatures which can support cyanobacteria. Some species of cyanobacteria can also "fix" nitrogen from the atmosphere. These nitrogen-fixing species are not dependent on other nitrogen sources. They have a competitive advantage over non-nitrogen-fixing species when water nitrogen concentrations are low. Because cyanobacteria are mostly photosynthetic, they may be limited by availability of light, although they can grow under relatively low light conditions.

When can blooms occur?

According to scientific literature, cyanobacteria most commonly occur in late summer and early fall when water temperatures reach 72°-80° F. At these temperatures, cyanobacteria grow rapidly and may create a bloom within a few days. Most blooms east of the Cascade mountains in Washington follow the classic scenario and are visually observed in late summer and early fall. Data collected in Western Washington lakes have shown cyanobacteria blooms during atypical water temperatures and times of the year. Although an individual lake in Western Washington may have a predictable bloom season, this season may vary dramatically between lakes. Based upon preliminary evidence, a bloom may be found somewhere in Western Washington nearly any month of the year and at water temperatures as low as 45°-50° Fahrenheit (7°-10° Celcius).

What can I do about blue-green algae?

While there are short-term treatment options for controlling algae, the long-term solution involves finding ways to reduce nutrient inputs into the water body. Finding and reducing external nutrient inputs to the water body is crucial for long-term success and lake recovery. There are best management practices that lake residents can follow and there are some in-lake management options that may help reduce nutrients or algae problems. Once a bloom has formed, treating it with algaecides is probably not a good idea. Most chemical treatments can cause the cells to break open, releasing toxins into the water. Many cases of human poisoning from algal toxins have occurred after chemical treatments of blue-green blooms.


Most of the above information is from Toxic Cyanobacterial Blooms - A Field/Laboratory Guide. This guide was written by Dr. M. A. Crayton from Pacific Lutheran University, Tacoma, Washington and edited by Dr. F. Joan Hardy, Washington State Department of Health. Used with permission.