Blue-Green Algae (Cyanobacteria)

Blue-green algae, or cyanobacteria, are bacteria that often live in water and get their energy through photosynthesis, like plants. Although algae is in their common name, blue-green algae are actually a type of bacteria separate from true algae. Similarly confusing, they are most often green in color instead of blue.

When conditions are right, certain types of blue-green algae have the potential to produce toxins and form harmful algal blooms (HABs). HABs can be toxic to aquatic life, animals and people. HABs have occurred in various lakes throughout Wisconsin, including in the Great Lakes.

HABs are occurring more frequently in Wisconsin because blue-green algae thrive on excess phosphorus in the water, and phosphorus pollution is a growing problem. Phosphorus pollution originates from sources like agricultural runoff, municipal discharges and household products.[1]

Key Points

  • Toxic blue-green algae blooms close beaches, keep people out of the water, and can threaten the health of people and the environment.
  • In 2009 and 2010, there were a total of 57 illnesses related to blue-green algae reported in Wisconsin waters.[2]
  • Blue-green algae blooms are becoming more frequent due to excess phosphorus pollution in our waters.
  • Reducing phosphorus pollution can reduce the number of blue-green algae blooms that occur in Wisconsin.
  • It is estimated that freshwater eutrophication in the U.S. has economic impacts of between $5 and 7 billion per year.[3]
Clean Wisconsin's Work

The excess phosphorus from agricultural runoff, household products, municipalities and utility discharge is a major pollution threat to Wisconsin’s waterways. It contributes to the harmful algal blooms of cyanobacteria capable of producing toxins dangerous to human beings, especially children. We must reduce these harmful blooms as much as possible to assure Wisconsin has clean and safe waterways in the future.

Clean Wisconsin has been involved in fighting for clean waters since we supported the EPA and DNR water quality standards in 1987. Since 1992, with the passing of Wisconsin’s Nonpoint Pollution Act, we have been working hard to gain many victories for phosphorus pollution reduction in Wisconsin. Our most recent victory in 2010 was the passing of Wisconsin’s innovative new phosphorus rules package.  We are supporting these laws in hopes that they will prevent future harmful blue-green algal blooms.

Clean Wisconsin Media and Materials


Questions and Answers

What are blue-green algae?

Blue-green algae, or cyanobacteria, is a group of many different types of organisms that often live in water and get their energy through photosynthesis, like plants. They make up one of the largest groups of bacteria, and are widely credited with generating the current oxygen atmosphere of the Earth some 2.4 billion years ago.[4] They can be present as unicellular individuals or as a colonial species in filaments, sheets or hollow balls.

The common name “blue-green algae” is confusing, since cyanobacteria are not in fact considered to be algae. This is because algae have eukaryotic cells, which feature internal membranes and can be quite complex. Cyanobacteria, on the other hand, are prokaryotes, meaning that their cells lack a membrane-bound nucleus. Additionally, blue-green algae are not often blue in color.

What problems are associated with blue-green algae blooms?

Blue-green algae can cause many changes in an aquatic system such as discolored water, reduced light penetration, taste problems, odor problems, dissolved oxygen depletions and toxin production. These effects alter the ecosystem, often making conditions impossible for aquatic life to survive. Invasive species may take the place of native species, creating an unstable ecosystem. The unsightly colors and disagreeable odor of blue-green algae make waters less appealing for fishing, tourism and recreation.[1]

The toxins produced by some blue-green algae can result in sickness in humans and animals. Common illnesses include allergy-like reactions with rashes, eye/throat/nose irritation, asthma, headaches, fever and gastroenteritis (nausea, stomach cramps, vomiting or diarrhea). Children are more at risk than adults because they are more likely to transfer the algae from their bodies to their mouths and are smaller in size, making effects more drastic. Pets are vulnerable as well because they are more likely to swim in and drink unsightly water.[1]

What makes cyanobacteria reproduce so quickly?

Relative to true algae, the growth rates of cyanobacteria are often quite slow. However, under certain conditions, cyanobacteria are capable of reproducing explosively. The conditions that fuel blue-green algae growth can include dramatically increased access to sunlight (as can happen when invasive mussel species cause an increase in water clarity) or increased levels of nutrients such as nitrogen and phosphorus in the water. In freshwater ecosystems, phosphorus pollution has the biggest impact on cyanobacteria growth. As a result, agricultural runoff is a major contributor to the rapid growth of these blue-green algae.

What else can environmental changes do?

In addition to causing rapid growth, changes in environmental conditions can also include a shift in cyanobacteria community composition from primarily nontoxic to toxic strains, and an increase in toxin production. This is most often caused by changes in the chemical composition or physical characteristics of a waterbody. However, other factors can also impact cyanobacteria toxicity; for example, invasive zebra mussel populations promote toxic growth by selectively rejecting toxic strains and choosing to consume more palatable species instead.[5]

What can be done to reduce the harmful blue-green algae blooms in Wisconsin?

Harmful blue-green algae blooms can be reduced in the environment through multiple methods. The best and most efficient way is to prevent them from occurring in the first place by limiting the amount of phosphorus entering our waterways. The Wisconsin Department of Natural Resources has revised the state Administrative Codes to make them more effective in fighting phosphorus by reducing the amount allowed to runoff from agricultural sources as well as limiting the phosphorus levels in discharge from plants.[5]

When blooms are already occurring, there are multiple ways to attempt to reduce their severity. The first is physical disruption. For example, an area can have air bubbles added, an increase in water flushing rate, or a decrease in water retention time, to disperse and reduce the blooms. The area could be biologically altered as well. This would generally include removing selective grazers such as zebra mussels that promote the growth of toxic blooms. Algal grazers can also be introduced, as well as lytic bacteria or viruses, to consume or otherwise destroy the blooms.[6] While these methods have potential to reduce the current blooms though, they could also have unforeseeable effects on the aquatic ecosystems in the area. Because of this uncertainty, blooms are often left to die out or disperse naturally (often due to changes in weather), and actions to preventing the blooms from reoccurring become that much more important.

Quick Facts
  • Blue-green algae blooms degrade water quality, alter food webs and can produce toxins.[6]
  • Blue-green algae blooms are a widespread problem, and toxin concentrations when they are present can exceed levels safe for humans and wildlife.[7]
  • Blue-green algae blooms are most often seen on the surface of waterways that have high phosphorus levels, appearing as “pea soup” or “green paint” in the water.[1]
  • Some health effects resulting from inhalation or ingestion of cyanobacteria include eye, ear and skin irritation, vomiting, diarrhea, liver damage, respiratory illness, nerve damage and cardiac arrest.[7]
Wisconsin Data, Trends, and Legislation

Wisconsin has many beautiful waterways across the state. An increase of phosphorus levels from both point and nonpoint sources has resulted in many impaired waters. Lakes in Madison, Winnebago, Menomonie close each summer due to dangerous levels of blue-green algae.[8]

The levels of phosphorus greatly contribute to the harmful blue-green algae blooms that make the waters unsafe for humans and put the aquatic ecosystem at risk. In the Great Lakes, levels of microcystin, a toxin produced by some blue-green algae species, have been known to exceed those recommended by the World Health Organization of 1 μg/L (0.001 mg/L). This is very problematic because we depend on the Great Lakes for drinking water. The Great Lakes are the largest supply of freshwater in the world, comprising 80% of the U.S freshwater supply and providing drinking water for 40 million people in the U.S. and Canada.[6]

  • In 2009 and 2010, there were a total of 57 algae-related illnesses reported in Wisconsin waters: 22 in Dunn County, 17 in Adams County, and 8 in Dane County.[2]

Current Law

There is no legislation in Wisconsin specifically addressing cyanobacteria. However, cyanobacteria toxic blooms occur more when there is an increase in a waterway’s eutrophication. In Wisconsin, this is a result of excess phosphorus entering our waters. There are multiple new and revised laws to reduce these phosphorus levels:

NR 151

NR 151 dictates runoff management and is used for nonpoint sources. It implements a system called the Phosphorus Index (P Index), developed at UW-Madison, to rate the risk of phosphorus runoff from various agricultural sites. There is a numeric limit to the P Index value to allow the certain high-risk sites to be targeted for improvement. There are also new tillage setback standards to protect surface waters.

NR 102

NR 102 is in place to protect surface waters and sets standards for factors affecting aquatic life such as temperature, pH, and pollutants. The new NR 102 places phosphorus criterion for different waters in units of mass per volume. For example, Lake Michigan has phosphorus criterion of 7 ug/L.

NR 217

NR 217 sets effluent (discharge) standards and limitations for phosphorus in our waterways. This rule targets point sources discharging wastewater such as treatment works by placeing limits in the permits to lower the amount of phosphorus that can be discharged.

Assembly Bill 281

Assembly Bill 281 was placed in Wisconsin statute in 2009. It lowers the amount of phosphorus allowed in household dishwashing soaps from 8.7% by mass to 0.5% by mass. It does not apply to non-household dishwashing soaps or those used to sanitize medical or surgical equipment.

Proposed Legislation

There is no proposed or pending legislation directly affecting blue-green algae.


Did we miss something? Let us know by emailing

You can support our work by becoming a member of Clean Wisconsin at

Please cite this resource as: Clean Wisconsin, Inc. “Blue-Green Algae (Cyanobacteria).” Clean Wisconsin Enviropedia. Retrieved from