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Algal bloom at Grand Lake St. Mary's, Ohio, 2010. Photo by Russ Gibson, Ohio EPA

Cyanobacterial Harmful Algal Blooms

Algae are natural components of marine and fresh water flora performing many roles that are vital for the health of ecosystems. However, when certain conditions are favorable, algae can rapidly multiply causing "blooms" or dense surface scums. When blooms are present in large quantities they can pose a significant potential threat to human and ecological health. In addition to algae, microorganisms like cyanobacteria, historically known as blue-green algae, are able to form blooms some of them with the presence of toxins and other noxious chemicals. These are known as cyanobacterial harmful algal blooms (cyanoHABs) and are of special concern because of their potential impacts on drinking and recreational waters.

EPA has compiled information on freshwater cyanoHABs including causes, detection, treatment, health and ecological effects, current research activities in the U.S.; and policies and regulations for cyanotoxins at the state and international levels.


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The most common cyanotoxins in the U.S.


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Sample collection and list of detection methods available for cyanotoxins.

Health and Ecological Effects

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Routes of exposure, adverse human health outcomes, some of the effects on aquatic ecosystems.

Research and News

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Current research activities on cyanotoxins in the U.S.

Causes and Prevention

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Major causes of HABs and ways to prevent their occurrence.

Control and Treatment

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How to mitigate and treat for cyanotoxins in freshwater and drinking water.

Guidelines and Recommendations

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Action levels and guidelines from the WHO and the U.S. for recreational and drinking water.

State Resources

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State-by-state list of resources and a link to the Inland HABs webpage.

More Information

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Links to resources, websites, and activities from EPA, CDC, NOAA, USGS, and international agencies.

For comments, feedback or additional information, please contact Lesley D'Anglada (, Project Manager, at 202-566-1125.