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Monitoring and Responding to Cyanobacteria and Cyanotoxins in Recreational Waters

The information provided on this website is intended for recreational waterbody managers, which may include public health officials, lake managers, or other state, local or tribal officials involved in monitoring water quality and protecting the health of people and animals that use waterbodies within their jurisdiction.

This information does not impose legally binding requirements on the EPA, states, tribes, or the public, nor does it confer legal rights. It does not constitute a regulation, nor does it change or substitute for any Clean Water Act provision or EPA regulation. Any mention of trade names, products, or services does not convey and should not be interpreted as conveying official EPA approval, endorsement, or recommendation for use.

What are Cyanobacteria and Cyanotoxins?

Cyanobacteria, commonly referred to as blue-green algae, are photosynthetic bacteria that occur naturally in waters used for primary contact recreation, such as swimming and waterskiing. Under certain conditions, cyanobacteria may grow rapidly to form dense accumulations known as cyanobacterial blooms. When the bloom is formed by a toxin-producing bacteria, it is generally referred to as a harmful algal bloom (HAB).

These blooms are considered harmful due to the production of irritants and/or toxins, called cyanotoxins (e.g., microcystins and cylindrospermopsin), which can pose health risks to humans and animals. For more information on health risks associated with exposure to the cyanotoxins, cylindrospermopsin and microcystins, see:

Animals such as pets, livestock, and wildlife may also be exposed to cyanotoxins if they drink water from toxin-contaminated water bodies, lick their fur after swimming in such waters, or consume toxin-containing algal scum or mats. Health effects from cyanotoxin exposure in animals can include vomiting, diarrhea, seizures, and death.

What Causes Cyanobacterial Blooms?

Certain environmental conditions, such as elevated levels of nutrients from human activities (e.g., nitrogen and phosphorus), warmer temperatures, still water, and plentiful sunlight can promote the growth of cyanobacteria to higher densities, forming cyanobacterial blooms.  Such blooms may result in a higher risk to human or animal health due to the production of cyanotoxins and other cyanobacteria-associated irritants. Although the presence of cyanobacteria does not necessarily mean that cyanotoxins are being produced, it is important to note that cyanotoxins may be present both before and after cyanobacteria are observed.

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What Are Some Visual Signs of a Cyanobacterial Bloom?

Visual signs of a bloom include: surface water discoloration (e.g., a green, white, brown, red, or blue tint); reduced transparency (e.g., water that looks like pea soup or lets limited light through); or, thick, mat-like accumulations of scum on the shoreline and surface. Cyanobacteria are also associated with unfavorable taste-and-odor compounds in lakes and reservoirs. The following  photos show visual signs of cyanobacterial blooms, including surface water discolorations, scum, or, mat-like accumulations along the surface.

Figure 1: Cyanobacterial Bloom - Sudbury River, MAFigure 1: Cyanobacterial Bloom - Sudbury River, MA (Photo by Susan Flint) Figure 2: Cyanobacterial Bloom - Lake AttitashFigure 2: Cyanobacterial Bloom - Lake Attitash (Photo by Nancy Leland) Figure 3: Cyanobacterial Bloom - Monponsett Pond, MAFigure 3: Cyanobacterial Bloom - Monponsett Pond, MA (Photo by Edward Broderick)

As discussed under “What Causes Cyanobacterial Blooms?” above, it is important to note that a cyanobacterial bloom may be present without producing cyanotoxins, and conversely, cyanotoxins can be present both before and after blooms are visible. Therefore, it is recommended that cyanotoxin levels be confirmed through laboratory testing of the water. Microscopic phytoplankton identification can provide information when blooms are present and not visually apparent.

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What Can Be Done to Reduce the Occurrence of Cyanobacterial Blooms?

Addressing nutrient pollution (i.e., nitrogen and phosphorus) can help reduce the occurrence of cyanobacterial blooms. As a long-term strategy, states may consider adopting numeric nutrient criteria and/or numeric interpretations of a narrative nutrient criterion into their water quality standards. The World Health Organization: Guidelines for Safe Recreational Water Environments (PDF)(253 pp, 1 MB) contains a chapter on algae and cyanobacteria in fresh water, which includes short-term and long-term management options with the goal of preventing or reducing the occurrence of cyanobacterial blooms in recreational waters. States may also consider cyanotoxin criteria, in addition to numeric nutrient criteria, and evaluate the need for (and effect of) the criteria and/or advisories in their recreational waters. For the results of a study on how the experimental limitation of nutrient supplies, after a lengthy enrichment period, aided in the diminishing of a cyanobacterial bloom, see Reversal of a cyanobacterial bloom in response to early warnings (PDF)(6 pp, 653 K).

For more information on what EPA is doing to reduce nutrient pollution and for tools to assist states and tribes, see:

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How Can Recreational Waterbody Managers Be Prepared to Respond to a Cyanobacterial Bloom in the Future?

Recreational waterbody managers can take several steps to prepare in the event of a future cyanobacterial bloom. The following actions will help managers protect people, pets and livestock from exposure to HABs:

  • Prioritize recreational waters based on risk. Recreational managers with limited resources may choose to take a risk management approach toward monitoring the recreational water bodies under their jurisdictions. Such an approach may include prioritizing water bodies based on the likelihood of a HABs event and its relative impact to the public. Prioritization may consider past occurrences of HABs, current environmental conditions (including the levels of nitrogen and phosphorus, temperature, availability of organic matter, light attenuation, and pH), and waterbody use (i.e., type of recreation and the number of users).

  • Develop a response plan. Some local and state governments have already implemented response guidelines in the event of a cyanobacterial bloom in recreational waters. These include: (1) identifying state-designated recreational water health advisory levels for analyzing the severity of a bloom (as measured by cyanotoxin concentrations or cyanobacteria cell counts); and, (2) taking specific actions, such as issuing public advisories, posting warnings, and closing waterways that exceed a predetermined threshold. For a summary of the U.S. states with health advisory values, see Guidelines for Cyanobacteria and Cyanotoxins in Recreational Water. It is also helpful to have a communication plan ready which includes key contacts and ways to notify the public. For a contact list template to help a recreational water manager initially respond to a cyanobacterial bloom, go to Cyanobacteria Bloom Response Contact List(1 pg, 34 K, July 2017, EPA 820-F-17-003) .

  • Develop a monitoring plan. For information and process steps that a recreational water manager may use to confirm a cyanobacterial bloom event and monitor cyanotoxin levels, including when to post and remove notification signs, go to [See Recommendations for Cyanobacteria and Cyanotoxin Monitoring in Recreational Waters ]

  • Develop a Control and Treatment Plan. Recreational water managers should reduce nutrient pollution and address other underlying factors that can cause cyanobacterial blooms; however, they should also know what technologies are available to control, mitigate and treat cyanobacterial blooms and cyanotoxins in an emergency. For more information, see Control and Treatment - Mitigation Measures for the Presence of HABs in Surface Water. Measures to treat blooms should only be undertaken after consultation with relevant authorities.

  • Develop signage and other communication methods to notify the public. For information on recommended communication tools, go to Recreational Water Communication Toolbox for Cyanotoxins and HABs. For an interagency toolbox for communicating with stakeholders and the public about water advisories (based upon research and identified practices), see Drinking Water Advisory Communication Toolbox. Although this drinking water advisory toolbox was not designed for recreational waters, the same principles and ideas can easily be adapted for recreational waters. 

  • Prepare to respond to media and public inquiries. See a list of Frequently Asked Questions - Cyanobacterial Blooms and Cyanotoxins(3 pp, 38 K, July 2017, EPA 820-F-17-008) about HABs.

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What Should a Recreational Waterbody Manager Do if a Cyanobacterial Bloom is Identified or Suspected?

Recreational water managers should take action to confirm the presence of toxin-producing cyanobacteria and/or cyanotoxins, and consider whether to notify partner agencies and the public, depending on the relative threat to public health. They should follow key steps of an emergency response plan including: 

1) Analyze samples from the recreational waters to assess whether the bloom is producing cyanotoxins at levels potentially harmful to human or animal health. 

2) Notify key partners at the local and state level to coordinate a response.  

  • For a list of possible partner agencies (such as poison control, veterinary facilities, and public health agencies), see Cyanobacteria Bloom Response Contact List(1 pg, 34 K, July 2017, EPA 820-F-17-003).

3) Issue public notifications (i.e., warnings, advisories, or closures) based on the cyanotoxin level and the risk it presents to human and animal health. 

4) Consider treatment options, if necessary, to bring cyanotoxins concentrations under control and down to safe levels.

5) Monitor and sample the recreational waters to confirm or modify the notification until cyanotoxins concentrations are at or below safe levels; notifications can be lifted once cyanotoxin concentrations are at or below safe levels. 

6) EPA recommends that state water recreation managers or appropriate state partners report suspected or confirmed harmful blooms and/or human and animal illnesses associated with cyanobacterial blooms to the One Health Harmful Algal Bloom System (OHHABS).  The Centers for Disease Control and Prevention (CDC) developed OHHABS as a voluntary reporting system available to state and territorial public health departments and their designated environmental health or animal health partners. 

  • For guidance about defining a bloom and how to report health and environmental data, see OHHABS.

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