Cyanobacterial Harmful Algal Blooms (CyanoHABs) & Water

Cyanobacteria are microscopic bacteria that live in all types of waterbodies. A large growth of these bacteria results in algal blooms. These blooms can pollute the water and may even be toxic to animals and people.

Table of Contents

What are cyanobacteria?

Cyanobacteria have been often referred to as blue-green algae; however, they are a group of microorganisms that share similar characteristics of algae, such as the ability to perform photosynthesis like green plants using the chlorophyll in their cells. Cyanobacteria may occur as single cells, thread-like filaments, or as colonies of various sizes and shapes composed of groups of many filaments or cells. They are naturally occurring in all waterbodies with some species growing in waterbody sediments, while other cyanobacteria can move within the water column. Some cyanobacteria have the ability to produce toxins, called cyanotoxins.

What are CyanoHABs?

When a dramatic increase in a cyanobacteria population occurs, this is called harmful algal blooms (HABs), or more accurately, cyanobacterial HABs (CyanoHABs). It often results in the waterbody turning bright green or blue-green, and forms a surface scum or a discoloration of the water column. CyanoHABs may also form a mat on the bottom sediments, which is more difficult to observe. The surface scums or discolored waters can extend several inches below the water surface, or accumulate near shorelines and in coves from onshore wind action. They often look like pea soup or spilled, blue or green paint; however, the color can also vary. CyanoHABs can cause dissolved oxygen (DO) swings that may result in plant and animal die-off, taste and odor issues, and can cause potential public health issues from the cyanotoxins they may release.

What are possible health effects associated with CyanoHABs?

Health concerns associated with CyanoHABs vary depending on the type of cyanobacteria, the route of exposure, and the amount of cyanotoxins present. Ingestion is the primary concern since ingesting small amounts of cyanobacteria or cyanotoxin can cause gastrointestinal symptoms while larger amounts may cause liver or neurological damage. Contact with cyanobacteria can cause skin or eye irritation. Inhaling water spray containing cyanobacteria can cause asthma-like symptoms. Small children and pets are more susceptible to the effects of cyanotoxins than adults.

Are cyanobacteria regulated contaminants in drinking water or in recreational water bodies?

No, cyanobacteria and the cyanotoxins they may produce are not currently regulated by the federal government or Massachusetts. However, the US Environmental Protection Agency (US EPA) and the Massachusetts Department of Public Health (MDPH) have issued guidance for evaluating potential health concerns.

Drinking Water

On June 17, 2015, US EPA released 10-day drinking water (DW) health advisory (HA) levels for two cyanotoxins – microcystins and cylindrospermopsin. HAs are non-regulatory concentrations of DW contaminants at or below which adverse health effects are not anticipated to occur by oral ingestion over specific exposure durations. HA values are determined by US EPA using the best available information on health effects, exposure and other relevant data. The HA levels for microcystins and cylindrospermopsin for children younger than 6 years old are 0.3 parts per billion (ppb) and 0.7 ppb, respectively. For children older than 6 years old and adults, the HA levels for microcystins and cylindrospermopsin are 1.6 ppb and 3.0 ppb, respectively. Since there are over 100 types of microcystins, the HA values apply to their sum or total.

In compliance with US EPA’s fourth round of the Unregulated Contaminant Monitoring Rule (UCMR4), certain Public Water Systems (PWSs) in the United States, including Massachusetts, will begin testing their drinking water for ten cyanotoxins (total microcystins, microcystin-LA, microcystin-LF, microcystin-LR, microcystin-LY, microcystin-RR, microcystin-YR, nodularin, anatoxin-a, and cylindrospermopsin) between 2018 and 2020. Data from the UCMR serves as a primary source of research information, which US EPA utilizes to develop regulatory decisions. Thus, data are not evaluated or acted upon in real-time relative to the HAs. For further information on UCMR4 and cyanotoxins assessment monitoring, please see:

Recreational Water

MDPH has health guidelines for evaluating the presence of cyanobacteria and microcystin in waterbodies used for recreation. Since 2008, MDPH has recommended that individuals be advised not to contact the water when a visible scum or mat is present, the total cyanobacteria cell count exceeds 70,000 cells per milliliter of water (cells/ml), or the microcystin level equals or exceeds 14 parts per billion (ppb). These recommendations are typically made to the local health department, which issues the advisory.

EPA is publishing their final Recommended Recreational Ambient Water Quality Criteria or Swimming Advisories for two Cyanotoxins, Microcystins and Cylindrospermin, under Clean Water Act section 304(a). Learn more about cyanobacterial HABs and how the EPA, states, territories and tribes are working to address them on the newly redesigned EPA Cyanobacterial HABs website. EPA updated and reorganized their online information about cyanobacterial harmful algal blooms (CyanoHABs) in water bodies, creating a new website dedicated to scientific information, EPA tools, and collaborative work on cyanoHABs in U.S. waters.

EPA is also providing information on the latest scientific knowledge about human health effects from exposure to cyanobacteria, discussion of other governmental guidelines for recreational waters, and incidents involving exposure of pets and other animals to cyanotoxins. More information on these recommendations:

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What should I do if I see a potential CyanoHAB?

The first step an individual should take if they observe a potential CyanoHAB is to AVOID CONTACT with the affected water. While most algae are harmless, cyanobacteria can be skin irritants and some are capable of producing and releasing cyanotoxins, which can make people and animals sick. It is impossible to visually tell if a bloom is toxic or non-toxic; this can only be determined through analyses performed specifically for cyanotoxins.

Who should I contact if I see a potential CyanoHAB?

Contact your local board of health or health department, and alert them to what you have observed, along with the name and location of the waterbody. Your local board of health may contact MDPH or the Massachusetts Department of Environmental Protection (MassDEP) depending on the type of waterbody. If you are a PWS surface water supplier and have observed a potential CyanoHAB in your source, you should contact the Drinking Water Program in your MassDEP regional office.

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Which state agency is responsible for responding to CyanoHABs?

Local health departments are the primary point of contact for responding to reports of CyanoHABs in recreational waterbodies.  MDPH provides technical support to local health departments and residents in response to reports of blooms. MassDEP is responsible for coordinating the response to reports of CyanoHABs in water bodies that are used as public water system (PWS) drinking water (DW) supplies. In some cases, a PWS drinking water source is also used for recreational purposes, such as the Merrimack or Charles Rivers. Responses to reports of CyanoHABs in these instances will be jointly coordinated by MassDEP and MDPH.

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How do I know if a CyanoHAB has already been reported?

MDPH maintains a list of cyanobacteria advisories it is aware of on its website. This website also provides additional public health information regarding CyanoHABs.

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Where can I get information on laboratories that perform cyanobacteria/cyanotoxin analysis?

A number of private and public entities conduct cyanobacteria and cyanotoxin analyses. MassDEP has developed a list of vendors that perform a variety of testing in this area, which is available here.

If you are a vendor and would like to be included in the list you should send an email to, subject: CyanoHAB Services List.

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Additional guidance for Public Water Suppliers (PWS)

MassDEP recognizes the far-reaching impacts CyanoHABs may have on PWSs with surface water supplies, which warrants additional attention particularly regarding source water protection and emergency response planning. MassDEP has developed a Guidance Document and other resources for PWSs utilizing surface water sources to help assess, monitor for, prevent and respond to CyanoHABs.

MassDEP Guidance: Cyanobacteria and Public Water Systems

MassDEP Presentation on Cyanobacteria March 15, 2018

Drinking Water Program Letter to Public Water Systems

Public Water System Fact Sheet: Cyanobacteria and Public Drinking Water Supplies in Massachusetts

Public Water System Bloom Tracking Form

Additional Resources   for Additional guidance for Public Water Suppliers (PWS)

Image credits:  Joan L. Beskenis, Ph.D., MassDEP

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