Cyanobacteria Monitoring Map

The 2022 cyanobacteria monitoring program is winding down for the season and the number of ponds being monitored consistently over the next few weeks has been reduced. The list of ponds we continue to monitor can be found here (PDF).

Remember, if you notice pond water is scummy, or discolored and may have a strong odor, avoid contact. If you see what you think might be a suspicious cyanobacteria bloom, notify your local health department and send a photo to noting the location, day and time.


How to use the map:

Pan around the map and zoom in/out to locate an area or specific pond you are interested in.
Use the legend by clicking the arrow on the upper left corner of the map window to identify the cyanobacteria status of the pond you are interested in.
Click on a pond to open a pop-up box with more info about our monitoring results. If the pond you are interested in is not colored and the pop-up box does not offer recent information this means it has not been sampled recently.
For more information about what the results mean, select from the list of terms below for definitions.
If you have witnessed a possible cyanobacteria bloom or other related event at a pond that is not included on this map, please follow the instructions in the box below titled, “WHAT TO DO IF YOU SEE A POSSIBLE CYANOBACTERIA BLOOM.”

Definition of Terms


Name of the lake or pond selected. Note that lakes and ponds may have multiple names. The name listed is the most commonly used.

Cyano Status: Potential for Concern

Monitoring results or the presence of cyanobacteria scum at the time and place of sampling indicate a potential for increased risk for exposure to cyanobacteria toxins approaching but below state standards. Conditions do not yet warrant the posting of a recreational human health advisory according to guidelines from the Massachusetts Department of Public Health (MDPH). While these conditions pose low health risks to adults, risks are higher for children or pets based on lower body mass, particularly if contaminated water is incidentally ingested. Children may inadvertently consume pond water while swimming and pet exposure can result from drinking or ingesting pond water or from grooming after swimming. Map color is yellow. Map color yellow with crosshatching indicates a municipal pet advisory has been issued. Formerly the Moderate Warning Tier.

Sample Location

The location of sample collection on or around the pond. Note that samples collected by boat are classified as “Pond Center” and are from the deepest point in the pond and not necessarily the geographic center. Samples collected at the shoreline are roughly indicated by North, South, East or West. It’s important to note that floating cyanobacteria are easily moved by the wind and tend to congregate on windward shorelines.


Indicates which type of cyanobacteria is most abundant in a given sample. A “mixed” dominance sample indicates roughly equal abundance of different types. This information is important because different types of cyanobacteria produce different types of cyanotoxins at different rates.

Cyanobacteria Bloom

Cyanobacteria blooms occur when cyanobacteria multiply quickly in a pond or lake. A cyanobacteria bloom may occur below the pond’s surface, or it may be visible on the pond surface as floating scum, foam, or a mat. Cyanobacteria blooms often occur when the water is warm, stagnant, and rich in nutrients from sources such as septic systems or stormwater. Cyanobacteria blooms can make people and animals ill (

Possible Pet Health Effects

Pet exposure can be from drinking pond water or grooming after swimming. Due to lower body masses, pets are more susceptible to impacts at lower concentrations than adult humans. Pets exposed to suspected cyanotoxins should be rapidly assessed by a veterinarian.


The town in which the pond exists. If the pond is within more than one town, the town indicated is the one where the sample was collected.

Cyano Status: Use Restriction Warranted

Monitoring results at the time and place of sampling indicate the pond is unsafe for recreation by humans and pets based on one or more of the following criteria: 1) presence of microcystin at or above state standards (8 ppb microcystin) as described in MDPH guidance, 2) presence of significant cyanobacteria scum layers according to MDPH guidance, 3) a municipal health agent issues a closure for any other reason related to cyanobacteria. Recreating at this site may cause harm to adults following exposure ( Recreational risks are especially high for children and pets following exposure through accidental ingestion of contaminated water. Children may inadvertently consume pond water while swimming and pet exposure can result from ingestion or directly drinking pond water or from grooming after swimming. Due to lower body masses, children and pets are more susceptible to cyanobacteria risks than adults. Map color is red. Map color red with crosshatching indicates a municipal advisory has been issued. Formerly the High Warning Tier.

Water Temp F

This reading is the temperature of water at the pond surface at the sample location. All temperature data provided here is recorded in degrees Fahrenheit.

Two Common Types of Cyanobacteria

Dolichospermum – A genus of cyanobacteria very common in Cape Cod ponds. It can produce hepatotoxins, dermatoxins, and neurotoxins, but is known to produce Microcystin at relatively low levels.

Microcystis – A genus of cyanobacteria that is common in Cape Cod ponds. It can produce hepatotoxins, dermatoxins, and neurotoxins, but is known to produce Microcystin at relatively high levels.

Cyanobacteria Growth Rates
APCC tracks changes in cyanobacteria concentrations between each sampling event. Rapid growth rates, defined here as net daily cyanobacteria growth rates greater than or equal to 0.05, may lead to a cyanobacteria bloom formation or microcystin exceedance. Alternatively, the cyanobacteria concentrations may peak and then decrease before a cyanobacteria bloom or microcystin exceedance occurs. APCC will recommend weekly testing of ponds where any APCC sample has a confirmed net daily cyanobacteria growth rate greater than or equal to 0.05. 
Children’s Exposure
Due to lower body masses, children are more susceptible to cyanobacteria toxins at lower concentrations than adults. See the link to the CDC page on “Harmful Algal Bloom Associated Illnesses” at the bottom of this page for more information on exposure and symptoms of health impacts from cyanotoxins.
Sample Date

The date the sample was taken.

Cyano Status: Acceptable

No concerning cyanobacteria results at the time and place of sampling. To the best of APCC’s knowledge and based on our monitoring results, regular recreational usage of the pond is safe with respect to cyanobacteria and toxins. Map color is blue. Formerly the Low Warning Tier.

Cyanobacteria Scum

This is a floating scum, foam, or mat composed of cyanobacteria. A cyanobacteria scum is one indicator of a cyanobacteria bloom and can be green, blue, brown, or even red in color ( ). Cyanobacteria scum can also collect along the shore as a scum line when winds blow the scum towards the shore.
The presence of a cyanobacteria scum can indicate high toxin levels at that location. MDPH recommends avoiding contact with the pond for a minimum of two weeks after a significant cyanobacteria scum has formed and issuance of an advisory ( Ponds with limited or less significant cyanobacteria scums will be marked in the Potential for Concern category on APCC’s interactive map. Cyanobacteria scums deemed significant by the Massachusetts Department of Public Health or a town health agent will be marked in the Use Restriction Warranted category on APCC’s interactive map. If a pond has a cyanobacteria scum at one location but no cyanobacteria accumulations at a different location, the location with the cyanobacteria scum may present a higher risk to pond users. However, as conditions in ponds can change rapidly, APCC recommends following the CDC and MDPH recommendation for ponds with unknown toxin risks: “When in doubt, stay out.” Other types of scum can be caused by pollen or harmless algae; in such cases APCC labels them as such in the notes section to avoid confusion. If you find a possible cyanobacteria scum near the shore of your pond, take a photo and inform your town health department and APCC at

Common Toxins Produced by Cyanobacteria

Dermatoxin – A toxin that is a skin irritant. The most commonly occurring dermatoxin, lipopolysaccharide, is unregulated by the state.

Hepatotoxin – A toxin that can cause liver damage.

Microcystin – A type of hepatotoxin that can cause liver damage and is the primary cyanotoxin regulated by the Massachusetts Department of Public Health.

Neurotoxin – A toxin that can cause neurological damage. Anatoxin and beta-N-methyl amino-L-alanine (BMAA) are known neurotoxins that are unregulated by the state, but are currently being studied by cyanobacteria researchers.

Town Advisory

The posting of advisories for cyanobacteria concerns are determined by a town’s health department. Typically, a town will post signage at a pond’s public access point(s). If a town posts an advisory and informs APCC it has taken such action, black hashmarks are added to the pond on the map. These hashmarks will be removed when APCC has been informed by the town that the advisory has been lifted. For Potential for Concern ponds (yellow), the town of Barnstable uses their own criteria to post “Pet Advisories.” If the town of Barnstable has posted a Pet Advisory and informed APCC of this decision, APCC will add black hashmarks to these ponds until APCC has been informed that the Pet Advisory has been lifted. See the Barnstable Water Resources website for more information on the Pet Advisory.

Program Overview

APCC’s Cyanobacteria Monitoring Program partners with officials at the town, county, state, and federal levels as well as local pond associations and residents to conduct cyanobacteria monitoring in Cape Cod ponds. Each season, data is collected biweekly and shared with local officials and the general public through reports, emails, and our interactive map of monitoring results. Results are communicated as either “Acceptable,” “Potential for Concern,” or “Use Restriction Warranted.” These terms are described in the Definitions section.


Our program goals are to:

  • Raise public awareness of the health and ecological risks posed by cyanobacteria blooms
  • Help inform proper responses to cyanobacteria concerns in order to promote public health
  • Monitor priority ponds across the Cape
  • Motivate public action to address the causes of harmful cyanobacteria blooms (HCB) by improving water quality

Photo Gallery

What are Cyanobacteria?

Cyanobacteria are an ancient group of photosynthetic microorganisms commonly found in freshwater systems on Cape Cod and worldwide. Under the right conditions, they can multiply rapidly and form harmful cyanobacteria blooms (HCBs). According to the Centers for Disease Control and Prevention (CDC), certain common cyanobacteria genera can produce toxins known as cyanotoxins that can be harmful to humans. HCBs have increased worldwide, including in the U.S., because of nutrient enrichment and rising water temperatures due to warming resulting from climate change.

What to do if you see a possible cyanobacteria bloom:

  1. Avoid contact and don’t let your dogs or children near the water.
  2. Take photos and make note of pond name, date, time, and location of the possible bloom.
  3. Report your observations to the local town department of health or natural resources.
  4. Inform APCC by emailing the above info about the possible bloom to so we can consider adding the pond to our monitoring program if it’s not already included.

Monitoring Program Methodology

APCC’s cyanobacteria monitoring program provides scientifically sound data on cyanobacteria composition, concentrations, and predicted toxin concentrations. Our program uses and follows the EPA’s Quality Assurance Project Plan (QAPP) for cyanobacteria monitoring, developed by EPA for the Cyanobacteria Monitoring Collaborative or CMC. The QAPP was developed by EPA Region 1 scientists, including Hillary Snook and others, with the goal of encouraging and facilitating widespread monitoring of cyanobacteria. The QAPP is based on methods created by EPA scientists and other cyanobacteria specialists, including Dr. James Haney, who is affiliated with the University of New Hampshire Center for Freshwater Biology, and Nancy Leland of Lim-Tex, Inc. The method involves taking concentrated samples of “Bloom Forming Colonies” (BFCs) through a 3-meter student plankton net tow and unconcentrated samples of “Whole Lake Water” (WLW) through a meter integrated tube.

APCC also utilizes the CyanoCasting method developed by Nancy Leland, which builds on the methods described in the QAPP by including metrics that allow for the forecasting of potential imminent cyanobacteria blooms. The forecasting ability of this method provides valuable advance warnings of potential HCBs to inform proactive responses, such as increased frequency of specific toxicity testing or precautionary advisories and postings of ponds to warn the public of imminent cyanobacteria blooms. The ability to predict and anticipate potential HCBs based on frequent monitoring is a unique and valuable feature of APCC’s program and stands in contrast to reactive responses involving measurement of cyanobacteria concentrations after a bloom has occurred.

Beginning in 2022 as a complement to APCC’s well-established monitoring program, the Barnstable County Department of Health and the Environment (BCDHE) Water Quality Lab will expand its cyanobacteria toxin testing capabilities to provide local officials with precise toxin measurements from ponds pre-identified by APCC as potentially containing cyanobacteria toxin levels of concern. APCC’s proactive cyanobacteria monitoring data will be used to flag ponds that will receive confirmatory toxin testing by the BCDHE Water Quality Lab to determine whether an advisory is warranted. The BCDHE Water Quality Lab will then pass on its results and recommendations to local officials and APCC. These results will supplement data collected and communicated by APCC.

Use Restrictions and Advisories

Use restrictions and advisories are issued at the discretion of the municipal health agents. As of this date, there is no commonly utilized set of guidelines in use by health agents across the Cape that provides consistency in posting criteria. As a result, members of the public are advised to contact the health agent in their town (Cape and Islands Health Agent Contact List PDF) to determine the official status of the pond in which they are interested. Ponds exceeding MDPH standards will be marked in red on APCC’s map, but this coloration does not always mean that a use restriction has been issued by the town. APCC will update our list of restricted ponds as we are informed by the respective towns, but APCC does not speak for the towns unless otherwise and explicitly noted on our posting map.

APCC’s recommendations for removing a recreational use advisory will mirror the reopening guidance from MDPH. For a microcystin toxin exceedance or cyanobacteria scum, APCC will recommend lifting a recreational use advisory or closure after two consecutive tests a week apart show microcystin concentrations less than 8 parts per billion (ppb) and little to no presence of cyanobacteria bloom material, depending on the basis for the original restriction. Health agents are solely responsible for the issuance and removal of recreational use advisories or closures related to water clarity, such as clarity less than 4 feet.

Interpretation of Results

Combining APCC and BCDHE data with MDPH guidance, APCC will communicate and display results on the interactive map according to the table below.

APCC 2022 Cyanobacteria Risk Categories

Information and Resources:

Following are some links to various agencies who provide more information about cyanobacteria, harmful cyanobacteria blooms (HCBs) and cyanotoxins.

Massachusetts Department of Public Health
Center for Disease Control
U.S. Environmental Protection Agency
Cyanobacteria Monitoring Collaborative
University of New Hampshire – Phycokey
University of New Hampshire – Dirty Dozen

Program Partners

Town of Barnstable MASS Seal
Town of Brewster MASS Seal
Town of Chatham MASS Seal
Town of Wellfleet
Town of Yarmouth
Town of Falmouth
Town of Mashpee
Town of Harwich Seal
Town of Sandwich Seal
Town of Orleans Seal
Town of Truro Seal
Town of Provincetown Seal
United States Environmental Protection Agency
Falmouth Water Stewards
Friends of Chatham Waterways
Brewster Pond Coalition
Center for Freshwater Biology
Massachusetts Bays

Funding generously provided by:

Mary-Louise & Ruth N. Eddy Foundation, Cape Cod Healthcare, Kelley Foundation, and Horizon Foundation.