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'How Do I Know if My Local Pond Has a Cyanobacteria Bloom?' And 11 More Frequently Asked Questions

Gerald Beetham; Association to Preserve Cape Cod
A cyanobacteria bloom covers Walkers Pond in Brewster on June 25, 2020.


1. What does a cyanobacteria bloom look like? 

Often, cyanobacteria blooms can look like bright green paint on the water’s surface. They’re also described as looking like pea soup, but colors can range from blue, to brown, to red, according to the Centers for Disease Control (CDC).
Cyanobacteria blooms can also look like scum or mats, that are often found at the shoreline.

It’s important to remember that you may not be able to see a cyanobacteria bloom. Sometimes blooms stay below the water’s surface, so there’s no visual cue at all.  Or, even when there is a visual cue, wind can push that suspicious-looking scum on the water’s surface to the other side of the pond in under an hour. Even if you can’t see a bloom anymore, the water could be just as toxic.  


2. Are there other signs that my pond has a toxic bloom?

It’s a good idea to look for life around the pond. Do you hear birds chirping? Are there dead fish or frogs near the shore? Take a cue from how the ecosystem around you looks, said APCC ecologist Kevin Johnson. If a pond is unusually quiet, it might be best to follow the advice of the Massachusetts Department of Public Health: When in doubt, stay out. 

Though not all cyanobacteria blooms produce toxins, there’s no way to tell by looking at a bloom whether it’s toxin-producing or not. Toxins are invisible and can persist in the water, sand, and sediment when a bloom is gone.

3. Is there anything I can do to determine whether there’s cyanobacteria in my local pond or just an overabundance of more beneficial types of planktonic algae?

The Kansas Department of Health and Environment offers several suggestions on this score. These two tests are not 100% perfect. Additional precautions may still be necessary.

The Jar Test:

1. Find an appropriate clear glass jar with a screw-top lid. A Mason (canning) jar or a store-bought pickle jar with the label removed works well.

2. For safety reasons, use rubber or latex gloves to collect a sample of water from the pond in question to prevent skin exposure

3. Collect the water just below the surface of the water. (NOTE: Do not collect the sample directly from the surface but collect just under the surface to avoid collecting just the scum on the top layer of the water)

4. Fill the jar about ¾ of the way full with the pond water.  (Note: Do not  fill the jar completely to the top. Algae can release gases and may cause of buildup of pressure inside the jar, causing it to break.)

5. Wipe off any scum that may be on the outside of the jar.

6. Screw the lid onto the jar.

7. Be aware that the jar may be contaminated with any toxins present in the water. To avoid contamination of other surfaces, place the jar in a plastic bag or other containment device.

8. Place the jar in a cold refrigerator and leave it completely undisturbed overnight.

 9. The next day, carefully remove the jar from the refrigerator and look to see where the algae have accumulated. (NOTE: IT IS VERY IMPORTANT that you do not shake or agitate the jar in any way. If you do, this will mix the algae into the water again and you will not get usable test results).

VERDICT: If the algae are all settled out near the bottom of the jar, then that is a likely indication that the lake does not have a lot of cyanobacteria growing in it.

If the algae have formed a green ring around the top of the water in the jar, or just seem to be collected at the air/water divide, there is a strong possibility that the pond does have cyanobacteria bloom present.

The Stick Test:

1. Find a sturdy stick.

2. Put rubber or latex gloves on before attempting to retrieve a sample of the green material from the pond to prevent skin exposure.

3. Thrust the stick into the surface mat and slowly lift it out of the water. (Make sure you do not fall into the water while attempting to retrieve material.)

4. Look at the end of the stick to see what came out of the water.

VERDICT: If the stick comes out looking like it has been thrust into a can of paint, the mat on the pond is likely to be cyanobacteria.

If the stick pulls out strands that look like green hair or threads, the mat on the pond is likely filamentous green algae. (Although filamentous green algae can be a nuisance when over-abundant, they do not pose a danger to your health.)

4. I think the algae I see matches the description of cyanobacteria. Who do I call? 

First, take some photos. As previously stated, blooms can move over the course of a single hour, so documentation is key.

“Everybody can contribute to collecting helpful data,” said Karen Malkus, the coastal health resource coordinator for the Town of Barnstable.

Next, call your town’s health agent, who can provide you with further information.

Then, reach out to the Association to Preserve Cape Cod. If the nonprofit environmental organization isn’t already working with your town, its ecologists may still be able still test the pond. APCC also tracks these blooms on its website.

5. Am I allowed to put a sign up on my pond warning my neighbors of a bloom before the health agent gets there?

“Only town officials have the authority to post public areas,” Malkus said. Associations and private areas have their own rules, but too many signs can be confusing or allow for misinformation to spread. Clarity of message is very important so people understand their risk and can make safe choices for themselves.

“Individuals can share information with friends and neighbors through e-mails and text if they think there is a bloom or area of concern,” she said.

6. How to I protect myself, my family, and my pets from a toxic bloom?

First, the CDC recommends you avoid swimming, wading, or boating in areas where the water is discolored or where you see cloudy water, scum or mats of algae on the water’s surface. Exposure can come from ingesting, inhalation, or skin contact.

If you, your family, or pets do spend time around water that might contain harmful cyanobacteria, rinse off with fresh water as soon as possible.

Pets and livestock are particularly susceptible to cyanotoxins, so don’t let them drink from areas of concern or lick their fur after exposure.

If you or your pet swallows water from where there is a harmful algae bloom, the CDC recommends you call your doctor, a Poison Center, or a veterinarian.

7. If I or my pet is exposed to a bloom, how soon could symptoms appear, and what kind of symptoms could I experience?

According to the CDC, you should call a veterinarian if your pet shows any of the following symptoms of cyanobacteria poisoning: “loss of appetite, loss of energy, vomiting, stumbling and falling, foaming at the mouth, diarrhea, convulsions, excessive drooling, tremors and seizures, or any other unexplained sickness after being in contact with water.”

The kind of symptoms humans experience depends on the kind of toxins they’ve come in contact with, and they may take hours or days to show up.

The EPA warns that when people are exposed to cyanotoxins, adverse health effects may range from a mild skin rash to serious illness, or in rare circumstances, death. “Acute illnesses caused by short-term exposure to cyanobacteria and cyanotoxins during recreational activities include hay fever-like symptoms, skin rashes, respiratory and gastrointestinal distress.” Exposure to contaminated drinking water could cause liver and kidney damage.

8. Can I spread the sickness onto others?

No. Symptoms cannot be passed from animal to person or person to person according to the Iowa Department of Public Health.

9. What’s the treatment for exposure?

There is no specific treatment, but supportive care can be helpful. The CDC recommends that anyone who ingests contaminated water should replenish fluids and electrolytes.

10. How long should I wait before going back into the water after I see a bloom in my pond?

“Two weeks after the visible signs of a toxic bloom have disappeared is a guideline, but monitoring for toxins is important for the safest approach,” Malkus said. 

In its guidelines, the Massachusetts Department of Public Health says an advisory can be lifted after two successive and representative sampling rounds one week apart demonstrate cell counts or toxin levels below those which an advisory would be posted.

11. How can I help reduce cyanobacteria blooms from forming?

To help reduce cyanobacteria from forming, the CDC recommends only using the “recommended amounts of fertilizers on your yard and gardens to reduce the amount that runs off into the environment.” Household septic systems should also be properly maintained, and nitrogen-reducing systems are strongly recommended. 

New data also says that cyanobacteria benefits from pesticides, herbicides and many CEC’s.

“Reducing these in our wastewater and lawn and garden practices can be helpful, too,” Malkus said. 

Lastly, those with ponds bordering their property can maintain “a buffer of natural vegetation around ponds and lakes to filter incoming water,” according to the CDC. 




Karen Malkus, coastal health resource coordinator for the Town of Barnstable.

Association for the Protection of Cape Cod.

U.S. Centers for Disease Control.

U.S. Environmental Protection Agency.

Massachusetts Department of Public Health.

Iowa Department of Public Health.

Kansas Department of Health and Environment.

Eve Zuckoff covers the environment and human impacts of climate change for CAI.