Cyanobacteria (Blue-Green Algae)

Film on surface of water from blue-green algae can look like spilled paint

Thick green foam may wash ashore during a bloom

Blue-green algae on a lake — Blue-green algae may make the water look like pea soup

Cyanobacteria, also known as blue-green algae, are naturally found in many freshwater ecosystems. A combination of excess nutrients, sunlight, and high temperatures can lead to a rapid increase in cyanobacteria, called a “bloom.” Blooms of cyanobacteria generally occur in late summer into the early fall when water temperatures are warmest and an abundance of sunlight and nutrients are available. Some species of cyanobacteria can also produce toxins. These toxins are harmful to people and pets. There are no visual properties of a cyanobacteria bloom that indicate the algae are producing toxins. It is only possible to determine if toxins are present with laboratory tests. If a cyanobacteria bloom is observed, it is best to take caution and stay out of the water to avoid any potential exposure to toxins.

Please let us know if you've discovered a blue-green algae bloom. Please submit photos whenever possible.

Report a bloom

Click here for more information from RIDOH on cyanobacteria blooms.

What you should do

Do not swim, play, or fish in water that appears to have a bloom.
Do not let your pets swim or play in water experiencing a bloom.
If you or your pet comes into contact with waters experiencing a bloom, wash with soap and water immediately.
If you or your pet becomes ill after contact with waters experiencing a bloom, contact your health professional.
Visit the RI Department of Health's website to learn more

The Rhode Island Department of Health (RIDOH) and the Rhode Island Department of Environmental Management (RIDEM) work cooperatively to detect/respond to the presence of cyanobacteria blooms, evaluate the potential risks to the public, and, when necessary, issue health advisories notifying the public of health concerns. The agencies jointly issue health/recreational advisories when conditions indicate a cyanobacteria bloom poses a risk to public health.