Cyanobacteria (Blue-Green Algae)

Film on surface of water from 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

Thick green foam may wash ashore during a bloom

Blue-green algae

Blue-green algae may make the water look cloudy with a green tint

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.

Report a bloom

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

Report a bloom

Click here for more information from RIDOH on cyanobacteria blooms.

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.

Current Advisories

Past Advisories

On 12/22/2023, the Rhode Island Department of Health (RIDOH) and the Rhode Island Department of Environmental Management (DEM) lifted the recreational advisories at Waterman Reservoir in Glocester/Greenville, Little Pond in Warwick, Blackamore Pond in Cranston, and Indian Lake in South Kingstown.

Almy Pond in Newport, Roger Williams Park Ponds in Providence, J.L. Curran Upper Reservoir, Spectacle Pond, and Mashapaug Pond in Cranston remain under advisory due to continued visual evidence of blue-green algae.

Blue-green algae are generally less active as seasonal changes reduce light and temperature in the winter. However, the possibility of recurring blooms and/or toxins represents potential risks, even in iced-over conditions.  Sampling is over for the 2023 season however, we encourage people to report suspected 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.
  • Visit the RI Department of Health's website