Blue-Green Algae Blooms

Also known as "cyanobacteria blooms" or "harmful algae blooms (HABs)"

Blue-green algae, also known as cyanobacteria, occur naturally in lakes and ponds throughout Connecticut.  These microscopic organisms often go unnoticed and cause no harm.  However, when nutrient loading exceeds certain levels, a waterbody can experience nuisance blue-green algae blooms that may produce and release toxins.  When blue-green algae blooms release toxins, people and animals using the water body for recreation can be affected.  It is important to note that not all algae blooms are harmful algae blooms (HABs), but as it is not possible to determine the type of algae within the bloom without a more detailed evaluation.
What Does a Blue-Green Algae Bloom Look Like?
In Connecticut, most algae blooms occur from midsummer to early fall.  During a bloom the following conditions may be observed in surface waters:
  • The water may be cloudy or even thick like pea soup.
  • It may look like someone spilled paint on the water.
  • The water will likely be green or brown.
  • There may be a mat of algae, scum or foaming on the water surface.
algal bloom closeup#1algal bloom aerial #1
Above: Photographs of past blue-green algae blooms in Connecticut     


How to Report a Suspected Bloom

If you believe that you have observed an algae bloom, follow the guidance listed above and contact your Local Public Health Agency.  You may also contact CT Department of Public Health (860-509-7758), or CT Department of Energy and Environmental Protection (860) 424-3028, or send an email to

Learn More about Cyanobacteria and Harmful Algae Blooms

General Information:

Guidance Documents:

CT Department of Public Health Resources

Potential Health Effects from Exposure to Algal Toxins
Several different types of blue-green algae that can produce toxins commonly occur in lakes and ponds in Connecticut.  Assessing the potential health effects from blue-green algae blooms is complicated because the blue-green algae in a bloom may not be producing toxins.  Therefore, chemical analysis of the water is needed to verify if a blue-green algae bloom is releasing toxins.

People who recreate in waters when a blue-green algae bloom is present may be exposed to toxins by ingesting water, dermal contact, or inhalation of water droplets.  Potential health effects to such exposure could include:

  • Irritation of the skin, nose, eyes and respiratory tract.
  • Gastrointestinal upset such as vomiting or diarrhea upon ingestion.
  • Liver or nervous system effects, if relatively large amounts of the algae are ingested.
Dogs are especially at risk from exposure to blue-green algae blooms.  They can be exposed to the toxins in a similar manner as people.  However, dogs may also drink from the tainted water and be exposed when they groom themselves after leaving the water.

Fish living in waters affected by a blue-green algae bloom may accumulate algal toxins in their muscle tissue and internal organs.  The health risk posed by consumption of such fish is uncertain.  Toxin levels are usually higher in internal organs than in the muscle tissue.  General precautionary advice to anglers to reduce exposure includes:

  • Avoid fishing in areas with visible algae blooms due to potential incidental contact with the water.
  • Eat fish from water bodies with blue-green algae blooms in moderation (1-2 meals or less per week.)
  • Remove skin and internal organs before cooking. Wash fillets before cooking or freezing.
Activities which could lead to potential exposures to blue-green algae blooms are listed below.
Level of Potential Exposure Recreational Activity Primary Exposure Pathway of Concern
Swimming / wading
Water skiing / wake boarding
Wind surfing
Jet Skiing
Ingestion / inhalation
Ingestion / inhalation
Ingestion / inhalation
 Moderate Fish consumption
Motor boating
Inhalation / skin
Inhalation / skin
Inhalation / skin
Inhalation / skin
 Low / None Catch and Release Fishing
Not applicable
Not applicable
Not applicable

Recommended Precautions

Before going out on or into the water:
  • Be sure to comply with all posted signs regarding recreational access to the water body and read any advisories on closures.
  • To obtain the latest information on ongoing blue-green algae blooms, call your Local Public Health Agency, CT Department of Public Health at (860) 509-7758, or CT Department of Energy and Environmental Protection at (860) 424-3028.
If you accidentally come in contact with a blue-green algae bloom:
  • Thoroughly rinse yourself and your pet(s).
  • If you experience any signs or symptoms post-exposure, contact your health care provider or Poison Control Center for advice.
  • Immediately contact your veterinarian if your pet shows any symptoms post-ingestion.  
For additional information please contact:
  • CT Department of Public Health: (860) 509-7758
  • The CT Department of Energy and Environmental Protection: (860) 424-3028
  • Email inquiries:
Content last revised August 03, 2022