Reporting Blue-Green Algae Blooms 

• If you see or suspect a bloom, contact the Ministry of the Environment, Conservation and Parks (MECP) online or by phone.
• The MECP will investigate and test the water for blue-green algae.

Current Waterways Affected by Blue-Green Algae

What is blue-green algae (cyanobacteria)?

• Microscopic organisms naturally found in lakes, rivers, and ponds.
• Thrive in warm, slow-moving water that is rich in nutrients such as nitrogen and phosphorous.
• Normally present in the environment at low levels but can grow into dense blooms when conditions are right.
• Some blooms can produce toxins that can be harmful to humans and animals.

How to spot a bloom:

• Water looks discoloured - green, blue-green, turquoise, or olive.
• Surface may appear coated with scum, foam, mats, or paint-like streaks.
• Strong odours like rotting garbage, rotten eggs, or decaying plants.
• Dead fish or animals along the shoreline.
• You cannot tell if a bloom is harmful just by appearance. Stay out of the water and keep pets away if you suspect one.

Symptoms and Illness

It's possible to get sick if people:

• Swim, wade, play, or water ski in or near contaminated water.
• Drink or breathe in contaminated water.
• Eat contaminated fish, shellfish, or supplements.

Exposure to contaminated water may cause:

• Stomach pain,  vomiting, and diarrhea.
• Headaches, muscle weakness, fever, and dizziness.
• In severe cases, liver or kidney damage may occur.
• Skin, eye, nose, throat, or lung irritation.

Severity of Illness Depends On:

• Length of exposure.
• Amount of toxins present.
• Type of exposure (i.e.: Ingestion or skin contact).
• Children are more at risk because they tend to spend more time in the water and are likely to swallow it.

Contact your healthcare provider if you develop symptoms after being in water with blue-green algae.

Swimming, bathing, or showering in areas without visible blooms is generally safe and not expected to cause health effects.

Protect Dogs and Other Animals

• Animals can get sick or die from drinking contaminated water.
• Don't let pets swim, play in or drink water in areas with a bloom.
• Watch for symptoms such as vomiting, stumbling, or trouble breathing.
• Wash pets immediately with clean water if exposed.
• Call a veterinarian if symptoms occur.

Drinking Water Systems

Municipal Drinking Water Systems (Town or City water):

• Regularly monitored for blue-green algae blooms.
• Treatment systems remove algae and their toxins.
• Sampling, monitoring, and reporting plans include testing raw and treated water, plus visual checks near intake lines.
• Most municipal systems in Ontario have never had unsafe toxin levels in treated water.
• Residents will be notified if toxins are found above the Ontario Drinking Water Quality Standard.
• Risk of toxins in treated municipal water is very low.

Private Wells and Drinking Water Systems:

If you use a private water supply:

• Check the lake or river often for blue-green algae, especially in summer and fall.
• Treatment systems may not remove toxins, and water may not always taste or smell bad.

If your private water supply may be affected by a bloom:

• Do not drink the water or use it for cooking or baby formula.
• Use bottled water or another safe source.
• Do not boil the water. Boiling does not remove toxins and can increase them.
• Avoid using chemicals like herbicides, copper sulphate, or algaecides which can release more toxins.

What should I do if I spot blue-green algae?

• If you're not sure, stay out!
• Assume toxins are present whenever you see a bloom.
• Stay out of the water and keep pets away.
• Wash with soap and clean water right after exposure.
• Contact your healthcare provider if you develop symptoms after being in water with blue-green algae.
• Report suspect blooms to the Ministry of the Environment, Conservation and Parks (MECP). Incidents can be reported online or by phone.

For more information:

• Blue-Green Algae - Ontario Ministry of the Environment, Conservation and Parks
  Learn how Ontario monitors and responds to blue-green algae blooms.
• Cyanobacteria in Freshwater - National Collaborating Centre for Environmental Health
  Overview of cyanobacteria, health risks, and public health guidance.
• Recreational Water and Health: Cyanobacteria and their toxins - Health Canada
  Information on the risks of cyanobacteria, toxins, and how to stay safe.
• Cyanobacteria Toxins in Drinking Water - Health Canada
  Information on cyanobacteria in drinking water and how they are managed in Canada.
• Guidelines for Canadian Recreational Water Quality: Cyanobacteria and their Toxins - Health Canada
  Details on cyanobacteria in recreational waters and related health concerns.
• Harmful Algae Blooms (HAB): Associated Illness - U.S. Centers for Disease Control and Prevention (CDC)
  U.S. public health information on harmful algal blooms and related illnesses.

Updated July 17, 2026