Blue-Green Algae: If In Doubt, Stay Out

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The Simcoe Muskoka District Health Unit is encouraging local residents to exercise caution around Little Lake, Midland after the first blue-green algae bloom of 2023 was confirmed through testing.

Blue-green algae, or cyanobacteria, occur naturally in freshwater. Blue-green algae can grow rapidly to form dense blooms when the weather is warm and the water is shallow, nutrient-rich, slow moving and warm. Blooms are usually smelly and can make the water appear green, blue, turquoise, or olive green. They often form scum, foam, or mats on the surface of the water and may appear as though turquoise paint was spilled in the water.

Many species of blue-green algae have the potential to produce toxins that are harmful to the health of humans and animals. If you suspect a blue-green algal bloom, be cautious and assume that toxins are present to avoid exposure. If you spot a bloom, you can report it to the Ministry of the Environment, Conservation and Parks online or by phone at 1-866-MOE-TIPS (663-8477).

The Simcoe Muskoka District Health Unit is recommending all persons take the following precautions when they spot blue-green algae:

  1. Do not drink the water or use it for food preparation, including infant formula.
  2. Do not boil and drink water that may contain blue-green algae because toxins are not destroyed by boiling and boiling may increase toxins.
  3. Use an alternate source of safe drinking water, such as bottled water, for the duration of the bloom.
  4. If you see a bloom, stay out of the water, and keep your pets out of the water. You cannot tell if a bloom is harmful by looking at it, so it’s best to use caution and stay away.
  5. Do not swim, boat, or play water sports in areas where there are harmful algae or cyanobacteria.
  6. Do not use herbicides, copper sulphate or other algaecides that may break open algae cells and release toxins into the water.

Symptoms following contact or consuming toxins from a blue-green algae bloom can include irritation of the skin, eyes, nose, throat, or lungs, stomach pain, headaches, diarrhea, vomiting and in severe cases, liver damage. Symptoms can be more serious if water is swallowed in large quantities.

For a list of affected waterways and to learn more about blue-green algae, and the precautions to take before swimming in or consuming water where there has been an algae bloom, visit


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