greenish-tinged water containing blue green algae

This Version Posted: June 28, 2022

If you’re out on the water, be on the lookout for Harmful Algae Blooms (also known as Blue-Green Algae).


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What are Harmful Algae Blooms (HAB)?

HAB, also known as cyanobacteria, are primitive microscopic organisms that occur naturally in lakes, bays, ponds and inlets. Normally, the algae are barely visible. In warm weather when conditions are right, the algae can rapidly grow to form a large mass (or bloom).

Blooms most commonly occur in late summer and early fall. They thrive in areas where the water is shallow, slow moving and warm (but they may also be present in deeper, cooler water). A common cause of HAB blooms is agricultural and stormwater runoff, as well as leaching from septic systems.

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What to Look For

Dense harmful algae blooms can make the water look like a bluish-green pea soup, or a shiny paint slick. Very dense blooms may form solid-looking clumps. Fresh blooms often smell like fresh cut grass, while older blooms can stink like rotten garbage.

In general, harmful algae blooms can be placed into three categories:

  • Category 1: Water appears cloudy, but you can still see through it. While there is no health effect expected at this stage, be careful when using the water, especially if there is a dense algal bloom and you cannot see into the water. Wait until the bloom has subsided and the water is clear.
  • Category 2: Water colour changes in appearance, and algae may be in clusters or flakes in the water or appear like a pea puree. Do not swim in a Category 2 algal bloom or any bloom which obscures your view into the water. Doing so could lead to skin rash or eye irritation.
  • Category 3: This is a dense bloom, resembling a paint spill or forming a scum on the surface of the water. The algae is easily swept by the wind and deposited near the shore. Do not swim in a Category 3 algal bloom or any bloom which obscures your view into the water. Doing so could lead to skin rash or eye irritation.

When harmful algae blooms fall into categories 2 or 3, you should avoid swimming AND never use water for drinking, cooking, rinsing foods, washing dishes, or any other purposes. Keep your pets from going into the water or drinking it. Most algae blooms are short-lived and will break down in a few days or weeks.

Do NOT boil the water, as it kills the algae resulting in the release of more toxins into the water.

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Health Risks of Harmful Blue Algae

While many forms of blue-green algae are harmless, some types produce toxins that can be harmful to humans and animals. The risk depends on the type and amount of exposure to the toxins. If you see HAB in a waterway, do not swim, drink or use the water.

Longer exposure to more severe toxins in the water through drinking, swimming, bathing, cooking or washing may lead to health problems. This includes: itchy, irritated eyes and skin; headache; fever; diarrhea; abdominal pain; nausea; and vomiting.

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If You Detect or Suspect Harmful Blue Algae:
  • Assume toxins are present
  • Avoid using, drinking, bathing or swimming in the water.
  • Restrict pets from getting into the water.

If you spot a bloom, report it to the Spills Action Centre at 1-800-268-6060.

For more information about HAB, call the Health Unit toll-free at 1-866-888-4577, ext. 5006.

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Additional Resources
Government of Ontario:
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How to Prevent Harmful Algae Blooms

Human activities (like fertilizer runoff, agricultural runoff and faulty septic systems) can increase levels of phosphorus and nitrogen in the water. This can lead to the growth of HABs. To reduce the nutrient levels in water that promote algae growth, do the following:

  • Use phosphate-free laundry detergents.
  • Avoid using fertilizers.
  • Reduce surface runoff by maintaining a naturalized shoreline on waterfront properties.
  • Ensure septic systems work properly and are maintained so sewage does not leak.
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