greenish-tinged water containing blue green algae

This Version Posted: May 6, 2021

If you’re out on the water, be on the lookout for Blue-Green Algae.


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What is Blue-Green Algae (BGA)?

BGA, 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 be present in deeper, cooler water. A common cause of BGA blooms is agricultural and stormwater runoff, as well as leaching from septic systems.

What to Look For

Dense blue-green 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 blue-green algae blooms often smell like fresh cut grass, while older blooms can stink like rotten garbage.

In general, blue-green algae can be placed into three categories (click here for a detailed poster):

  • 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 blue-green algae falls into categories 2 or 3, you should not only avoid swimming, but also NOT use water for drinking, cooking, rinsing foods, washing dishes, or any other purposes. Pets should also be prevented from entering or drinking the water. 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.

What to do if You Detect BGA

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 Suspect a BGA Bloom:
  • 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 BGA, call the Health Unit toll-free at 1-866-888-4577, ext. 5006.

Additional Resources

HKPR District Health Unit

Government of Ontario: