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Blue Green Algae

We are currently working with residents and organizations to identify and prevent Blue-Green Algae on Deer Lake. 

How to recognize Blue-Green Algae

  • Blue-green algae is not normally visible in the water, but populations can rapidly increase to form a large mass or scum called a bloom when conditions are favorable.

  • 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.

  • Dense blue-green algae blooms may make the water look bluish-green, or like green pea soup or turquoise paint. Very dense blooms may form solid-looking clumps.

  • Fresh blooms often smell like newly mown grass, while older blooms may smell like rotting garbage.


  • Leaking septic tanks may be causing prolonged blue-green algae blooms.

  • One key factor contributing to the growth of blue-green algae is the amount of available nutrients such as phosphorus and nitrogen.

  • Blue-green algae blooms can be caused by agricultural and stormwater runoff as well as leaching from septic systems.

  • In Ontario, phosphorus tends to be the nutrient that influences the growth of algae.

If you spot it

  • Take a cautious approach, as some varieties of this algae can produce toxins that are harmful to both humans and animals.

  • If you suspect a blue-green algal bloom:

    • assume toxins are present

    • avoid using, drinking, bathing or swimming in the water (call your local health unit for swimming advisories)

    • restrict pet and livestock access to the water


Home treatment systems may not remove toxins and can get easily overwhelmed or clogged, so they should not be relied on. Do not boil the water, or manually treat the water with chlorine or other disinfectants, as this could increase the toxin levels.


If you spot blue-green algae blooms, contact the

Ministry of the Environment, Conservation and Parks


Send an email, noting date and location, to:

Take these steps to prevent the growth of blue-green algae:


  • Have your septic system checked, and replace if needed

  • Use phosphate-free detergents, personal care and household cleaning products

  • Avoid using fertilizers on lawns, especially fertilizers that contain phosphorus

  • Maintain a natural shoreline on lake and riverfront properties

  • Reduce agricultural runoff by planting or maintaining vegetation along waterways and minimizing fertilizer use

  • Contact your local health unit for more information.

On June 8th, 2019, Members of Friends of Deer Lake were invited to attend a Blue-Green Algae Workshop organized by the Eagle Lake Conservation Association (ELCA) and Machar Township. 

Please read the’s article that provides highlights from the workshop, and recommendations on what each of us can do to help prevent blue-green algae on Deer Lake.

Click here: Blue-green algae, climate change and what it means to Almaguin


Ministry of the Environment Blue-Green Algae

North Bay Parry Sound Health Unit - Blue-Green Algae

State of Ontario's Biodiversity - Blue-Green Algae Blooms in the Great Lakes

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