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Lake Simcoe Region Conservation Authority

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An picture showing a blue-green algae bloom., with boats and a dock in the background. The water is bluish-green.
Blue-green algae

Learn about blue-green algae – bacteria that can be harmful to humans and animals – and what you should do if you spot it.

What is it?

Blue-green algae are microscopic, plant-like organisms that occur naturally in ponds, rivers, lakes and streams. They are not actually a type of algae, but a bacteria known as cyanobacteria. Although often blue-green, they can also be olive-green or red.

How to recognize it

Normally, blue-green algae are not visible in the water, but when conditions are favourable, algal populations can rapidly increase to form a large mass in the water, called a bloom. 

Blooms are commonly found nea​r docks and shoreline areas in the 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.

Causes

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

Blue-green algal 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. Although many varieties of blue-green algae are harmless, some can produce toxins that are harmful to the health of 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

Contact your local health unit for information on health risks associated with blue-green algal blooms.

Report blue-green algal blooms

If you spot blue-green algal blooms, call the Spills Action Centre:

1-866-MOETIPS (6638477)
TTY: 1-855-889-5775

Prevention

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

  • 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
  • check septic systems to ensure they do not leak into the water source

Contact your local health unit for more information:

York Region Health​
Simcoe Muskoka District Health Unit
Durham Region Public Health​
Haliburton, Kawartha, Pine Ridge District Health Unit​