Harmful Algae Blooms


Harmful algae blooms (cyanobacteria), also known as blue-green algae or pond scum, form on lakes and can create toxins that are harmful to humans and animals. 

Residents of a lake with a harmful algae bloom may be affected depending on their closeness to the site of the algae, the size of the lake, wind directions, water flow, and other environmental conditions that cannot be predicted or controlled. Due to the many factors involved, government authorities are unable to determine where and when there are no toxins. Users are recommended to use their judgment. For further details on the location of the algae bloom, contact the Ministry of the Environment, Conservation and Parks - Spills Action Centre at 1-800-268-6060.

 If you reside near where the bloom was detected or are visiting the lake and encounter the algae, follow these safety measures:

  • Do not use the water. This includes: drinking, cooking, bathing, and brushing teeth. Note: Using a private water system or boiling the water will not destroy the toxins.
  • Do not swim and avoid water sports where there is harmful algae.
  • Limit the amount of fish flesh you eat. Some toxins can build up in fish and shellfish. Do not eat the liver, kidneys and other organs. Be careful not to cut the organs when filleting.
  • If skin contact does occur, wash with soap and water or rinse thoroughly with clean water to remove algae.

 More information about cyanobacteria (harmful algae bloom) found here.

Harmful Algae Blooms (HAB) categories and photos



Category 1 

Category 2

 Category 3


The water appears cloudy, but you can still see through it.  

Particle density can vary greatly in the water column.   

The water colour appears changed due to increased number of cyanobacteria.   

Algae may be in cluster or ball or flakes in the water, like a pea puree. 

This is a dense bloom: it may resemble a paint spill. Accumulated cyanobacteria on the surface of the water may form a scum.                                

Algae is easily swept by the wind and deposited
near the shore.



Do not swim in any bloom which obscures your view into the water. Even if it is not blue-green algae, it is unsafe to swim or allow children to play in any dense algal bloom, since you cannot see into the water. Wait until the bloom has subsided and the water is clear.   
Health effects are not expected at this stage   Sensitive individuals may develop mild symptoms of skin rash or eye irritation.

Drinking Water Precautions 

Health effects are not expected at this stage Ingestion of water in this category may result in a range of health effects.  Ingestion of water in this category will likely
result in a range of health effects. 
  Do not use the water from a Category 2 or 3 bloom for drinking cooking or other consumption.
Be observant of when the water has cleared. If you treat surface water for cooking or consumption, wait at least a week after the bloom has subsided before normal use of
the treated water. The toxin is naturally reduced by dilution, degradation by other bacteria in the water, and sunlight. 

Assess your water supply if you are in an area where there are frequent algae blooms. Since there is no confirmed domestic water treatment available yet, consider a drilled well. 

 *Adapted from Peterborough Public Health. 

Category 2

Blue Green Algae Category 2

Blue Green Algae Category 2

Category 3

blue-green algae category 3

 Harmful Algae Category 2

Harmful Algae Category 3

 Water Advisory Sign



When is it safe to swim after a harmful blue-green algae boom advisory has been release?

It is safe to swim in an area where harmful blue-green algae advisory was released when the bloom is not present. The advisory does not close the beach, but rather advises swimmers that they need to take caution in the area. 

When will the public be made aware when an advisory is lifted?

Harmful blue-green algae can come and go throughout the season. The advisory does not close the beach, rather informs swimmers to be aware that there could be a bloom in the area. Therefore, the advisory stays in place all summer, however, this does not mean the bloom will be there all summer. Look for signs of the bloom and if none are present swimming my take place. 

Why are there signs on the beach? 

The sign reminds swimmers and individuals using the area to watch for blooms before using the water. 

Ontario's 12-point plan on harmful algae blooms

Ontario is concerned about the effects of harmful algae blooms in our water.  Ontario has a 12-point plan outlining how we are working with our many partners, to fight algae blooms in the Great Lakes and other lakes and rivers. 

For more information on Ontario's 12-point plan on harmful algae blooms click here.

Canada's drinking water guidelines during harmful algae blooms

Health Canada and the Federal-Provincial-Territorial Committee on Drinking Water are updating advice for parents of young infants when it comes to using tap water to prepare infant formula during harmful algae blooms.

For more information on Canada's drinking water guidelines during harmful algae blooms click here.

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