Google Translate Disclaimer

Translation on this website is provided by Google Translate, a third-party automated translator tool. The Simcoe Muskoka District Health Unit assumes no responsibility for the accuracy of translations performed by Google Translate, or for any issues or damages resulting from its use.

water image
print header

Safe Water

Blue-green algae

If you suspect a blue-green algae bloom, contact the Ministry of the Environment, Conservation and Parks (MECP).  Incidents can be reported online or by phone.

The MECP will gather information, conduct field visits, as necessary, and facilitate testing to confirm if a blue-green algae bloom is present.


Affected Waterway


Date Notified


Little Lake
(Little Lake Park Beach)

Town of MidlandJuly 4, 2023RESOLVED - August 11, 2023
Orr Lake Township of SpringwaterJuly 25, 2023RESOLVED - October 11, 2023
MacLean Lake
(Southwest Shoreline)
Township of SevernAugust 14, 2023RESOLVED - November 24, 2023
Three Mile Lake including Hammell's Bay
Township of Muskoka LakesAugust 21, 2023
(updated September 20, 2023)
RESOLVED - December 4, 2023
Stewart Lake
(Southwest Shoreline)
Township of Georgian Bay August 24, 2023RESOLVED - December 4, 2023

Midland Bay
(Gawley Park Beach)

Town of MidlandAugust 25, 2023RESOLVED - September 1, 2023
Hogg Bay
(Northeast Shoreline)
Tay TownshipAugust 25, 2023RESOLVED - January 8, 2024
Bass Lake
(Northwest Shoreline)
Town of GravenhurstAugust 29, 2023RESOLVED - March 12, 2024
Penetang Harbour
(East Shoreline)
Town of PenetanguisheneSeptember 1, 2023RESOLVED - January 8, 2024
Young Lake also known as Trout Lake
(Northeast Shoreline)
Township of Muskoka LakesSeptember 14, 2023RESOLVED - March 14, 2024
Longs Lake
(Southwest Shoreline)
Town of HuntsvilleSeptember 15, 2023RESOLVED -  March 14, 2024
Bass Lake 
(Northeast Shoreline) 
Township of 
October 5, 2023RESOLVED - February 20, 2024
Mary Lake
(Northwest Shoreline) 
Town of HuntsvilleOctober 10, 2023ACTIVE

Tea Lake
(Northeast Shoreline)

Township of SevernOctober 30, 2023RESOLVED -  March 12, 2024


Affected Waterway


Date Public Notice Issued


Farlain Lake
(Southeast and Southwest Shoreline)

Township of Tiny

August 2, 2022

RESOLVED - October 5, 2022

Lake St. John
(Southwest Shoreline) 
Township of RamaraAugust 12, 2022RESOLVED - December 13, 2022
Three Mile Lake
(including Hammell's Bay)
Township of Muskoka Lakes

August 29, 2022 (updated September 2, 2022)

RESOLVED - December 14, 2022
Lake St. George
(South Shoreline)
Township of SevernSeptember 6, 2022RESOLVED - October 26, 2022
Penetang Harbour
(West Shoreline) 
Town of PenetanguisheneSeptember 9, 2022RESOLVED - December 13, 2022
Bass Lake
(Southwest Shoreline) 
Township of Muskoka LakesSeptember 20, 2022RESOLVED - December 13, 2022
Mary Lake
(Northwest Shoreline)
Town of HuntsvilleOctober 13, 2022 RESOLVED - December 13, 2022
Leonard Lake (Northwest Shoreline)Township of Muskoka LakesOctober 28, 2022RESOLVED - December 13, 2022

Smith's Bay (Northwest Area)

City of OrilliaNovember 10, 2022RESOLVED - December 13, 2022
Muldrew Lakes (Middle Muldrew Lake Area) Town of GravenhurstNovember 10, 2022RESOLVED - December 13, 2022

Blue-green algae, technically known as cyanobacteria , are microscopic organisms that are naturally present in lakes and streams. They are usually present in low numbers but can rapidly increase when the water is warm, slow moving and full of nutrients such as nitrogen and phosphorous. When this happens, they can form dense blooms and some blooms can produce toxins.

How to spot a potential blue-green algae bloom:

  • Water appears green, blue, turquoise, or olive green. 
  • Looks like scum, foam, mats, or paint floating on the water’s surface. 
  • Smells like rotting garbage, rotten eggs, or rotting plants.
  • Dead fish or other animals on the shoreline. 

You can’t tell if a bloom is harmful just by looking at it so it’s best to use caution and stay out of the water and keep your pets out of the water.

There are several types of blue-green algae that are not harmful to human health, but some blue-green algae can produce toxins.  Large amounts of these toxins, also called cyanotoxins, can be harmful.

It's possible to get sick if people:

  • Swim, wade, or play in or near contaminated water.
  • Drink contaminated water.
  • Eat contaminated fish, shellfish, or supplements.

People exposed to cyanotoxins by swallowing contaminated water may experience the following symptoms depending on the cyanotoxin involved:

  • Stomach pain
  • Headache
  • Neurological symptoms (for example, muscle weakness, dizziness)
  • Vomiting
  • Diarrhea
  • Liver damage

People exposed to toxins through swimming or breathing in droplets of contaminated water may experience irritation of the:

  • Skin
  • Eyes
  • Nose
  • Throat
  • Lungs

If you think you may have symptoms caused by blue-green algae, contact your healthcare provider.

Any mammal can be poisoned by swimming in or drinking water containing cyanotoxins or eating cyanobacteria, fish or other animals containing toxins.  Depending on how they're exposed they can show signs of vomiting, staggered walking, difficulty breathing or even death.

Dogs are at a higher risk because of their behaviours, which can include swimming in contaminated waters, drinking contaminated water, eating dead fish and other animals found near a bloom, licking algae or scum from their fur after swimming.

If your pets have been in contaminated water, immediately wash them off with clean water.  Call a veterinarian if your animal shows signs of illness after being in contact with water.

  • Check local advisories and the active blooms listed on this webpage before visiting a body of water.  Follow advisories to reduce your chances of getting sick.
  • If you see a bloom, stay out of the water, and keep your pets out of the water.  You cannot tell if a bloom is harmful by looking at it, so it's best to use caution and stay away.
  • Do not swim, boat or play water sports in areas where there are harmful algae or cyanobacteria.
  • Do not go into water that:

    -Smells bad

    -Looks discoloured

    -Has foam, scum, mats, or paint-like streaks on the surface

    -Has dead fish or other animals washed up on its shore or beach

  • Do not fill pools or spas with water directly from lakes, rivers, or ponds.

Municipal Drinking Water Systems:

Municipal drinking water systems are routinely monitored for blue-green algae and microcystin, a specific cyanotoxin with known health effects.  Both the raw water and treated water are tested regularly in addition to visual monitoring of the lake and intake lines.  Most municipal systems in Canada have never exceeded the maximum acceptable concentration of microcystin in treated water.  This tells us that the complex treatment systems used by most municipal drinking water systems are effective at removing blue-green algae and their toxins.

Residents will be notified immediately if microcystin test results for treated water exceed the Ontario Drinking Water Quality Standard.  The risk of exposure of cyanotoxins from municipal drinking water is very low.

Residential and Private Drinking Water Systems:

During peak season, usually June-October, home and cottage owners should visually monitor lakes and rivers for evidence of blue-green algae bloom.  Home treatment systems may not remove toxins from the water and the water may not always taste or smell bad, so be cautious and take the following precautions:

  • Do not drink the water or use it for food preparation, including infant formula
  • Use an alternate source of safe drinking water, such as bottled water, for the duration of the bloom
  • Do not boil and drink water that may contain blue-green algae because toxins are not destroyed by boiling and boiling my increase toxins
  • Do not use herbicides, copper sulphate or other algaecides that may break open algae cells and release toxins into the water
  • Bathing or showering with water not visibly affected by a blue-green algae bloom is not expected to cause health effects.

Recent studies showed that cyanotoxins do not accumulate at high levels in fish fillets.  This means there is a very low risk to human health, even from eating fish caught during peak blue-green algal bloom condition.

One exception is that elevated levels of microcystins have been found in the livers of fish from waterbodies that regularly experience blue-green algal blooms, even if a bloom is not present.  To eliminate this risk, follow the general advice to avoid eating any fish organs.

Ministry of Natural Resources and Forestry.  Guide to Eating Ontario Fish.  Government of Ontario: King's Printer for Ontario; 2023.  Available from:

If you spot a blue-green algal bloom, be cautious and assume that toxins are present.  Avoid exposure to blue-green algae.  If you have been in contaminated water, wash with soap and water or rinse thoroughly with clean water to remove algae.

If symptoms such as skin, eye or throat irritation, allergic reactions or breathing difficulties occur after contact with water, contact your health care provider.

Swimming, bathing, or showering with water not visibly affected by a blue-green algae bloom is not expected to cause health effects.

If you spot a bloom, report it to the Ministry of the Environment, Conservation and Parks (MECP).  Incidents can be reported online or by phone.

For information on the potential health risks associated with blue-green algal blooms, please contact Health Connection at 705-721-7520 or toll-free at 1-877-721-7520 to speak with a public health inspector.

For more information:

Adapted from the Centre for Disease Control and Prevention.  More information is available on their website.

Did you find what you were looking for today?
What did you like about this page?
How can we improve this page?

If you have any questions or concerns that require a response, please contact Health Connection directly.

Thanks for your feedback.
Failed to submit comment. Please try submitting again or contact us at the Health Unit.
Comment already submitted ...