Affected lakes in Ontario include Ramsey Lake near Sudbury, parts of Sturgeon Lake near Peterborough, and Colonel By Lake near Kingston.
In Alberta, Pigeon Lake and Wizard Lake, two popular recreational lakes south of Edmonton, have been placed under a health advisory due to potentially toxic algae blooms.
Those are just the lakes affected this year; blue-green algae blooms have caused problems in virtually every province in Canada over the past two decades. Last year, algae blooms in Manitoba's Lake Winnipeg were so large, they were visible from space.
Blue-green algae is caused by microscopic organisms called cyanobacteria.
Cyanobacteria are naturally present in freshwater lakes and don't usually pose a health threat, explains Dr. Chris Sikora with Alberta Health Services.
However, when conditions are right, the organisms reproduce rapidly, producing the thick, bluish-green scum that in high amounts can make the water toxic to humans and animals.
The weather plays a big role in producing algae blooms.
“Cyanobacteria thrive on hot summer days with direct sunlight on the water -- the exact same conditions where you and I want to be out at the lake with our families,” Sikora says.
Excessive rainfall can also wash fertilizer and other contaminents into the water, providing the cyanobacteria with plenty of phosphates and nitrogen on which to feed.
When the blooms die, they can release toxins into the water that if ingested can cause serious illness, including headaches, fever, malaise, vomiting, diarrhea and stomach pain.
Even swimming in contaminated water can cause skin rashes and irritation.
People are encouraged to avoid drinking water or eating fish from contaminated areas, and to prevent their pets from doing the same.
The algae blooms are usually short-lived, Sikora says, but can drift to different areas of a lake, meaning once a health advisory has been issued for a certain lake, it tends to stay in place for the rest of the summer.