The Northwestern Health Unit says a toxin-producing blue-green algae bloom has been confirmed in Abram Lake in Sioux Lookout.
The water has been sampled and tested by the Ministry of Environment, Conservation and Parks.
Health Officials are asking people to be cautious as blue-green algae can pose a health risk to people and pets when they are exposed.
Avoid, drinking, swimming or bathing in discoloured water that has been recently impacted by a blue green algae bloom or has scum on the surface.
Residential water treatment systems may not remove toxins and should not be relied on during a bloom event.
Do not boil the water, as this can cause an increase in toxin levels.
Municipally treated water supplies can be used normally unless you are notified otherwise.
It is recommended to wait at least two weeks after the bloom has disappeared before resuming the normal use of lake waters.