Huron Perth Public Health says a bloom of potential toxic blue-green algae has been found in its coverage area.
Health official say the algae bloom was detected in the Wildwood Reservoir at Wildwood Conservation Area, near St. Marys.
Blue-green algae can produce toxins which may be harmful to health through direct contact or consumption.
The Health Units advise that the Wildwood Reservoir is unsafe for swimming.
Health officials say drinking water supplies from wells in the area have not been affected by this algae bloom.