People are being urged to stay out of Conestogo Lake and not use its water because of the potential presence of blue-green algae.
The Grand River Conservation Authority says samples taken from the lake show the possibility that the algae is present.
Blue-green algae can contain microcystin, a toxin which can make people and animals sick.
According to the GRCA, the lake should not be used for swimming until the situation is cleared up. Additionally, water from the lake should not be consumed and fish caught in the lake should not be eaten.
The lake is safe for boating, although the GRCA recommends that boaters avoid touching any algae directly.
The algae blooms were first noticed earlier this week. Separate blooms of blue-green algae were detected at Conestogo Lake last summer.
Microcystin typically breaks down within one week.
