The Capital Regional District is advising people to avoid swimming or letting pets drink or swim in Prior Lake because of a blue-green algae bloom.
The lake is located at the north end of Thetis Lake Regional Park.
The CRD says the algae usually produce a blue-green sheen that appears on the surface but warns that not all blooms are easy to see.
Ingesting water containing the toxins produced by the algae can cause a range of symptoms in humans including headaches and abdominal pain.
It can lead to lethal liver damage in dogs.