Blue-green algae was discovered by the Grand River Conservation Authority near Belwood Lake in the Grand River.
The GRCA says it was found at the Wellington Road 26 bridge in Belwood Village.
Along with Public Health, they are advising lake users not to swim in the water and to keep children and pets away from the algae.
Eating fish and drinking water from the lake should also be avoided as the algae is poisonous.
Boating is still allowed but boaters are asked to be careful and avoid contact with the water.
