Signs have been posted at the Conestogo Lake beach and park gatehouse warning of toxic algae detected in the water.

People with cottages around the lake have also been notified about the presence of blue-green algae, the Grand River Conservation Authority says.

As blue-green algae nears the end of its life, it releases a toxin known as microcystin, which can cause illness if ingested by people or animals. It can also cause rashes if it comes into contact with skin.

The GRCA is advising people who use the lake to avoid contact with the algae, not swim or otherwise come into direct contact with the lake’s water, and not eat fish caught in the lake.

Blue-green algae was also detected at nearby Belwood Lake earlier this month, prompting similar precautions to be taken for about a week, until further testing showed no detectable levels of the algae.

The toxic microcystin typically breaks down in five to seven days.