Contestogo Lake is being tested for blue-green algae, which can contain microcystin — a toxin harmful to humans and animals.

"Algae blooms were first noticed this week when small mats of decaying algae started appearing on the shoreline," said the Grand River Conservation Authority in a news release Friday.

"When the algae die, the toxin is released in the water where it naturally breaks down in about five to seven days."

The Ministry of Environment took samples from the lake on Thursday evening. Initial testing is expected to be done over the weekend to confirm the blue-green algae, but the full results, indicating if the toxin is present, aren't expected until next week, GRCA spokesperson Cam Linwood told CBC News.

Until then, people are being told not to swim in Conestoga Lake, avoid touching the water and not to eat any fish caught there. Boating is fine, but boaters are asked to stay clear of the algae blooms.

Last August the reservoir was also closed to swimming and fishing because of a suspected blue-green algae bloom. In that case Ministry of Environment tests came back positive for microcystin.

Linwood says if anyone swimming in Conestogo Lake earlier this week feels ill, they should seek medical attention — but a soapy shower is often enough to wash away the algae and any toxins.