Dog owners and families with children are being warned to stay out of the water at Strathclyde Park.
The Motherwell loch is affected by blue-green algae as a result of the current high temperatures, and contact with the water is to be minimal due to health dangers.
Swimming, paddling, fishing and some watersport activities on the loch have been suspended.
Dog owners have also been warned not to allow their dogs in the water or for their dogs to drink from the loch.
Anyone who visits the loch should wash their hands thoroughly as a precaution.
Posters and signs have been place around the park warning residents of the danger.
Strathclyde Park is hosting rowing and triathlon events as part of the European Championships in Glasgow, however North Lanarkshire Council officials have said the water will be safe for the events as treatment is underway.
A council spokesperson said “As a result of the current high temperatures, Strathclyde Loch is affected by blue-green algae. This is a common issue in areas of fresh water, particularly during the summer, and we monitor the water regularly.
“When blue-green algae appears in the water, our Water Quality Protocol is activated to manage water-based activities on the loch and maintain public health.
“Swimming and other immersion sports are restricted and we recommend users wash their hands thoroughly after being at the loch. We also have notices around the loch advising visitors to avoid contact with the water and not to allow dogs to enter or drink the water.
“We can use curtain barriers in place at the south end of the loch to allow the algae to be treated for major events. The barrier and treatment of the water will be used ahead of the rowing and triathlon events at European Championships Glasgow 2018 which take place at Strathclyde Park in August."
