Rother Valley Country Park has been forced to close both the main lake and the northern lake for all water-based activities, however all land-based activities are safe and open as usual.
In a warning letter issued in partnership with Rotherham Council, they said: “This algae may cause illness in humans and animals including pets.
“To keep you and your loved ones safe: please keep pets and children away from the water’s edge.
“Do not touch scum either in the water or on the shore.”
The site is known to be a hot-spot for activities including sailing and windsurfing and is the home of the Sheffield Cable and Aqua Park.
Visitors are asked to go to the park office, cafe or water sports kiosk for more information.