The waters at Yeadon Tarn, near Leeds, are out of bounds to local sailing clubs and activity groups after toxic blue-green algae was spotted at the site.
The algae can be harmful to humans and could kill wild animals, livestock and domestic pets.
Warnings from Leeds City Council and the Environment Agency have been put up around Tarnfield Park advising people to keep away from the blooms.
Experts say the toxic algae could cause skin rashes and illnesses if swallowed.
Local groups who use the site have been warned about the dangers of the blooms and the surrounding waters. Activities at Leeds Sailing and Activity Centre have been stopped until the algae has disappeared.
A spokeswoman from Leeds City Council said that samples of water from the tarn will continue to be taken to monitor the algae. She said that the tarn would be open again once the water has been clear for at least two weeks.
An Environment Agency spokesman said: “We have tested samples from Yeadon tarn and can confirm the presence of toxic blue-green algae.
“Our advice is that people and animals should keep well away until the problem is resolved.”
The Environment Agency was called to investigate the tarn two years ago after a spate of fish deaths were caused by a shortage of oxygen.