Visitors to the main lake at Stodmarsh Nature Reserve are being advised to avoid contact with the water following the discovery of a widespread bloom of toxic blue-green algae.
Environment Agency sampling revealed high levels of the algae which can cause illness in humans and can be fatal to animals.
Natural England has added appropriate signage around the lake advising people to stay away from the water until further notice.
Other lakes and ponds in the district may be affected so caution should be taken when visiting.
You can find Environment Agency advice on blue-green algae here.