Dog walkers are being advised to keep their pets away from the water at Ashland Lake following the discovery of blue green algae.
The Parks Trust has also suspended fishing at the lake, following confirmation of the algae by the Environment Agency.
Blue-green algae is a group of bacteria, called cyanobacteria. Unless they clump together, they can't be seen by the naked eye.
The prologed heatwave may be the cause of the outbreak - the algae is most common in non-flowing fresh water during hot weather when there is less rainfall.
Exposure to the algae is often fatal in dogs, and cats, but even those that survive can be left with long term health issues.
In humans, the algae can cause rashes, sickness, stomach pains, fever and headaches.