People are being advised to stay out of the water at Clatto park after harmful levels of algae were discovered.
Signs telling people to keep out of the reservoir have been put in place by the council warning of the dangers blue-green algae posses towards humans and animals.
People exposed to the toxic substance can suffer from sickness, muscle pains, sore eyes and stomach upsets.
It can also be extremely harmful to pets.
A council spokesperson said: “When in high concentrations in the water this naturally occurring algae can multiply to cause algal blooms which may then release toxins.
“The behaviour of algae is erratic and the levels of its toxicity can fluctuate. It can appear one day, be dispersed by the wind and re-accumulate at any time.
“Contact with algal scum or water close to it can cause skin rashes. More serious health effects can include stomach upsets, eye irritations and pain in muscles and joints. These symptoms are usually mild, but in some cases can be severe.
“If anyone has been in contact with the water and experiences health problems they should contact their GP or else NHS 24 on 111.”