“Extremely dangerous” blue green algae that can cause diarrhoea, blistering and rashes to those who come into contact with it could keep Frensham Great Pond's bathing area off limits until November.
The Environment Agency confirmed the presence of blue-green algae, which occurs naturally in freshwater ecosystems, during a regular assessment of Frensham Great Pond. As such, Waverley Borough Council has issued a warning against entering the water.
The algae is potentially harmful to humans and animals – and particularly dogs.
It can develop suddenly when warm, still, weather is interrupted by heavy rains.
Once established, there is no way of removing the algae from the water, the council said.
Speaking to the full meeting of Waverley Borough Council on Tuesday was Councillor Steve Williams, portfolio holder for environment and sustainability.
He said: “Visitors to Frensham Common are being warned not to enter the water at Frensham Great Pond because of the occurrence of blue green algae – which was confirmed by the Environment Agency at its regular assessment on July 7.
“Blue green algae, as we probably know, is extremely dangerous for anyone, especially to dogs and children, and it can cause dermatitis, diarrhoea, irritation, rashes, blistering, abdominal pain, and nausea, and it’s pretty unpleasant.
“We’ve put up warning signs on the site, we have messaging on social media, and on the Waverley website, and our rangers are on site providing advice to visitors."
He added: “It is a seasonal thing, and we await the all clear from the Environment Agency, but on past form this is not likely to happen until certainly well into the autumn, and November as it was last year."
Frensham Great Pond is managed by Waverley Borough Council as is one of only a handful of non-coastal sites to have been graded for its excellent water quality.
Its popularity can create its own issues, with overcrowding, litter, and traffic damaging its special environment.
Anyone who believes they have been affected by blue-green algae are advised to seek medical advice or in the case of their pets, from a veterinarian.