People and dogs are being warned to stay out of Carr Vale Pond in Bolsover after blue-green algae has been found in the water.
Blue-green algae is a naturally occurring organism which is more common in ponds during hot weather. The algae can produce toxins which may be harmful to animals and cause rashes and illness in people.
Derbyshire County Council said it is working with the Environment Agency and Carr Vale Angling Club to understand the problem and install signage to warn visitors.
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Carr Vale Angling Club have closed the pond to anglers.
If you or your dog have been in contact with the pond water recently and are concerned, seek advice from your GP or vet.
Derbyshire County Council’s interim assistant director for environment, Claire Brailsford, said: “Many areas of water are susceptible to blue-green algae during the warm summer months. It can persist for several weeks but usually disperses naturally when the weather gets colder.
"Blue-green algae is not always harmful but as a precaution, we are advising people to keep dogs out of the pond water until further notice.”