People are being warned not to swim or let their pets go into water where blue-green algae has been discovered.
A spokesman for the Environment Agency said they have carried out tests at Thrapston Lakes and confirmed that blue-green algae has been found there.
Toxins from the algae can kill wild animals, farm livestock and domestic pets.
In humans, it can cause rashes after skin contact and illnesses if swallowed.
Information from the Environment Agency says not all blue-green algae blooms and scum are toxic, but you can’t tell by just looking at them so it is best to assume they are.
A spokesman for East Northants Council said: “We are advising people not to go swimming in the lake.”