Dog walkers are being urged to keep their animals out of the water at Chasewater after the appearance of blue-green algae.
The blooms have begun to appear on the lake. The common form of bacteria occurs naturally in inland waters and can cause stomach upsets in humans and more serious illnesses in pets.
Sailing and water skiing activities are unaffected as the algae is only around the edge of the lake.
County Councillor Gill Heath, Cabinet member for the environment at Staffordshire County Council, said: “Blue-green algae is a natural occurrence and is fairly common.
“It tends to come and go by itself, and we suspect the growth is likely just a natural result of the warmer weather we’ve had. The signs are up around the site advising people to take sensible precautions.
“Chasewater is not a swimming lake and people shouldn’t be diving in for a swim at any time, but it does get popular when the weather is hot like it is currently.
“The algae will only cause upset if it is ingested, so we’re just advising people not to swim or let their pets swim in water where algae blooms are obviously present.”