People are being warned off going in reservoirs due to 'toxic' algae.
Bristol Water manages Chew Valley Lake, Litton Lake, Cheddar Reservoir, also known as Cheddar Lake, and Barrow Tanks.
And in recent days it has issued a warning over high levels of blue-green algae reported in the waters.
The algae, which blooms in hot weather, can cause vomiting and diarrhoea.
On Thursday, at Cheddar Reservoir, Bristol Water's head of recreations, Steve Smith, issued advice to around 300 people in four hours.
They were over not sticking to designated footpaths, failing to maintain social distancing and taking a swim.
Mr Smith said: “Just like the wildlife is flourishing, so is the algae that naturally occurs in the regions lakes and rivers.
“It’s really important to stay safe and stay out the water. It’s never safe to swim in our lakes due to hidden machinery in the water and cold water shock, to name a few, but the presence of algae adds an extra layer of risk. It’s toxic to both dogs and humans.
“I am so delighted to see the lakeside full of people once again.
"But please enjoy it safety, stick to designated footpaths, maintain social distancing and don’t go for a dip.
"On Thursday, I spoke to over 300 people failing to enjoy the lakes safely. Rules might seem boring, but we promise they’re there for a reason.”
Bristol Water says algae can make the water look bright green or brown tinged, it may also foam towards the surface.
It can often be mistaken for sewage as it can have an earthy like smell but does not affect water supplies as water goes through an intensive treatment process before it reaches your tap.
Bristol Water will be activity monitoring the lakes over the weekend, with regular security patrols in place.
Signs of algae poisoning include:
- Vomiting
- Diarrhoea
- Seizures or fits
- Loss of energy
- Drooling
- Difficulty breathing
