An urgent warning has been issued after potentially fatal blue-green algae was found at a park in Rochford.
The Environment Agency confirmed the toxic algae was present in the lake at the Cherry Orchard Park.
The bacteria can result in sickness and skin rashes for humans and prove deadly to pets if ingested.
A warning sign has been erected telling people to stay away from the water.
It reads: "Warning. Blue-green algae. Avoid contact with the water, it may cause skin irritation and stomach upset.
"The algae is toxic to humans and may be fatal to pets."
The bacteria cannot be seen with the naked eye unless they clump together.
When this happens, blue-green algae can look like green flakes, greenish bundles or brown dots in a pond, lake or stream.
When the algae blooms, it can look like a blue-green scum has appeared on the surface of the water.
Rochford District Council have warned residents about the infestation and asked dog walkers to be careful when walking near to the park lake.
A spokesman said: “Due to confirmed blue-green algae at Cherry Orchard Country Park, we are asking users of the park to stay clear of the ponds and to keep their dogs on leads when they are anywhere near them.
“The algae can produce toxins which can harm, or even kill, animals and cause skin-rashes, eye irritation, vomiting, diarrhoea, fever and muscle and joint pain in people.
“The algae tends to disperse naturally when the weather gets colder. Until then, we would ask the public to ensure they and their pets avoid contact with the water in the ponds.”
The algae is especially harmful to animals, as Brentwood dog owner Francesca Buckley found when she let her dog Archie swim in the lake at Hatfield Forest.
The 23-year-old let her dog cool off in the lake when a member of staff alerted her to the algae.
"He said to not let our dogs in the water or let them drink it," Francesca said. "So we told him that they already had.
"He said they have blue-green algae in the lake and he could get a really upset tummy. We didn't let him in anymore."
Francesca and her mother Julie were not too worried, thinking that an upset tummy was not so serious.
However, as they were walking back to the car park, Julie noticed a warning sign, the first they had seen all day.
"I didn't even see the sign, it was tiny," Francesca added.
"Mum spotted it. It said something like, 'Blue-green algae is fatal to dogs'."
After frantically searching for information about the algae on the internet, Francesca was horrified to find out that her dog could be in real danger.
She rang her local vet and the receptionist told her to bring Archie in immediately.
Archie had to stay at the vets for two nights before he was discharged and sent home as the vet said he was fit and healthy.
Hatfield Forest say blue green algae occurs in summer regularly
Henry Bexley, head of operations at Hatfield Forest for the National Trust, said: “We are sorry to hear that one of our visitors did not have the day they had hoped for.
"The lake area is enjoyed by thousands of people who visit us each year and it offers a peaceful and quiet haven for wildlife.
“Blue-green algae appears in the lake at Hatfield Forest regularly in the summer and is commonly found in inland water bodies around the UK.
“We have clear signage at every main entrance point to the lake area to warn visitors about the algae and work closely with the Environment Agency to limit the risk to wildlife.
“In addition, we have operated a complete dogs-on-leads policy in the lake area for several years, with clear signage to ensure visitors know about this.
“No dog should be allowed into the main lake or decoy lake at any time at Hatfield Forest.”
Gudrun Ravetz, British Veterinary Association’s senior vice president, added: "While not all blue-green algae are poisonous, it is impossible to tell the difference visually, so it is better to be safe than sorry.
“If you suspect your dog has been exposed to blue green algae, rush it to your local vet without delay.”
Penny Down, secretary of the Friends of Cherry Orchard Park, said the issue has been a regular occurrence over the years.
“It is not unusual in any body of still water," she explained.
“It is not a huge inconvenience but dog walkers have to keep safe.
“If their dogs jump in the water, it could make them seriously ill.
“We do also see people actually use the lake to swim in and that would be a very silly thing to do as it could have the same very serious affect.”
