Visitors to Chasewater Country Park are being warned not to swim in its waters as tests are carried out to find out whether a dangerous toxin is lurking.
Signs around its lake went up alerting potential swimmers and dog walkers that blue green algae had been found in its reservoir at the weekend and samples are now being analysed.
Blue-green algae can kill animals and could cause humans rashes, eye irritation, diarrhoea, fever and pains to muscles and joints.
It grows naturally in inland waters, estuaries and the sea, the Environment Agency said, and blooms when lots of it collects together.
Gill Heath, cabinet member for communities and the environment at Staffordshire County Council, said: “We are now having the water tested to see what kind of algae it is.
“As a precautionary measure we are asking people not to go into the water and for dog owners to keep their pets away from the water’s edge.
“There are lots of different types of algae that gather on water.
“Once we have the results back and know which type of algae we are dealing with we will take the appropriate action.”
“Children and adults alike were enjoying Chasewater, between Brownhills and Burntwood, yesterday morning.
“They included a man who was jet skiing on the reservoir’s water despite the warnings.
About 150,000 people visit Chasewater, which is set across 890 acres of land, every year.
Burntwood North councillor and Labour group leader on Staffordshire County Council Sue Woodward said: “It seems to me that the staff on the ground, the rangers acted quite promptly but it does not look like the follow through has been as it might have been.
“With the summer holidays just starting, Chasewater is such a lovely asset to our area.”
The Environment Agency has issued warnings about the algae in a booklet, Blue-Green Algae.
They stated: “Illnesses including skin rashes, eye irritation, vomiting, diarrhoea, fever and muscle and joint pain have occurred in people who’ve swallowed or swam through algal scum.
“These haven’t led to long-term effects or death but, in some cases, the illnesses were severe.
“Although algal scum isn’t always harmful, avoid contact with it and the water close to it.
“The toxins the algae may produce are also toxic to animals and can cause severe illness and death.
“Farmers and pet owners should keep their animals away from affected waters.”
The leaflet continues: “Blue-green algae photosynthesise during the day –adding oxygen to the water – but consume it at night. This means oxygen levels can be very low in the early morning and can suffocate fish and other creatures.
“When the bloom has subsided, bacteria causing the decay can also remove large amounts of oxygen.
“Scums form during calm weather when several bloom-forming species rise to the water surface. This can look like paint, jelly or form small clumps.
“Scum colour varies because algal pigments differ between species, and even within single species, depending on the nutrient supply, light intensity and age of the bloom.
“Scums may be blue-green, grey-green, greenish-brown or occasionally reddish-brown.”
In the 1990s gangs suspected of dumping chemicals into waters were blamed for the deaths of 200 dolphins, which washed ashore in Mexico.
But those deaths were subsequently found to have been caused by algae, which was found to have been growing in the seas they were swimming in.
A huge algae bloom in America recently closed one of its largest freshwater lakes after 100 people became ill after using it.
Utah Lake was covered with blue green algae, leaving it with an unappealing scum along the shore and turning it a bright green colour.
Similarly, if left untreated in warmer climates, it can cause greater disruption.
One of America’s Great Lakes, Lake Erie, was swamped with the bacteria in 2014. That left about 400,000 people without water for two days.
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