The blue-green algae present at the marina can be highly poisonous.
Mid Ulster District Council has issued a 'no swim' notice at Ballyronan Marina just a week after it was awarded a Blue Flag.
The presence of blue-green algae (pictured), which can be highly poisonous, was confirmed in multiple areas of Lough Neagh, including Antrim and Craigavon.
Mid Ulster Councillor Christine McFlynn, who highlighted the issue on social media, urged water users to heed the advice of the no swim signs.
“Concerned residents contacted me on Monday night regarding the algae which has appeared in the Marina Ballyronan,” she said.
“I immediately raised the concerns with the Chief Executive, and Northern Ireland Environment Agency (NIEA) have conducted tests which have confirmed that the blue-green algae is highly toxic so my main concern was for anyone who would go in the water, paddle boarding, kayaking and for residents walking their pets.
“The Council and NIEA acted quickly in erecting warning signage which people should follow.”
She said that while the council owns the land in and around the marina, not the lough itself, it has a duty of care for residents.
“I'd also point out that the Battery harbour has the same issue at the minute and also other areas along the Lough have been affected, Antrim and Craigavon area.
“The marina has just recently been awarded Blue Flag status so it's important the area is kept to a very high standard to encourage tourists into the area, it's important all issues are resolved quickly,” she said.
Mid Ulster District Council released a statement after its tests confirmed algae's presence.
“Following tests carried out yesterday, Tuesday June 6, Northern Ireland Environment Agency (NIEA) has confirmed a presence of blue-green algae at Ballyronan Marina and Battery Harbour,” it said.
“It is advised that all contact with the water is avoided until further notice. Farmers and pet owners should also ensure that their animals do not have access to this water, as the algae can produce toxins that are highly poisonous and often fatal to pets.
“The Council has erected signage warning the public of the presence of the algae at these sites and advise that dogs are kept on leads at all times.
Last month a dog died within an hour of interacting with the water at Ballyronan Marina.
The marina was recently awarded the internationally recognised Blue Flag for the 14th year in a row. The criteria for the flag included water quality, safety, accessibility, cleanliness and environmental education.
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