Blue-green algae is back at 2023 levels in Lough Neagh.

Large green mats have formed around the lough, particularly in the north-east.

A “very sharp uptick” in the amount of potentially toxic algae has been noticed in recent days by scientists monitoring the lough.

Blue-green algae has already been detected in the Bann River system and scientists have said it could travel further up the north coast, where it closed bathing waters in Portstewart and Portrush last year.

Causeway Coast and Glens Council confirmed this week that blue-green algae had been most recently detected at Cloonavin in Coleraine.

While the algae may well make it to the coast, it cannot survive in salt water, but bathing bans or advisories may be used as a precaution until it has confirmed to have broken down.

The council has advised that people should not enter the water as a precautionary measure near Cloonavin and also advised pet owners to ensure their animals do not have access to this water.

Dr Matthew Service, from the Agri-Food and Biosciences Institute (AFBI), said blue-green algae spreads rapidly.

“It starts to double every seven to 14 days,” he said .

“So once you get to where you can see it, it’s just going to keep going until either the climatic conditions change, there’s not enough light for it to reproduce or it just simply breaks up with the autumn and winter weather,” Dr Service said.

AFBI scientists have detected the algae out in the middle of Lough Neagh, showing the wind is moving the very buoyant species around and causing it to clump together – or “aggregate”.

From the shoreline at Antrim, the water is visibly green.

Regular walkers Karen and Allistair Gordon said they were “shocked” by the sight and the smell of the lough and the Sixmilewater river flowing into it.

“I just don’t understand how it’s got to this stage,” said Mr Gordon, who is an amateur wildlife photographer.

“It’s just heartbreaking to see.”

Ms Gordon described it as “a very sad day”.

When BBC News NI visited Antrim Lough Shore Park on Thursday, a number of people voiced their displeasure at the blue-green algae levels.

Teresa Greer, who is part of a walking group, said the algae on Lough Neagh was “absolutely ridiculous”.

“The smell is disgusting,” she added.

Ms Greer said some members of the walking group had to stop their stroll, as the “smell was so foul at Rea’s Wood”.

Rea’s Wood is a nature reserve located at the north-east corner of Lough Neagh.

Last month, the Department of Agriculture, Environment and Rural Affairs (Daera) advised people against bathing at Rea’s Wood due to “the presence of a thick algal scum”.

One of Ms Greer’s walking companions, Anne Gregg, also expressed her concern about the algae.

“Dog walkers don’t come up to Rea’s Wood, near the water,” she added.

“My son and brother won’t walk their dogs there just in case.

“There is no quick fix,” she added.

John and Gwen Dowling said they regularly make trips to Antrim Lough Shore Park, from their home in Lisburn.

Mr Dowling said he “would rather see the algae away”, as it is “bad for wildlife”.

“You want the wildlife to thrive,” he said.

Ms Dowling said it “didn’t look right” and “it is the first thing you notice”.

“There is too much algae,” she added.

Lough Neagh is the largest body of fresh water in the British Isles.

The lough supplies half of Belfast’s drinking water and 40% of Northern Ireland’s overall.

The Northern Ireland Executive approved an action plan for the future of Lough Neagh at a meeting earlier this month.

On a private visit to Antrim Lough Shore Park on Thursday, NI Minister of Agriculture, Environment and Rural Affairs, Andrew Muir, said it was “distressing and sadly unsurprising ” to see the algae at such levels in the lough.

The lough has been affected by blue-green algal blooms in recent months.

Scientists and officials agree it will take a long time to resolve the challenges of the lough.

“Unless the fundamentals change, it’s unlikely to get any different result,” Dr Service said.

“The fundamentals in the lough have not changed – the inputs are staying the same.

“The levels of nutrients are still high and of course the zebra mussels are still there.”

The ownership has been in the Earl of Shaftesbury’s estate since the 1800s.

The rights to the lough were given a price tag of £6m 10 years ago, but civil servants at the time advised the government not to go ahead with the sale. It is not known why.

Nicolas Ashley-Cooper, who is the 12th member of his family to hold the title, told BBC News NI in October that a sale was something he was willing to discuss again.

However he has since said he would like to transfer his estate’s ownership of Lough Neagh “into a charity or community trust model, with rights of nature included”.

He said the proposal “may take time”.