A vast stretch of Lough Neagh's shore line this week was cloaked in what locals insist is toxic blue-green algae that has once again surfaced following heavy rain and warm weather.
But the underlying cause of the increasing amount of the algae appearing on the lough is being blamed on pollution, including sewage and agricultural run off.
Many of those living on the lough for generations, said the algae this summer, which they insist has noticeably increased in recent years, is the worst they have ever seen.
It was reported algae could be seen on the lough surface, on the shore line and in other connecting water ways all along the western shore and all the way to the Toome Canal on the north side.
Further, Armagh, Banbridge and Craigavon Borough Council (ABC) confirmed blue-green algae was detected on the south shore from Bartin's Bay to Oxford Island. Fermanagh and Omagh council also reported a blue-green algae bloom in lower Lough Erne in the vicinity of Castle Archdale.
Mick Hagan, of Western Shore Angling, said the warm weather and increased sunlight contributes but the underlying reason was pollution and overall neglect of the lough.
"It has definitely been getting worse this year, certainly from a few years ago," Mr Hagan said.
Signs were posted earlier in the summer warning people to avoid the water, particularly if they have pets. But the blue-green algae still remained as "billions of two penny pieces", the angler said.
"It took some rain, then a bit of warm weather," he said, adding the agricultural and other run off down the rivers has led to what many in the area are describing on social media as an "ecological disaster".
Nitrogen and phosphate in waste run off feeds the algae which then blooms when the weather warms and the sun shines.
Mr Hagan also noted the recent arrival of the Zebra Mussel in Lough Neagh has contributed to the problem as they are expert at filtering the water to make it clearer, allowing sunlight to penetrate to greater depths.
Some locals reported a strong odour along parts of the shore, with Md Ulster District Council identifying complaints from the Derrytresk / Derryloughan area.
Following consultation with the fire service, "the council has no current concerns regarding gas", a spokesperson said.
"In relation to the query of potential odour from the presence of blue-green algal blooms, this matter has been referred to the NIEA," the council added without directly addressing reports of the algae cloaking a long stretch of the lough within the district.
Algae appeared in Lough Neagh during the warm weeks of June at levels not seen since the 1970s, the Agri-Food and Biosciences Institute (AFB) said. Beaches on the north coast were closed following the appearance of the algae.
Mark Horton, of The Ballinderry Rivers Trust, also chief executive of the Ballinderry Rivers Trust, said: “Blue-green algae, also known as cyanobacteria, is not actually an algae, but a bacterium which can pose serious risks to human and animal health and the environment.
“The rivers trust is dismayed at the increase of these toxic algae blooms in water bodies in Northern Ireland because they are a clear and dangerous indicator of increased pollution in our rivers and marine environments.
"Increased nutrient levels create favourable conditions for the rapid reproduction of cyanobacteria. Excessive nutrients, particularly phosphorus and nitrogen, in the water can promote the growth of blue-green algae.
"These nutrients often come from human activities such as agricultural runoff, sewage discharges, and the use of fertilisers."
