Agencies say toxic blue-green algae, thought to be driven by farm runoff and sewage, recorded at levels not seen since 1970s.

Northern Ireland’s Lough Neagh, the largest lake in the British Isles, has been hit by recorded levels of potentially toxic blue-green algae that regional agencies say have not been seen since the 1970s.

Campaigners say the lake is “dying in plain sight” as vast algal blooms choke the aquatic life and bird and insect wildlife plummets.

The blooms are thought to be driven by slurry and other runoff from nearby farmland, as well as discharges of human sewage at the body where Northern Ireland sources nearly half of its drinking water.

A study showing that the temperature of the lough’s water has risen 1C since 1995, however, suggests that climate change and clearer waters as a result of an invasive zebra mussel species may also be contributing factors. Environmentalists highlight the additional impacts of industrial sand extraction, activity that has been described as an “outlier” case within Europe by UN experts and which may be redistributing toxins in the lough bed.

The lough, which at nearly 400 sq km has a surface area bigger than Malta, and its surrounding wetlands hold a considerable number of domestic and international conservation protections. Campaigners point to a serious lack of regulation and sharply declining wildlife as signs of long-term neglect and unintegrated thinking as regards the lough’s management.

A 2013 study by researchers at Queen’s University Belfast found that the number of migratory winter birds had dropped by nearly 80% in a decade. It also highlighted a 66% fall in certain insect and snail species inhabiting the lough bed.

“Lough Neagh is dying and it’s dying in plain sight,” said James Orr, Friends of the Earth NI’s director. “So interrelated and long-term are Lough Neagh’s problems that if it were a person it would be described as suffering from multiple organ failure.

“How an ecosystem that is so immense, so strategic, so beautiful, so rich in heritage has been ignored, forgotten, and abused for decades is the story of Wild West exploitation.”

About two dozen organizations – spanning devolved government departments, councils, and charities – have varying degrees of responsibility for Lough Neagh’s management. But these groups are fragmented and siloed, campaigners say, with there being no agency or body that exercises executive control over the lough.

The last major research facility at Lough Neagh, operated mainly by the University of Ulster at Traád Point near Ballyronan along the lough’s north-western shores, closed more than a decade ago.

The soil, bed, and banks of the lough are owned by the Shaftesbury estate, whose claim to the water body dates back to the Plantation of Ulster in the early 1600s. The current Earl of Shaftesbury, Nicholas Ashley-Cooper, has in recent weeks indicated he would be open to selling the asset.

Devolved administrations have explored public ownership of Lough Neagh a number of times since the 1970s, with the most recent bid floundering a decade ago.

But it is thought that a restored power-sharing executive at Stormont would be required in order to advance any fresh bid. And Northern Ireland’s devolved governing institutions have been lying dormant for more than a year, owing to a prolonged standoff over post-Brexit customs arrangements in the Irish Sea.

The Sinn Féin MP for Mid Ulster, Francie Molloy, who brought forward a debate over the lough’s future in 2012, said a new model for managing it was needed.

“I think there’s a greater awareness now of the lough and our dependency on it,” he said.

“There needs to be a proper management structure in place and we need to establish: what are the responsibilities of the Shaftesbury estate to ensure the safety of people using the lough? Is there a liability on the Shaftesbury estate for the condition of the lough?

“If you’re claiming ownership of it, then you have to take responsibility for it as well.”

Ashley-Cooper, the 12th Earl of Shaftesbury, said: “We should all be concerned about the algae bloom which has occurred not just on Lough Neagh but also on the Craigavon lakes and the beaches at Portstewart and Castlerock, as it can pose serious risks to human and animal health.

“I share the concerns about the management of Lough Neagh, as regulating all of these associated factors is currently within the remit of NI government bodies. The estate has agreed to contribute to projects developed by the Lough Neagh Partnership, which is a non-profit charity responsible for the integrated management and protection of the lough, and with whom the Shaftesbury estate of Lough Neagh Ltd has worked closely for many years as one of its members.”

A spokesperson for the devolved Department for Infrastructure and Department of Agriculture, Environment and Rural Affairs said: “DAERA has a range of significant programs underway to improve water quality … [and] the Northern Ireland Environment Agency (NIEA) also undertakes extensive monitoring and inspection programs as well as investigating all reported pollution incidents.”

The agency conceded that blue-green algae had been detected in multiple locations across Northern Ireland including Lough Neagh and Lough Erne, largely because excess nutrients were entering water bodies, primarily from agricultural activities and wastewater pressures.

It said: “Algal blooms occur naturally due to the combination of factors such as water temperature, water clarity, sunlight and nutrient availability. However, excess nutrients can compound blooms and lead to the growth of blue-green algae, exacerbated by invasive Zebra mussels leading to clearer water conditions.

“Whilst it is recognized that significant pieces of work are progressing, improvements in water quality will take a considerable period of sustained effort over many years, and DAERA and NIEA cannot deliver this on their own. Every person in Northern Ireland needs to consider how their behavior impacts on the water environment. We all have a part to play in this long-term effort to positively contribute to a sustained improvement in the status of our water bodies.”