• Two recent pieces of research support this claim.
  • The NEW Harmonica project, a collaboration between AFBI and various universities, suggests 61% of the lough’s phosphorus comes from agriculture while a total of 30.3% is from the wastewater system and septic tanks. A full report from this project is due in the new year.
  • A study cited in Stormont’s Lough Neagh action plan says that 62% of the phosphorus in all NI’s waterways – not just the lough – stems from agri sources while a further 36% is from wastewater and septic tanks.

During an Assembly debate on Lough Neagh on 9 September 2025, SDLP MLA Patsy McGlone said:

“The only way in which to address [the Lough Neagh crisis] is by drastically reducing the excessive levels of nutrients — phosphorus and nitrogen — in the lough. Over 60% of the phosphorus in the water originates from agriculture, while one third comes from waste water treatment facilities and septic tanks.”

This claim has two aspects:

  1. Over 60% of the phosphorus in Lough Neagh originates from agriculture.
  2. A third of it comes from waste water treatment facilities and septic tanks.

Both parts of this claim are supported by evidence.

Data from an ongoing project called NEW Harmonica, a collaboration between the Agri-Food and Biosciences Institute and various universities, suggests 61% of the lough’s phosphorus comes from agriculture, while 29% comes from wastewater and 1.3% from septic tanks (so 30.3% in total). It is worth noting, however, that NEW Harmonica is set to publish a full report in the new year.

The Department for Agriculture, Environment and Rural Affairs, together with other departments, produced a Lough Neagh Report and Action Plan in 2024, which cites research from 2020 suggesting that the sources of the phosphorus in NI’s waterways (in total, not just Lough Neagh) are 62% from agriculture, 24% from wastewater systems and 12% from septic tanks (so 36% in total).

Both of these pieces of research provide evidence to support the claim – although anyone interested in this topic should keep an eye out for the full report from NEW Harmonica due in 2026.

For more detailed information, read on.

  • Source

FactCheckNI contacted Mr McGlone to ask him about this claim but, at the time of writing, had received no response.

  • The lough

Lough Neagh is the largest freshwater lake in the UK and Ireland, and provides over 40% of Northern Ireland’s drinking water. It is a Special Protection Area as a result of its importance for various bird species and a Ramsar Site, denoting it as wetland of international importance for waterfowl and fish.

In 2023, Lough Neagh drew widespread attention when blue-green algae (cyanobacteria) turned it bright green, sparking public concern over water quality. 

The blue-green algae is damaging the ecosystem of the Lough. Exposure to the water can cause illness in humans and be toxic for animals. The algae forced the suspension of Lough Neagh’s eel industry in 2024 and caused other negative impacts.

In 2025, the Chartered Institution for Water and Environmental Systems called the situation an emergency. But what is behind the blue-green algae in Lough Neagh?

Is phosphorus to blame for the algae? And is Mr McGlone correct to say that 60% of the Lough’s phosphorus comes from agriculture?

  • Causes

The July 2024 Lough Neagh Report and Action Plan, developed by the Department of Agriculture, Environment and Rural Affairs (DAERA) in conjunction with officials from the Department of Infrastructure (DfI) and NI Water (NIW) explains that various factors combined in 2023 to cause the spread of blue-green algae in the Lough:

  • Pollution (excess phosphorus and nitrogen) entering waterways from agriculture, wastewater treatment works, domestic systems and industry.
  • Climate change resulting in record high water temperature.
  • Zebra mussels – an invasive species known to disrupt ecosystems by making water clearer (which helps algae grow).

The report states that while the team considered the impact of all three factors, they focused on reducing pollution in waterways for their action plan as a “first step.”

  • Phosphorus significance

Phosphorus is required for all life and is a key nutrient in the fertilisers and feeds used in crop and livestock production. 

Too much phosphorus, however, is harmful for ecosystems. According to the UK Environment Agency (UKEA), an overabundance of phosphorus and nitrogen can cause “eutrophication” – an excess of nutrients.

The Environmental Protection Agency (EPA) in Ireland says the “general rule is that phosphorus is of most concern in our freshwaters, and nitrogen is of most concern in estuaries and coastal waters.”

The Agri-Food and Biosciences Institute (AFBI) describes how excess phosphorus acts as a fertiliser for algae, causing large blooms.

DAERA’s public consultation on the proposed Nutrients Action Programme for 2026-29 states that Northern Ireland has a significant phosphorus surplus. Phosphorus has built up in the soil and “resulted in diffuse phosphorus losses to rivers and lakes which are a main cause of poor water quality across Northern Ireland.”

  • Where does the phosphorus come from?

The Lough Neagh Report and Action Plan cites figures from the RePhoKUs Report of October 2020.

RePhoKUs (The Role of Phosphorus in the Resilience and Sustainability of the UK Food System) is a collaboration between various universities and AFBI, funded by various Research Councils and the Scottish Government.

The study found three main sources for high phosphorus levels in Northern Ireland waterbodies and apportioned them as follows:

  • 62% from agriculture
  • 24% from wastewater treatment works
  • 12% from septic tanks

These figures, however, relate to all of Northern Ireland’s waterbodies, not just Lough Neagh.

More recent modelling was published as an infographic by NEW Harmonica project on 1 June 2025. NEW Harmonica is a collaboration between AFBI, Wageningen University, the University of Lancaster and the Flanders Environment Agency funded by the EU.

The team ran a model for the reference year 2021 using  available data, taking account of direct inputs into Lough Neagh as well as all the rivers that flow into it. They call this the Neagh-Bann catchment, which covers an area of over 8000 square kilometres, and spans both sides of the Irish border.

  • Neagh-Bann catchment

Figure 1 – source: NEW Harmonica

The model’s work apportioned the different sources of phosphorus coming into Lough Neagh as follows:

  • 61% from agriculture
  • 29% from networked wastewater (sewage + industry)
  • 1.3% from septic tanks
  • 9% from non-agricultural sources

The 2025 infographic further specifies that most of the surplus phosphorus in the Lough Neagh catchment comes from cattle slurry. 

A full report will be published in the year by NEW Harmonica. However, based on the information available so far it is reasonable to conclude that there is evidence to support the two parts of Mr McGlone’s claim, namely that:

  1. Over 60% of the phosphorus in Lough Neagh comes from agriculture. The findings from NEW Harmonica suggest it is 61% while RePhoKus put this at 62% for the whole of Northern Ireland, not just Lough Neagh.

A third comes from waste water treatment facilities and septic tanks. NEW Harmonica says this is 30.3% in total, which it is reasonable to describe as one third. RePhoKus suggested a figure of 36% for NI’s waterways overall.