Lough Neagh’s freshwater emergency

29 August 2025

How blue-green algae on the UK’s largest freshwater lake is disrupting ecosystems and threatening public health

In the summer of 2023, blue-green algae blooms in Lough Neagh, Northern Ireland, made international headlines – they were so severe that they were visible from space. The unsettling event disrupted the lough’s ecosystems, halted recreation and fishing, threatened public health and caused fear in surrounding communities. It also publicly exposed shocking, fundamental weaknesses in water governance, land management and climate resilience in Northern Ireland.

The following year, the lough continued to suffer from significant algal blooms after a spring heatwave and high temperatures triggered a resurgence of blue-green algae. With record temperatures and extended dry periods again this year, by August 2025, Lough Neagh is back in the news as both political relations around the issue and water quality have now further declined. Commercial eel fishing was halted early in the season, and distressing footage recently emerged on social media showing juvenile eels, starved of oxygen, dying in thick algae.

Notably, this species was awarded protected geographical indication (PGI) status in 2011 to protect its identity and heritage. The Lough Neagh Fishermen’s Co-Operative Society has described the eel fishery as a “casualty of significant environmental change within the lough”.

Partly in response to the ongoing Lough Neagh crisis, in April this year, over 250 people gathered online for The Rivers Trust’s conference on the causes and costs of blue-green algae. Experts from across government, academia, utilities and environmental NGOs shared insights into the rising frequency and severity of blue-green algae blooms across the UK, Ireland and beyond, highlighting the role of excess nutrients, rising temperatures and systemic land-use pressures.

The message was clear: long-term, catchment-based solutions rooted in data, collaboration and investment are urgently needed to tackle the causes of this growing problem.

Lough Neagh BGA Bloom from Copernicus Satellite imagery shows the vast extent of the blue-green algae bloom on Lough Neagh during summer 2023.

As the largest freshwater lake in the UK and Ireland, Lough Neagh is both ecologically and economically vital. Yet starting in earnest in 2023, it showed all the hallmarks of a failing water body: visible pollution, declining biodiversity and widespread public concern. These symptoms were neither sudden nor unforeseeable. Rather, they’re the result of decades of compounding pressures, including nutrient enrichment, rising temperatures, invasive species and underinvestment in infrastructure, all magnified by policy fragmentation and a lack of coordinated political action or urgency.

Understanding harmful algal blooms

Blue-green algae, or cyanobacteria, are naturally occurring microorganisms found in both freshwater and marine environments. Under certain conditions, especially warm temperatures, high light levels and elevated nutrient concentrations, they can reproduce rapidly, forming harmful algal blooms (HABs). These blooms produce cyanotoxins, which pose serious risks to human and animal health. HABs can also deplete oxygen in waterbodies, causing fish kills and long-term habitat degradation.

The recurring algae blooms in Lough Neagh have led to widespread surface mats and prompted repeated public health warnings about contact with, or consumption of, contaminated water. Monitoring by Northern Ireland’s Agri-Food and Biosciences Institute (AFBI) confirmed that nutrient levels, particularly phosphorus, have persistently exceeded safe ecological thresholds in the lough. Phosphorus is the primary nutrient for cyanobacterial growth and a key driver of HAB formation. The AFBI estimates that 56 per cent of phosphorus entering Lough Neagh originates from agriculture, 31 per cent is attributed to discharges from wastewater treatment plants, and 1 per cent comes from domestic septic tanks.

The Neagh Bann River Basin District covers an area of around 5,740 km sq. It includes Counties Armagh, Antrim, Londonderry, Down, and Tyrone, as well as a small area in Fermanagh and County Monaghan across the border in Ireland. Lough Neagh is fed by six major rivers and supplies drinking water to over 40 per cent of Northern Ireland’s population. Despite multiple environmental designations, including special protection area, Ramsar site, and area of special scientific interest, the lough’s health continues to decline. Invasive zebra mussels, first recorded in the lough in 2005, have further destabilised its ecological balance. By filtering out competing plankton species, they increase water clarity, allowing deeper light penetration, which creates favourable conditions for cyanobacteria.

An illustrated diagram explains how nutrient runoff, warm temperatures, and untreated waste lead to harmful algal blooms, threatening aquatic life and public health.

The impacts of a changing climate are amplifying the frequency, duration and severity of harmful algal blooms across many freshwater systems. According to the UK Met Office, 2023 was the hottest year on record for Northern Ireland. This period was marked by prolonged dry spells followed by short, intense rainfall events, conditions that significantly increase nutrient runoff. An increasing body of research links extreme weather events, particularly in intensively farmed landscapes, with elevated phosphorus runoff and a heightened risk of harmful algal blooms. These trends are further exacerbated by a changing climate, which acts as a co-stressor by warming, acidifying and deoxygenating aquatic ecosystems, thereby creating conditions that favour the frequency and severity of HABs. In Lough Neagh, these shifting climate patterns are compounding existing nutrient pressures, creating warm, nutrient-rich, stagnant surface waters that provide ideal conditions for cyanobacterial blooms to thrive.

Governance and infrastructure gaps

Northern Ireland has faced historic chronic underinvestment in its water and wastewater infrastructure. NI Water has publicly acknowledged that current capital funding is insufficient to meet regulatory standards. Combined sewer overflows, many of which lack real-time monitoring, continue to discharge untreated sewage into the catchment during heavy rainfall events. Agriculture is a cornerstone of Northern Ireland’s economy, with roughly 75 per cent of land used for farming, most of it dedicated to grassland and livestock. While the sector underpins rural livelihoods, it is also a major contributor of nutrients to the water environment.

Despite regulatory frameworks, nutrient surpluses persist, especially where organic manures are not applied according to crop demand or soil capacity. Combined with increasing rainfall, warming temperatures and invasive species, the fundamental pressures of underfunded infrastructure and nutrient-intensive land use have now tipped the ecological balance of the catchment.

Lough Neagh’s failure reflects a wider pattern of systemic governance challenges. While river basin management plans (RBMPs) and cross-border cooperation structures exist, implementation has been inconsistent. Only one-third of surface water bodies in Northern Ireland currently meet the standards of the Water Framework Directive, according to the Northern Ireland Audit Office. Reversing this trend requires coordinated action across sectors. Wastewater infrastructure requires investment to modernise, including increasing treatment capacity, reducing dependence on combined sewer overflows, and deploying real-time spill monitoring.

In agriculture, nutrient management needs to be improved through funding of water-friendly farming intervention schemes, updated legislation and grassroots support and empowerment for change. Nature-based solutions such as wetland restoration, riparian buffers and sustainable drainage systems should be mainstreamed into rural and urban land use planning. Catchment-scale RBMP delivery must be reformed to prioritise climate resilience, transparent performance tracking and meaningful community co-design.

Water-friendly farming in practice

In practical terms, The Rivers Trust is delivering the Sustainable Catchment Programme, which is funded by NI’s Department for Agriculture, Environment and Rural Affairs in several river catchments feeding into Lough Neagh. Working through local river trusts, and in partnership with farmers, communities and agencies, the programme supports water-friendly farming that tackles nutrient pollution at source.

Interventions include the creation of riparian buffer zones, livestock exclusion fencing, alternative livestock watering facilities, and clean-dirty water separation measures. By combining local stakeholder engagement with data-led planning, the programme delivers tangible environmental improvements while supporting farm productivity and compliance. As pressure mounts to reduce diffuse pollution, this integrated, co-designed approach offers a scalable model for sustainable land management across the Lough Neagh catchments and beyond.

In addition to The Rivers Trust’s catchment-based work, several other bodies and initiatives are actively responding to the blue-green algae crisis at Lough Neagh. The Lough Neagh Report, published by the Northern Ireland Executive in 2024, sets out 37 targeted measures across enforcement, education and investment, with delivery overseen by a multi-agency programme board and supported by a minister-led stakeholder forum. To tackle pollution at source, the Sustainable Utilisation of Livestock Slurry (SULS) scheme is targeting Northern Ireland’s manure surplus, while the Soil Nutrient Health Scheme, now in its final year, is testing every field in Northern Ireland to support more efficient fertiliser use and reduce phosphorus loading.

The Lough Neagh Partnership, a long-standing local stakeholder group, continues to advocate for joined-up solutions and community engagement. While two small business research initiatives are currently trialling innovative technologies, including a solar-powered ‘bubble barrier’ designed to direct algal blooms, and the use of drones and satellite imagery to improve real-time detection and forecasting. Collectively, these efforts reflect a growing recognition that science, innovation and cross-sector collaboration are critical to restoring Lough Neagh’s ecological health.

Looking ahead

The Lough Neagh algal bloom of 2023, and recurring events since, should be viewed as a sentinel warning. It marks the intersection of climate pressures, land use intensification and institutional shortcomings. This is not a problem unique to Northern Ireland and many innovative projects are now urgently seeking to develop solutions.

For example, across the UK and Ireland, harmful algal blooms are being reported with increasing frequency. The Bloomin’ Algae app, developed by the UK Centre for Ecology & Hydrology, provides a live map of confirmed sightings, highlighting the widespread scale of the issue. Researchers at Queen’s University Belfast are at the forefront of efforts to understand and tackle blue-green algae, using cutting-edge tools from molecular diagnostics to ecosystem modelling. Drawing on cross-disciplinary expertise, their work includes rapid toxin detection, DNA sequencing and food web analysis to inform long-term, data-driven solutions.

In July 2025, researchers at the University of Birmingham developed an advanced mass spectrometry technique that can quickly and accurately detect toxin-producing blue-green algae in freshwater before harmful blooms emerge, offering a powerful early-warning tool for safeguarding water quality and public health.

Protecting freshwater systems like Lough Neagh is not only essential for nature, but for the communities, farms and businesses that depend on clean, resilient water resources. Recovery will take time, sustained investment and smarter, joined-up action – the good news is that the tools, partnerships and knowledge already exist. By tackling the root causes of pollution through integrated, catchment-based approaches, we can restore ecological health while supporting a thriving rural economy. With decisive, collaborative effort, this crisis can become a turning point towards safeguarding the water we all rely on and building a more balanced, sustainable future for generations.

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The CIWEM Northern Ireland branch are hosting their annual autumn morning conference, taking place this year at Lough Neagh Discovery Centre, near Lurgan. To learn more about the conference, and book your spot, visit the event page.

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Mark Horton is the all-Ireland director at The Rivers Trust