Protected Waterfoot seagrass among every NI meadow exceeding pollution warning level

Underwater seagrass meadow.

A protected seagrass meadow off the County Antrim coast at Waterfoot has been found to contain more nitrogen than scientists consider healthy, after researchers discovered every seagrass meadow tested across Northern Ireland had exceeded an internationally recognised pollution warning threshold.

The Queen’s University Belfast study examined the amount of nitrogen stored inside seagrass leaves — a natural record of the pollution the plants have been exposed to over time. Unlike a single water sample, seagrass absorbs and stores nutrients from the surrounding water as it grows, allowing scientists to build a longer-term picture of water quality.

Researchers found that all nine meadows studied, including the protected site at Waterfoot Bay, contained more than 1.8% nitrogen in their leaf tissue. Scientists say this is the point at which seagrass begins to show signs of stress from nutrient pollution.

Once levels rise above 2.8%, the plants deteriorate much more rapidly, with even small increases in pollution causing disproportionately large losses of seagrass.

The findings, published in Marine Pollution Bulletin, suggest nutrient pollution linked to sources such as agricultural run-off, wastewater discharges, aquaculture and urban drainage is affecting protected seagrass habitats across Northern Ireland, despite them having legal protection.

Why seagrass matters

Seagrass meadows are among the most important habitats in coastal waters.

They provide nursery grounds for young fish and shellfish, support biodiversity, stabilise seabed sediments, help protect coastlines from erosion and storms, and capture significant amounts of carbon, making them important in tackling climate change.

They are also closely linked to the heritage of many coastal communities but have been declining worldwide, with nutrient pollution recognised as one of the biggest threats to their survival.

What the nitrogen figures actually mean

Rather than measuring nitrogen in the water itself, researchers analysed the amount stored inside the seagrass leaves.

Because seagrass continuously absorbs nutrients from the surrounding sea, its leaves effectively act as a natural pollution diary, reflecting water quality over weeks and months rather than conditions on a single day.

Scientists use internationally recognised thresholds to assess the health of seagrass.

A nitrogen level of 1.8% within the leaf tissue marks the point where plants begin to show measurable signs of stress. The research identified 2.8% as a critical tipping point, where seagrass biomass declines at its fastest rate and ecosystems become much more vulnerable to collapse.

The study validated these thresholds using data from seagrass meadows across 13 countries in the Northern Hemisphere.

Waterfoot remains healthy — but researchers urge vigilance

Waterfoot Bay, near Cushendall, was the only open-coast site included in the Northern Ireland study and forms part of the Waterfoot Marine Conservation Zone.

Researchers recorded a nitrogen level of 2.14% in the seagrass leaf tissue there. While this exceeds the 1.8% early warning threshold, it remains below the 2.8% level associated with the most rapid ecological decline.

Unlike some other locations, scientists found no immediate evidence that the Waterfoot meadow had entered serious decline. They believe the lower amount of seagrass recorded there was more likely due to the timing of sampling later in the growing season and because samples were collected from the edge of the meadow rather than its centre.

However, the study warns that continued nutrient enrichment could place increasing pressure on the site over time if pollution entering coastal waters is not reduced.

Every protected site exceeded the benchmark

The researchers sampled nine seagrass meadows across Northern Ireland:

• Ballyhenry Bay

• Castle Espie

• Castle Ward Bay

• Dundrum Bay

• Lough Foyle

• Mount Stewart

• Newtownards

• Strangford Marina

• Waterfoot Bay

Every site exceeded the internationally recognised 1.8% benchmark, despite all being located within Marine Protected Areas.

The paper concludes this indicates widespread human-related nitrogen pollution affecting Northern Ireland’s coastal waters.

Lough Foyle and Dundrum Bay raise the greatest concern

The strongest warning signs emerged from Lough Foyle and Dundrum Bay.

Lough Foyle recorded the highest nitrogen level of all the sites at 3.72% — more than double the recognised healthy threshold.

Researchers also detected isotope signatures consistent with nitrogen originating from sewage or manure. Previous sanitary surveys have identified direct sewage discharges and diffuse agricultural run-off as significant pollution sources within the wider catchment.

Although the meadow still supports relatively high seagrass biomass, scientists say the biochemical evidence points to an “early warning signal of ecological decline” that is not yet obvious through traditional monitoring.

Dundrum Bay recorded 3.14% nitrogen and was identified as the clearest example of a meadow already suffering ecological degradation.

Historical surveys have documented extensive losses of seagrass there over recent decades, with large areas replaced by dense mats of green algae.

Researchers believe the meadow may already have crossed an ecological threshold where reducing pollution alone may not be enough to restore it without active intervention.

Current monitoring may not detect problems early enough

One of the study’s most significant findings is that existing environmental monitoring may be identifying damage too late.

Researchers note that Lough Foyle was classified as having “High” ecological status under the Water Framework Directive, while Dundrum Bay received a “Good” classification.

However, they argue these assessments rely mainly on measuring how much seagrass remains, rather than whether the plants themselves are already under physiological stress.

The study concludes that biochemical indicators, such as tissue nitrogen, can reveal deterioration well before visible habitat loss occurs, giving environmental agencies an opportunity to intervene earlier.

Castle Espie offers hope

Not every site told the same story.

Castle Espie in Strangford Lough was highlighted as an example of how effective land management can help protect seagrass.

Although nitrogen pollution was still present, researchers found lower nutrient levels than at other intertidal sites, alongside comparatively healthy seagrass and no evidence of light stress.

The study suggests the nearby Wildfowl and Wetlands Trust reserve, with its reedbeds, willow systems and other natural filtration features, may be reducing nutrient pollution before water reaches the lough.

Researchers call for earlier intervention

Heidi McIlvenny, a PhD student from the School of Biological Sciences at Queen’s University Belfast who led the research, said:

“Seagrass takes up nutrients straight from the water around it, so the chemistry of the leaves gives us a longer-term record of pollution than a one-off water sample could. To work out where the nitrogen thresholds leading to plant loss are, we drew on tissue data from studies across 13 countries. We then tested meadows in Northern Ireland against these thresholds, and not one came back below the limit.”

She added:

“Dundrum Bay, on the County Down coast was the starkest example. On paper it’s classified as healthy. In reality, nitrogen levels there were nearly double what seagrass can tolerate, and surveys over the past decade show dense mats of green algae smothering what is left of the meadow.

“But Castle Espie, just up the coast in Strangford Lough, tells a completely different story. The seagrass there is thriving, and the difference is the wetland reserve next to it. The reedbeds and willows act as natural filters before any of that water reaches the Lough. It shows recovery is possible, but only if we deal with what is coming off the land.”

Dr Sarah Helyar, Senior Lecturer from the School of Biological Sciences at Queen’s University Belfast, said the research provides environmental agencies with practical tools to detect problems sooner.

“This study makes clear that lines on a map don’t protect ecosystems. The same species, with the same legally protected status, can thrive or collapse depending on what is happening in the catchment.

“Monitoring still focuses on how much seagrass is left, but once a meadow starts shrinking, the damage has already been done.

“Tissue chemistry can detect stress much earlier, and the nitrogen thresholds identified in this study offer a practical early warning system for environmental agencies. Meadows at or above 1.8% nitrogen warrant closer monitoring and intervention to reduce nutrient inputs, while those exceeding 2.8% require urgent action to reduce nutrient inputs from catchments — before they pass the point of recovery.”

A warning beyond protected waters

Researchers say the findings demonstrate that legal protection alone cannot safeguard marine ecosystems if pollution continues to flow into the rivers and coastal waters that feed them.

The study concludes that tackling agricultural run-off, improving wastewater treatment and reducing nutrient pollution across entire catchments will be essential if Northern Ireland’s seagrass meadows are to recover and continue providing the environmental benefits on which both wildlife and coastal communities depend.