Every seagrass meadow examined in a new Northern Ireland study contains nitrogen concentrations above the internationally recognised level at which the plants begin to suffer from nutrient pollution. Researchers at Queen's University Belfast analysed tissue from nine sites, all within Marine Protected Areas, and found each one exceeded the 1.8 percent nitrogen threshold.

The findings, published in Marine Pollution Bulletin, show that legal designation alone has not shielded these habitats from agricultural run-off, wastewater discharges and other land-based pollutants. Seagrass leaves absorb and store nitrogen over weeks and months, providing a longer-term record than single water samples.

Waterfoot Bay, a Marine Conservation Zone off the County Antrim coast near Cushendall, recorded 2.14 percent nitrogen. While this is above the early-warning level, it remains below the 2.8 percent mark at which declines accelerate sharply. Researchers attributed the bay's lower seagrass density to sampling timing and location rather than acute damage.

The most elevated reading came from Lough Foyle, where tissue nitrogen reached 3.72 percent, more than double the healthy limit. Isotope analysis pointed to sewage or manure as probable sources. Dundrum Bay in County Down registered 3.14 percent and showed the clearest signs of ecological breakdown, with historical surveys documenting seagrass replacement by dense green algae mats.

Scientists warned that current monitoring under the Water Framework Directive may be missing stress signals. Lough Foyle holds a "High" ecological status and Dundrum Bay a "Good" rating, yet both meadows already exhibit biochemical damage. The study argues that tissue nitrogen offers a practical early-warning tool for agencies.

One site bucked the trend. Castle Espie in Strangford Lough, adjacent to a wetland reserve managed by the Wildfowl and Wetlands Trust, had lower nutrient loads and no evidence of light stress. Filtration through reedbeds and willow systems appears to intercept pollutants before they reach the lough, indicating recovery is possible with catchment-level measures.

The researchers concluded that reducing nutrient inputs from agriculture, improving wastewater treatment and addressing diffuse pollution across entire catchments are essential to prevent further seagrass loss and to restore the coastal ecosystems on which fisheries, carbon storage and shoreline protection depend.