‘Protected’ seagrass meadows aren’t necessarily healthy – because pollution doesn’t stop at the shoreline

I spent last summer wading through seagrass meadows across Northern Ireland, from the sheltered waters of Strangford Lough to the exposed coast at Waterfoot Bay. I was collecting seagrass leaves and testing them for nitrogen pollution. Every meadow I visited sits inside a marine protected area – a stretch of sea that’s been given legal protection to safeguard the wildlife living there. And every single one was polluted beyond the limit for healthy seagrass.

Seagrass meadows are among the most valuable habitats in our coastal waters. They store carbon, nurture young fish and shellfish, stabilise sediment and buffer shorelines from storms. They are also woven into the heritage of coastal communities who have fished and foraged around them for generations. But they are disappearing worldwide, and nitrogen pollution from farming, sewage and urban runoff is one of the biggest reasons why.

It’s easy to assume that designating an area as “protected” keeps the habitat inside it safe. My research shows that, for seagrass, this assumption is dangerously wrong. Physical protection from anchors and dredging means little when pollution flows freely across the boundary from the land.

What matters most for seagrass is not lines drawn on a map, but what happens on shore.

To understand how much nitrogen pollution seagrass is absorbing, we can measure nitrogen content in the leaves themselves. Seagrass continuously takes up nutrients from the surrounding water, so the chemistry of its tissue works like a long-term pollution record. And my results showed that every meadow in Northern Ireland exceeded the pollution limit.

But knowing the pollution level is only useful if you know how much is too much, and what it means for the health of the meadow. To answer that, we pulled together data from 13 countries across the northern hemisphere and found a clear pattern.

When nitrogen in the leaves rises above 1.8%, seagrass starts to suffer and loose plant growth. Above 2.8%, the decline accelerates rapidly, and in this danger zone small increases in pollution trigger disproportionately large plant losses. Think of it as a traffic light system: green is below 1.8% where meadows can cope; amber is between 1.8% and 2.8%, where managers should be watching closely and acting to reduce pollution; and red is above 2.8%, where urgent intervention is needed before the damage becomes irreversible.

The starkest example of a meadow in the red zone comes from Dundrum Bay, on the County Down coast. According to government assessments, it’s healthy. But my data tells a different story. Nitrogen levels here were nearly double the pollution limit of 1.8%. Surveys over the past decade paint an even bleaker picture: where lush meadows once thrived, dense mats of green algae now smother what little remains. This meadow has likely crossed a tipping point, and may never recover even if we clean up the pollution.

A few miles up the coast we see a very different picture. At Castle Espie, beside a wetland reserve in Strangford Lough, a seagrass meadow is thriving. The plants here belong to the same genetic population as struggling meadows elsewhere in the lough. But the difference is that the reserve’s reedbeds and willows act as natural filters, cleaning the water that runs from the land before it reaches the sea.

The same species with the same level of marine protection, but dramatically different outcomes. The difference is what happens on land. But current monitoring methods aren’t designed to spot this kind of trouble before it’s too late.

An early warning system

Current monitoring methods tend to measure how much seagrass is still there. But by the time a meadow visibly shrinks, the damage may already be done. The tissue chemistry approach we used picks up stress signals much earlier, while there is still time to act.

The nitrogen thresholds my research identifies could give environmental agencies a practical early warning system: meadows at or above 1.8% need closer watching, and those at or above 2.8% need urgent action to reduce nutrient pollution from catchments.

Seagrass meadows can recover but only if we tackle the pollution at its source. That means better management of urban and agricultural runoff, investment in sewage treatment and recognising that marine conservation cannot stop at the high tide mark. If we lose these meadows, we lose their carbon stores, their fish nurseries, the coastal protection they provide, along with a piece of our coastal heritage.

Heidi McIlvenny receives funding from the National Environment Research Council as part of the QUADRAT DTP. DAERA and Ulster Wildlife also part funded this PhD research. Heidi McIlvenny is a member of Ulster Wildlife, RSPB and the National Trust, and Director of the Irish Ocean Literacy Network.