Rising Seas Impair Salt Marsh Growth in Response to Nitrogen Supply, Global Study Finds

People have had powerful but changing views about salt marshes over time. They were once seen as wastelands best drained for “useful” purposes (a view tinged in part by worries that their often-noxious smell indicated “bad air” and disease.) That opinion has changed as research has revealed that salt marshes are critical ecosystems that provide a long list of human and environmental services, including stabilizing coastal erosion, storing carbon, intercepting nitrogen runoff from land, supporting fish and shellfish stocks, and preserving water quality. 

Today, salt marshes are threatened by rising sea levels. Marsh vegetation cannot survive if submerged for too long.  

“The trouble with being a grass in a salt marsh is that roots and rhizomes are growing in anoxic sediment,” said Distinguished Scientist Ivan Valiela of the Marine Biological Laboratory ( �黨�ǿ���Ƶ). Openings on leaves allow grasses to take in air and pump oxygen down to subsurface tissue, he explained, but if those openings are underwater for longer periods, it means less oxygen and less growth. 

A new data analysis from the Marine Biological Laboratory ( �黨�ǿ���Ƶ) presents new evidence that accelerating sea level rise, in addition to impacting vegetation through increased submergence, may also hamper the ability of salt marsh plants to use available nitrogen—a nutrient crucial for their growth. 

Valiela and �黨�ǿ���Ƶ Research Assistant Kelsey Chenoweth, along with John Day of Louisiana State University, drew the conclusions after synthesizing data from 43 studies that added nitrogen to salt marshes around the world. Their paper is published in Science of the Total Environment (Valiela et al. 2025). 

For some time, it has been clear that nitrogen supply is ecologically important in marine systems. In the early 1970s, John Ryther and other researchers at Woods Hole Oceanographic Institution (WHOI) demonstrated that nitrogen is a key nutrient regulating biological production in coastal ecosystems. At the same time, alarms were being sounded that humans were discharging increasing quantities of nitrogen into coastal waters and wetlands due to septic system leakage, runoff from grass and agricultural fertilizers, and other effects of urbanization. 

In 1970, Valiela and WHOI scientist John Teal set out to define the effects of increased nitrogen in salt marshes, by pioneering agricultural-style experiments in which they added different doses of nitrogen to salt marsh plots. Similar experiments were subsequently done by many other researchers in salt marshes along temperate coasts of the world.  

In the 43 studies synthesized in this new paper, different researchers tested salt marsh responses to a range of nitrogen levels that broadly overlap with the ranges estimated to be entering the world’s estuaries, both through human activities and atmospheric deposition. That alignment between experimental additions and real-world conditions gives a “certain confidence that we're talking about some very generalized responses that apply to most places where there are salt marshes,” Valiela said. 

Their analysis showed that while there was variation among the different marsh sites, overall, above-ground salt marsh vegetation (leaves and stems) increased as nitrogen supply increased. Below-ground vegetation (roots and underground stems) showed no demonstrable trend. 

The data compilation also included estimates of sea level rise in each of the sites. Plotting the vegetation response to nitrogen supply against the sea level rise at each of the research sites revealed an unexpected twist.

For above-ground marsh vegetation, growth responses to nitrogen supply initially increased slightly as sea level increased, but beyond a sea level rise of 3.4 mm per year, growth responses to nitrogen supply decreased. Rising sea levels appear to diminish the ability of salt marsh vegetation to respond positively to supplies of nitrogen. 

The results imply that above-ground responses to nitrogen supply could vanish if sea level rise continues to rise, as it is forecast, and exceeds 14.7 mm per year. Assuming greenhouse gas emissions remain within levels predicted by the Intergovernmental Panel on Climate Change, average global sea level rise could reach 15 mm per year by 2100.  

Below-ground vegetation responses to nitrogen in the compiled data showed a “weak but concerning” downward trend, the authors wrote, and may disappear at about 9 mm of sea level rise per year. 

The authors encouraged review of various salt marsh restoration methods—including sediment additions, tidal gate installations, and allowing marshes to shift further inland—in hopes of countering sea level rise. These may work, but without major shifts in the human-related drivers of sea level rise, many of the world's salt marshes seem unlikely to survive into the next century, Valiela said. 

Salt marsh vegetation evolved on the coast at a time when sea level rise was low, he explained. “And now we human beings have changed that regime,” with likely substantial effects on the future of salt marshes, he said. 

* * *

Sources: 

Ivan Valiela, Kelsey Chenoweth, and John Day (2025) Meta-analysis of vegetation responses to experimental nitrogen enrichments done in salt marshes under different sea level rise regimes reveal interaction of N supply and sea level rise. Science of the Total Environment, 975, DOI:

Ivan Valiela, Kelsey Chenoweth, Javier Lloret, John Teal, Brian Howes, Dale Goehringer-Toner (2023) Salt marsh vegetation change during a half-century of experimental nutrient addition and climate-driven controls in Great Sippewissett Marsh. Science of the Total Environment, 867, DOI: