Salt marshes, vital coastal habitats that buffer towns from flooding, store “blue carbon,” and support fisheries, are under increasing threat as sea levels rise. Yet, a new study led by the University of Massachusetts Amherst and the Massachusetts Geological Survey has uncovered a previously unrecognised source of the sediments essential to salt marsh survival. And it’s the ocean.

Published in Geophysical Research Letters, the research leverages innovative satellite-based methods to reveal a process that has been largely invisible – with immediate implications for coastal management.

Salt marshes can only survive if they gain enough elevation to keep pace with rising seas, a process that depends on a steady sediment supply. “Traditionally, scientists have assumed that most of the sediments for salt marshes came from rivers,” the paper’s co-author Brian Yellen, Massachusetts State Geologist and UMass Amherst faculty member said in a release.

“That assumption makes a lot of sense, but members of our team have previously shown that there’s not enough sediment in the rivers to keep marshes viable.”

What Yellen describes as a “wake-up call” led researchers to investigate another source: the ocean. Measuring sediment contributions from the coast, however, is challenging.

“You can deploy a sensor in the middle of a stream and get an accurate sense of how much sediment is flowing past it,” said Wenxiu Teng, lead author and postdoctoral researcher at UMass Amherst. “But along the coast, sediment is moved by waves, tides and storms across a wide, shifting area, making it nearly impossible to maintain enough instruments to measure those changes directly.”

To overcome this challenge, Teng and colleagues turned to satellite data. Using 40 years of observations from the LANDSAT Earth-observing system, Teng and his advisor, UMass Amherst Professor Qian Yu, developed a sophisticated dataset to estimate suspended ocean sediments.

Applying this dataset, the team examined 103 salt marshes along the Northeast coast, from New York City to the Canadian border.

Their analysis confirmed that the ocean provides a significant sediment supply for many regional marshes. It also revealed a stark north-south divide in marsh resilience: marshes from Cape Cod Bay into northern New England are generally keeping pace with rising seas, while those in southern New England show clear signs of stress.

Historical satellite imagery revealed that sediment supply along the southern coast has declined steadily since 2000. The researchers attribute this trend to human activities such as shoreline armouring, which prevents bluffs from eroding naturally, and offshore dredging, which removes sediment from the nearshore environment. Weakening wave energy in the region further reduces the ocean’s ability to transport sediment to marshes.

The authors warn that if sediment inputs continue to decline, many southern New England marshes may struggle to survive rising seas. They recommend that coastal managers consider sediment dynamics in restoration, dredging, and shoreline protection projects. To aid in this effort, the team developed SedXplorer, a web-based application allowing managers to monitor satellite-derived suspended sediment patterns worldwide.

“Coastal scientists have been debating whether salt marshes really need sediment,” said Teng. “This study adds one more piece of evidence that sediment is really important, and that a lot comes from the ocean in this region where rivers tend to run clear.”

“It also shows what a precious resource sediment is for maintaining coastal resilience in New England,” added Yellen.

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