UMass Amherst and Massachusetts Geological Survey researcher calls work a "wake-up call" about the critical role oceans play in helping marshes keep pace with rising seas
AMHERST, Mass. — Salt marshes, those critical habitats that protect coastal towns from flooding, store massive amounts of blue carbon , support fisheries and play a key role in ecological resilience, are struggling to survive as oceans rise due to climate change. A new study led by the University of Massachusetts Amherst and the Massachusetts Geological Survey reveals a previously undiscovered source of the sediments that are key to the salt marshes' survival: the ocean. The research, published recently in Geophysical Research Letters , relies on an innovative method of working with satellite data to see a process that has been hitherto invisible and holds immediate implications for coastal management.
Salt marshes can only survive if they gain enough elevation to keep up with sea-level rise—which requires a steady supply of sediment. "Traditionally, scientists have assumed that most of the sediments for salt marshes came from rivers," says paper coauthor Brian Yellen , Massachusetts State Geologist and UMass Amherst faculty member. "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" for the scientific community pushed researchers to look at the other possible source of sediment—the ocean. But it's very difficult to get a sense of how much sediment the ocean is flushing into salt marshes.
"You can deploy a sensor in the middle of a stream and get an accurate sense of how much sediment is flowing past it," says Wenxiu Teng , lead author of the paper and a 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.
So Teng, Yellen and their colleagues looked skyward for inspiration.
For 40 years, the LANDSAT Earth-observing satellite system has been taking fairly high-resolution images of the entire planet and sending them back to us. Teng and his primary advisor, UMass Amherst Professor of Earth, Geographic, and Climate Sciences Qian Yu , have developed a sophisticated dataset that uses LANDSAT observations to make highly accurate estimates of ocean sediments, and with it, he and his co-authors looked at 103 salt marshes across the Northeast, from New York City to the Canadian border.
Not only were they able to confirm that the ocean is a significant source of sediment for much of the region's salt marshes, they also found a clear north-south divide among those marshes that were getting enough sediment to keep up with sea level rise, and those that were drowning.
Marshes within Cape Cod Bay and into northern New England are generally doing well. But marshes in southern New England are showing signs of stress.
Using historical satellite imagery showed that Southern New England coastal sediment supply has been declining year over year since 2000, and the team hypothesizes that this dwindling supply could be due to coastal management that removes sediment from the coast. Sea walls and other armoring structures prevent bluffs from crumbling onto the beach to nourish coastal sediment. Dredging mud and disposing it offshore also starves the coastal ocean of sediment. In addition, wave energy in the region has been gradually weakening, reducing the ability of waves to stir up and transport sediment that would otherwise help sustain the salt marshes.
The authors warn that if this downward trend in sediment continues, many southern New England marshes may not be able to keep pace with rising seas. They recommend that coastal planners consider sediment impacts when designing restoration, dredging or shoreline protection projects. Notably, the authors developed a web-based application called SedXplorer that allows coastal managers to view satellite-derived suspended sediment dynamics themselves anywhere on earth.
"Coastal scientists have been debating whether salt marshes really need sediment," says 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," adds Yellen.
This study was supported by the Northeast Climate Adaptation Science Center (NECASC), part of the U.S. Geological Survey.
