ROWLEY — Navigating an unusually high tide through the submerged channels of the Great Marsh, Hillary Sullivan cut the engine and let her skiff drift onto a mud bank. She plopped into the mire, sloshing shin-deep in the brackish water, and pointed to an increasing threat.
There, a few feet from where she stood in the muck, a gouge had formed where a mud bank gave way, a telltale sign of how rising sea levels and pollution are slowly but steadily eroding one of New England’s most vital coastal ecosystems.
“We’re seeing a lot more of this slumping and cracking of the mud, which destroys the stability of the marsh,” said Sullivan, a research scientist at the Woods Hole Research Center who has spent years observing the changes in the Great Marsh, which stretches from the Crane Wildlife Refuge in Ipswich and Essex north to Newburyport and into New Hampshire, the longest continuous coastal wetlands in New England.
“We need to save these vital ecosystems before it’s too late.”
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Wetlands such as these are also crucial buffers against the damaging effects of rising sea levels from climate change. Yet the very forces unleashed by global warming are pounding away at the Great Marsh and other saltwater wetlands: higher tides — more than 8 inches here over the past century — and a 20 percent increase in precipitation over roughly the same period.
Fueled by more powerful storms, such as the succession of nor’easters that pummeled the region in recent winters, rising tides have intensified coastal erosion and threatened beachside properties. On Plum Island, a thin, 11-mile-long barrier of sand, a series of winter storms in 2013 destroyed six homes and damaged dozens of others. More storms last year washed away dunes, flooded 20 homes, and left about 10 others at risk of falling into the ocean.
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Moreover, the waters from the coast out into the Gulf of Maine have been warming faster than nearly any other body of water on the planet. That, combined with runoff from septic systems and other human-made pollution, has led to sharp declines of native species and the rise of invasive animals and plants, such as green crabs and reeds known as phragmites.
As a result, the native cord grass that pokes through the shallow water is becoming more top-heavy as its roots weaken, Sullivan’s team has found.
In concentrated areas where the team has simulated the changes it expects to come from further coastal development, it has measured a 40 percent decline in the submerged roots of the cord grass. Those changes are destabilizing the banks, making it harder for other species to survive. For example, the population of Mummichog, a small killifish that’s an important source of food for flounder and other larger fish, has fallen by half in recent years.
“The population crashed after the mud in the creek started to fracture,” said Justin Lesser, a PhD student working with Sullivan. “They lost access to where they would feed.”
Other salt marshes in New England, including those on outer Cape Cod and in Narragansett Bay, are showing similar signs of distress, undermining their ability to protect more developed coastal areas from rising sea levels and increasing precipitation that scientists expect to only accelerate in the coming decades. A 2018 report from the state government about the effect of climate change on Massachusetts projected seas rising by as much as 10.5 feet and annual precipitation increasing up to 16 percent by the end of the century.
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“A healthy marsh is intricately linked to the health of our coastal waters,” said Robert Buchsbaum, a staff scientist at Mass Audubon who has spent years studying the Great Marsh and other wetlands.
Salt marshes not only provide a buffer against coastal storms, they serve as a filter for pollutants, a nursery for juvenile fish, and a habitat for shellfish, migratory birds, and other species. But around the world, salt marshes have been transformed by rising seas and increasing coastal development; Massachusetts, for example, has lost about half of its marshland to dredging and fill of wetlands and other human-made changes to the environment.
Seawalls, which now line more than a quarter of the state’s 1,500-mile shoreline, also make it harder for salt marshes to cope with rising seas. Without the impediments, the marshes would naturally migrate further inland. Instead, they are increasingly submerged all the time, upsetting the delicate balance between the tidal cycle of drying and inundation, scientists say.
“The result [of these changes] will be a further loss of wetlands, with devastating impacts for fisheries,” said Jack Clarke, a spokesman for Mass Audubon and a member of the state’s Special Commission on Coastal Erosion. “Upland areas will also suffer, as the ability of marshes to absorb stormwater as natural sponges will be severely diminished.”
The Great Marsh has actually fared better than similar coastal wetlands because of limits on development in the area.
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Still, the most visible effect of the changes occurring here may be in the amount of water that lingers after high tides and storms, known as “marsh pooling.”
This has degraded the mud banks, where much of the ecosystem thrives, and has hurt plant life that needs time to dry out and birds that nest in drier areas. The pooling is exacerbated by human-made barriers to the natural ebb and flow of water, such as dams, roads, and culverts.
“The Great Marsh is the coastal jewel of New England, and we have an opportunity to get out in front of the impacts and keep it from being degraded, as have other systems in the country,” said Chris Hilke, senior manager of climate adaptation and resilience at the National Wildlife Federation, which has received millions of dollars in federal grants to study and help protect the Great Marsh from climate change.
For longtime residents, such as Michael Morris, who owned a cottage on Plum Island for more than a decade, the changes to the Great Marsh are far from abstract.
After the onslaught of storms in 2013, he and his neighbors formed a group called Storm Surge to urge state and federal officials to take the threats of climate change more seriously and help residents adapt.
A few years later, with seas rising faster, he decided it was time to sell his two-bedroom cottage — at a loss.
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“When there’s a disaster like what we experienced, property values plummet,” said Morris, chairman of Storm Surge.
As he has watched the landscape change in recent years, with phragmites replacing the natural grass and tunnelling crabs weakening the mud banks, he worries about the future.
“We’re just on the cusp of things about to happen,” he said.
David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.