Sinking in Saltwater: Maine’s coastal marshes at risk as sea levels rise

This story is part of a series on marshes in Maine co-published with the Portland Press Herald.

PORTLAND — It takes hundreds of years for a salt marsh to form, for fine sediment brought in on the tides to settle in sections of shoreline sheltered from the worst of the wind and waves. As salt-tolerant plants — smooth cordgrass, saltmarsh hay, saltgrass, black rush — begin to grow, their dense stems and roots trap more sediment, and the marsh builds more rapidly, up and out.

Crabs, shrimp and worms arrive, drawn to the rich food of dying marsh grasses, followed by a variety of fish — alewives, striped bass, smelt and Sea-run brook trout among them — many of which eventually migrate between the marsh and the sea.

Acre by acre, a healthy salt marsh anchors a food web “ more productive than most midwestern farmland,” according to a 2003 paper published by the University of Maine.

The same dense grasses that are so good at trapping silt also excel at ensnaring pollutants, pulling out nitrogen and nutrients that cause algal blooms, and burying toxic contaminants in the peat.

Once established, plants in salt marshes grow quickly, fed by the rich soil, and pull carbon from the atmosphere. Salt marshes are ten times more effective at storing carbon than tropical forests, and, left undisturbed, can trap the gas in the ground for centuries, a phenomenon scientists refer to as “blue carbon.”

Maine has some of the most extensive blue carbon reservoirs in the northeast — second only to Massachusetts, according to a study published by the Environmental Protection Agency in 2023.

But as sea levels rise and development presses in, these reservoirs, and the habitats they create, are at risk of disappearing.

An analysis by the University of Maine suggests that a significant portion of the Maine’s salt marshes — between 28 and 57 percent, depending on the sea level rise scenario — could be gone by the end of the century . They are also threatened by polluted runoff from pesticides, septic systems and agricultural waste.

“The decisions Mainers make over the next 10 years are going to determine whether these important ecosystems persist,” said Bates professor Beverly Johnson, who has been studying blue carbon for years, speaking to The Maine Climate Council in December.

Over the past 25 years, nearly 300 acres of Maine’s wetlands — both fresh and saltwater — have been impacted by or lost to development, according to a Press Herald/Maine Monitor analysis of data from the state’s In Lieu Fee Compensation Program. The program allows developers to fill or convert certain wetlands if they pay a fee, money that is used for conservation projects elsewhere.

In 2022, the latest year available, residential and commercial development accounted for the greatest impact on wetlands (59 percent), followed by energy supply and transmission (36 percent) for those projects that required permits and utilized the In Lieu Fee Program for their mitigation.

Developers have paid $33.7 million in fees to the program since 2008, according to state records, ranging from just a few cents per square foot for a project that removed vegetation in freshwater wetlands in Jackman in 2020 to fees in the $9-10 per square foot range for impacts in the southern Maine counties and to higher value wetlands.

Wetland loss may be “difficult to reverse”

For much of American history, the marsh has often been considered more of an impediment than an asset; something to be filled, ditched, dug and bulldozed into something more useful.

More than half of the wetlands that existed at the start of the Revolutionary War are gone, according to estimates from the U.S. Fish & Wildlife Service — much of them altered by farming, but also lost to houses, strip malls, marinas and other development.

The pace at which the country’s wetlands are disappearing has also accelerated, according to a study published by USFWS this spring.

Wetlands — 95 percent of which are freshwater — covered less than 6 percent of the lower 48 states as of 2019. The country lost roughly 70,000 acres of salt marsh between 2009 and 2019, impacts that the authors of the report note “are cumulative over space and time and may be difficult to reverse.”

Maine’s marshes are better off than most of the state’s New England neighbors, where development in and around wetlands ran rampant, particularly in the 1970s and 80s. Fully a quarter of Maine’s land — more than five million acres — remains wetland. That’s four times the combined area of the five other New England states.

While only 157,500 acres of those 5 million acres are tidal (tidal flats, salt marsh, brackish marsh, aquatic beds, beach bars and reefs), the state has also retained large stretches of salt marsh and sea grasses, which excel at trapping carbon.

Maine has almost 54,000 acres of salt marshes and sea grasses, or a quarter of the overall habitat in the northeast, and the most eel grass of any of the states surveyed in the EPA study, about 34 percent of the regional total.

A long history of human intervention

The wetlands that haven’t disappeared entirely in many cases have been altered so significantly by humans that they have become nearly unrecognizable (and in some cases, inhospitable) to the plants and animals that once lived in them.

In Maine, several experts could not think of a single sizable salt marsh that has not been filled in or ditched at some point over the centuries.

One of the most sweeping wetland alteration projects began during the Great Depression, when, in an effort both to control the mosquito population and put the unemployed to work, the federal government paid citizens to dig trenches in marshes from Maine to Virginia.

By 1940, an estimated 90 percent of the East Coast’s wetlands had been crisscrossed with a checkerboard of channels that allowed water to drain out more fully at low tide, leaving fewer pools in which mosquitoes could breed.

Long before this effort began, however, farmers were using the state’s coastal marshes to harvest salt hay, a natural marsh grass that was far easier to cultivate than painstakingly clearing forests to grow crops.

“We’re looking for evidence of agriculture, and we're finding it everywhere,” said Geoff Wilson, a marsh restoration specialist who has been working to rebuild marshes on the east coast for decades. “There’s definitely no place in Maine and there's no place on the east coast that we've found so far that hasn’t been impacted.”

In an unaltered salt marsh, seawater flows in and out through fissures resembling the branches of a mature oak — sprawling, curvy and tapered at the ends. Some parts of the marsh (the mud flats and other low-lying areas) flood twice each day, while others (the higher, grassier areas) are only occasionally truly flooded, during the highest tides and storms. The low marsh is salty; the high marsh, less so. Most plants, animals and insects are adapted to thrive in one area or the other.

Venture into your nearest salt marsh and you are likely to see the evidence of human interventions across the centuries: unnaturally straight channels, pooling water and long, lumpy berms underfoot, where soil from dug trenches was deposited to create embankments as water was directed one way or another.

Changing the way water flows in a marsh disrupts the fundamental nature of the marsh itself: where water once flowed, it pools; where it once lingered, it flows.

Eventually, the edges of unmaintained ditches crumble, clogging channels. Fish blocked from swimming up marsh are easy prey for the herons and egrets that follow them. Mosquitos thrive in stagnant pools, and invasive species take root in the newly oxygen-rich soils.

Marshes that are drained or full of pooled water because of ditch clogs cannot protect the shore beyond them from the sea — one of their functions most of interest to humans. Unaltered marshes act as sponges, absorbing and blunting the impact of wind and waves. Wetlands that are already saturated — often because collapsed channels prevent them from fully draining — lose some of that buffering capacity.

Development in marshes affects their ability to function or adjust to ri s ing seas . But even development in their vicinity can have an impact, polluting marshes with runoff that alters their chemistry or cutting them off from vital sediment deposits.

Fewer than 4 percent of Maine’s wetlands are tidal, and many are ringed by homes and roads, with little place to migrate as the sea encroaches — a phenomenon researchers refer to as “coastal squeeze.”

“We have a lot of marshes in Maine that either have roads across them or other structures that have limited tidal flow to the marsh,” said Jeremy Gabrielson, senior conservation planner at Maine Coast Heritage Trust. That means plants aren’t able to trap sediment and grow the marsh floor.

Sections of a marsh in Machias that have been impeded by a dike for more than a century stand 3½ feet lower than nearby tidal wetlands that haven’t been blocked, said Gabrielson. “Many of the marshes that we have now,” he said, “are actually starting at an elevation deficit.”

As the ocean has slowly risen over thousands of years, plants in marshes mostly have been able to keep up, trapping silt on the incoming tides and slowly shifting landward.

But if the sea rises faster than marshes can build, or if upland obstacles like roads or homes or rocks prevent them from migrating, said Gabrielson, “the plants that are in those tidal marshes drown.”

Over half a century, from the mid-1960s to 2016, the Atlantic Ocean crept up about 6 inches on Maine’s coast. Scientists expect the same amount of rise between 2017 and 2033, a pace drastically accelerated by melting glaciers and warming waters.

“The real question is, are [marshes] going to be able to keep up with the amount of sea level rise that we're expecting to see over the next 50 or 100 years?” asked Gabrielson. “The jury's out on that.”

Disclosure: Kate Cough’s husband, Caleb Jackson, works with Maine Coast Heritage Trust. He is not involved in marsh restoration.

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