Maine’s salt marshes play key role in fight against climate change, new report says

Beverly Johnson, a professor at Bates College, carries scientific equipment onto a marsh at Bates-Morse Mountain Conservation Area in Phippsburg to measure carbon dioxide and methane levels. Ben McCanna/Staff Photographer

With a 40-pound gas analyzer strapped to her back, Professor Beverly Johnson bounded nimbly through Sprague Marsh in Phippsburg on Friday, stopping here and there to measure the carbon storage capacity of the rich, spongy soil and swaying grasses beneath her feet.

The Bates College earth and climate science professor places the portable survey chamber on flagged test sites throughout the marsh to measure how much carbon dioxide is soaked up by the soil and vegetation. A healthy marsh is one of the most effective carbon sinks on the planet, Johnson said.

“Salt marshes are such beautiful, special places, but we are losing them fast,” Johnson said while waiting for a readout. “Development, runoff, and now sea level rise, they’re the reason why we’ve lost half of the seagrass habitat over the last century. We have to act fast if we want to save them.”

Maine has at least 54,000 acres, or 84 square miles, of these so-called blue carbon reservoirs: salt marsh, eelgrass beds, and even phragmites that store at least 1.7 million tons of carbon in the soil and vegetation. That much carbon is equal to the annual carbon emissions of 1.25 million passenger cars.

The Maine projections are part of a new first-of-its-kind inventory of New England’s coastal blue carbon reservoirs from the U.S. Environmental Protection Agency. Johnson not only helped to write this report; she and her Bates students also collected most of the Maine samples used to back it up.

Marshes can slow floodwaters, protect against erosion, improve water quality, and provide fish and bird habitat, but a healthy marsh can also absorb and store up to 10 times as much carbon as a forest, making them an invaluable buffer against climate change, Johnson said.

Maine is home to a quarter of New England and New York’s coastal blue carbon habitat, second only to Massachusetts’ 112,000 acres. The region’s 218,000 acres can store and hold at least 7.5 million tons of carbon, which is the emissions equivalent of powering 3.2 million homes for a year.

It works like this: The atmospheric carbon absorbed by salt hay or eelgrass beds during photosynthesis is stored in the leaves, stems, roots, and rhizomes. When the vegetation dies, the organic matter, or carbon, remains trapped within the marsh or, in the case of eelgrass, in the protected sediment underneath.

Beverly Johnson, left, a professor at Bates College, and Claire Enterline, a research associate at Gulf of Maine Research Institute, measure carbon dioxide and methane levels in a marsh at Bates-Morse Mountain Conservation Area in Phippsburg. Johnson was a lead author of a new report that maps out New England’s coastal blue carbon reservoirs, which are marshlands that store carbon. Ben McCanna/Staff Photographer

Since our carbon output, such as tailpipe and factory emissions, contributes to climate change, anything that soaks up carbon dioxide helps to offset what we put out into the world. Blue carbon reservoirs, such as marshes, and green carbon sinks, such as forests, slow climate change without doing a thing.

A struggling marsh – one cut off from the natural ebb and flow of the tide by a road, culvert, or irrigation ditch dug by a long-ago hay farmer – can’t store as much carbon. A degraded marsh may even turn into a greenhouse gas producer by emitting methane, a huge contributor to global warming.

The report relied on satellite data and existing maps to locate and quantify the size of the coastal carbon habitat and applied average storage values to each acre based on vegetation type: eelgrass, salt marsh, or phragmites. For example, salt marshes store more carbon than eelgrass beds or meadows.

Johnson hopes the report will help guide the restoration and preservation of the blue carbon reservoirs. She wants to see their full carbon storage value included in the state’s biennial carbon report, just like it is for Maine’s forests, to emphasize blue carbon’s role in helping Maine reach carbon neutrality by 2045.

It is likely a subject that Johnson, a member of the Maine Climate Council’s coastal and marine working group, will raise as the council starts its work this fall on its next edition of Maine Won’t Wait, the state’s climate action plan.

Her work is going faster thanks to the gas analyzer, a gift from Maine Sea Grant. It allows Johnson to do it in three minutes which used to take six hours. She is focusing on four marshes: Sprague Marsh, Spurwink Marsh in Cape Elizabeth, Jones Creek Marsh in Scarborough, and Drakes Island Marsh in Wells.

Johnson continues to gather her greenhouse gas emissions data from Maine’s marshes, fine-tuning and updating the rates of carbon absorption and methane emission rates to see how they vary from marsh to marsh, how they change over time, and how they are impacted by degradation.

For example, a gravel road through the Bates-Morse Mountain Conservation Area hinders the tidal flow into part of Sprague Marsh, making it drier than the part that has direct access to the ocean and causing it to sink. This still relatively healthy area stores less carbon and emits more methane.

The Sprague River meanders through a marsh at Bates-Morse Mountain Conservation Area in Phippsburg. Ben McCanna/Staff Photographer

Johnson points to Back River Marsh in Woolwich – which is visible to anybody driving by Taste of Maine who isn’t looking at the Route 1 eatery’s gigantic rooftop lobster – as an extreme example. A concrete box culvert has left one side of the marsh emitting methane readings too high to measure, she said.

Back River Marsh kicks off a lively debate among that day’s research crew over the pace of restoration.

“We spend so much time talking about saving the marshes, but we’re not doing nearly enough,” Johnson said as the analyzer chugged along. She pointed out to sea, then back at the cut-off marsh. “Sea level rise is happening, accelerating even, and we’re still talking. We’ll lose them all if we don’t get going.”

Maine has already experienced 8 inches of sea level rise. In the future, the Maine Climate Council is predicting another 1.5 feet of sea level rise by 2050 and 4 more feet by 2100. For context, 1 foot of sea level rise alone increases nuisance flooding over 15 times.

Restoring the health of the inland section of Sprague Marsh could probably be achieved by enlarging the culvert under the road and adding some sediment to lift the marsh, said Slade Moore, who runs a project that helps improve tidal road crossings in Maine called CoastWise.

“You’ve got to be careful about restoration, plan it carefully, or you end up doing more harm than good,” said Moore as he watches Johnson show the preserve’s caretaker, University of Maine graduate Amanda Ives, how to measure the salinity of the test site. “We want to do it quickly, but we want to do it right.”

Johnson nods and mutters “ditch plug,” eliciting a grimace from Moore. For a short time, the U.S. Fish and Wildlife Service encouraged people to push earthen plugs into wetland ditches to stop the drainage and create more wetlands but wound up drowning existing wetlands instead.

CoastWise has created a set of science-based best practices to improve Maine’s 800 tidal road crossings, mapped out in Maine Coastal Program’s new Tidal Restriction Atlas. Most of these crossings restrict the natural tidal flow, thus hindering the marsh from achieving its full carbon storage potential.

More complex and costly mitigation efforts are underway in Woolwich, where the state is raising Route 1 and the town is likely to remove the culvert and even dead-end a local road to prepare for future sea level rise and to restore Back River Marsh’s natural tidal flow and its carbon storage capacity.

“You want the same thing,” said Claire Enterline of the Gulf of Maine Research Institute. “Done fast, done right.” Enterline recently left the Maine Coastal Program to work with the GMRI.