The short version
Eelgrass meadows and salt marshes trap carbon in the mud beneath them and can hold it for decades to centuries, as long as the plants stay put. EPA’s New England office and a workgroup of federal, state and university scientists made an initial regional inventory of this “blue carbon” from Maine to Long Island, New York. They mapped about 218,000 acres of eelgrass, salt marsh and salt-marsh reeds holding roughly 7.5 million metric tons of carbon in just the top foot of sediment. More than half of it is in Massachusetts, and most of it is in salt marshes.
What the report covers
The New England governors and eastern Canadian premiers called in 2017 for protecting coastal “blue carbon” reservoirs. To give states a starting baseline, EPA brought together a volunteer workgroup to combine existing maps and sediment data.
- They gathered the most recent eelgrass maps from each state and a high-resolution regional salt marsh map, filling gaps (including around Buzzards Bay and Martha’s Vineyard) with Massachusetts wetlands data.
- They pooled sediment core measurements from 189 sites (114 in salt marsh, 40 in Phragmites reeds and 35 in eelgrass) and calculated the average carbon in each habitat’s top 30 cm (about 1 foot).
- They multiplied those averages by each state’s acreage and published the habitat and “carbon heat” maps on the Northeast Ocean Data Portal.
What they found
- A big coastal carbon bank. The region has 218,222 mapped acres of blue carbon habitat: 123,177 acres of salt marsh, 63,998 of eelgrass and 31,047 of Phragmites. Together they hold an estimated 7,523,568 metric tons of carbon in the top 30 cm.
- Massachusetts holds the most. The state has about 52% of the habitat (112,453 acres) and 57% of the carbon. Massachusetts salt marsh alone accounts for 37% of all the carbon in the region.
- Marshes do the heavy lifting. Salt marsh and Phragmites sediments hold about three times as much carbon per volume as eelgrass sediments. Salt marshes store nearly 70% of the region’s blue carbon, Phragmites 18% and eelgrass 12%.
- Putting it in context. Using EPA’s greenhouse gas equivalencies calculator, the authors say the stored carbon matches the emissions from the energy use of about 3,474,000 homes for a year.
- A deliberate underestimate. Peat in these habitats can run several meters deep, but most cores only reached 30 cm. The authors stress that
the sequestered carbon estimates represent a fraction of the actual total
(Colarusso et al., 2023). The top layer still matters most, because it is the first to wash away if the plants are lost. - Marshes at risk. If marshes can’t build up fast enough to keep pace with sea-level rise, they drown and erode, and the authors warn that they can switch from storing carbon to releasing it. They call for deeper cores, better maps and more research on how marshes can move inland.
What it means for Waquoit Bay today
The report gives regional and state totals, and it helps explain what Waquoit Bay has already lost. The bay once had eelgrass meadows, and nitrogen pollution has wiped out nearly all of them (see why Waquoit Bay lost its eelgrass). According to the report, carbon stays buried under a meadow as long as the meadow stays intact, and the top layer is most at risk once the plants are gone. Eelgrass is a smaller carbon store than marsh, so its main value to the bay remains as fish and shellfish habitat. But bringing it back by cutting nitrogen would also restore a place for carbon to build up again.
Salt marshes ring Waquoit Bay and its rivers, and on Cape Cod they face rising seas and, in many places, purple marsh crabs that strip creekbanks and speed erosion (see are crabs eating away Cape Cod’s marsh creeks?). This report suggests that protecting those marshes, and leaving them room to move inland, protects the region’s largest blue carbon store as well as its habitat and flood buffers. Anyone can zoom in on Waquoit Bay on the Northeast Ocean Data Portal to see the mapped marsh and any remaining eelgrass, along with the report’s carbon heat map.
Read the report
Colarusso, P., Libohova, Z., Shumchenia, E., Eagle, M., Christian, M., Vincent, R., & Johnson, B. (2023). The blue carbon reservoirs from Maine to Long Island, NY. U.S. Environmental Protection Agency, Region 1. https://www.epa.gov/system/files/documents/2023-11/blue-carbon-report-mar-v6-508.pdf
License: This report was published by the US Environmental Protection Agency (Region 1) and carries no copyright notice. Works of US federal employees are in the public domain in the United States. The report was written by EPA, USDA and USGS scientists with partners at the Northeast Regional Ocean Council, MIT Sea Grant and Bates College, and some photos in it are credited to non-EPA photographers. The PDF below is reproduced unmodified from EPA’s website. The card image is the report’s cover (photo: Phil Colarusso, EPA).
Drafted with AI assistance from the linked source.
