Carbon Dioxide Emissions Associated with the Restoration of a Tidal Salt Marsh in Boston, MA

Decades of land alterations had led to the encroachment of the invasive Phragmites australis in the Neponset River salt marshes in Boston, Massachusetts. An 11 acre area on the west bank of the Neponset River had been underlain by dredge spoil and was several feet higher in elevation than surrounding marsh; contributing to the domination of Phragmites which occurred at high enough densities to virtually exclude native vegetation species and posed as an ecological threat to the remaining marshlands. In 2005, restoration of this section involved excavating approximately 46,700 cubic yards of dredged materials; effectively lowering the marsh platform by 1.5 feet to reestablish tidal flushing. The removed materials were relocated to an area deemed unlikely for future restoration efforts on the northern portion of the site, containing relatively high elevations from previous dredge spoil deposits and dense strands of Phragmites. The mitigation has been considered successful as seawater inundation has promoted the replacement of the Phragmites with native Spartina alterniflora.

The excavation and relocation of dredge materials exposed previously buried marsh sediments to the atmosphere. Our research study focuses on determining how much carbon dioxide (CO₂) may have been released due to the disturbance of this sequestered carbon. Ten years after the restoration, in 2015, direct measurements of CO₂ fluxes from the soils in the remediated site, an unrestored area, and the dredge spoils reveal differing CO₂ emission rates between the three sites, measuring at 1.54 ± 0.70 μmol/m²/s, 5.48 ± 2.68 μmol/m²/s, 9.57 ± 2.09 μmol/m²/s respectively. Our measurements suggest that the restoration has resulted in a significant release of previously sequestered carbon to the atmosphere. Estimations of potential emissions and avoided emissions resulting from coastal restoration projects are necessary in evaluating mitigation policies and practices and managing conservation efforts of these essential ecosystems.