SALT MARSHES
With wide open views, natural beauty, and diverse wildlife, salt marshes are one of the most productive ecosystems on the planet. Their grasses, flooded and drained by tides, accumulate captured sediment and decomposing plant matter to form peat and provide growing space for roots, allowing marsh to naturally build. They also protect our shorelines from storms and flooding, provide habitat, and help preserve water quality.
The Islands have nearly 1,800 acres of marsh [CIT. 2] that sea level rise threatens to outpace (only Gosnold appear to be spared significant marsh loss). Unlike large marshes found on the North Shore or Cape Cod, most of these island marshes fringe estuarine areas. Some marshes may have a chance to migrate landward, and survive, where the absence of built barriers and natural topography allow. It will take concerted strategizing on an island-wide basis to determine where the best chances of restoration and protection reside. We must also educate Islanders and visitors about the importance of salt marshes to the ecosystem as a whole if we are to get everyone onboard with the value-based choices that must be made.
WHAT THE DATA SHOWS
LOSS AND GROWH OF MARSHES Based on state data, significant marsh loss may occur on the Islands by 2050 due to sea level rise [CIT. 2]. At the same time, some locations will experience new marsh growth or expansion because with rising sea level, marsh will be able to expand to area that was previously dry land.
Overall, total marsh loss on Martha’s Vineyard may exceed 266 acres [CIT. 2] — about one-third the area of Central Park in New York City.
On Nantucket, 568 acres (66%) of high marsh may be lost [CIT. 2].
At the same time, more than 600 acres of new marsh growth or migration could occur where land is undeveloped, particularly in Nantucket (438 acres) and Edgartown (89 acres).
While high marsh could recede in many areas, we may see a dramatic expansion of other coastal areas. This is good news for new shellfish habitat, even as the Islands benefit from the expansion of aquaculture.
On Martha’s Vineyard, estuarine open water may increase by 152 acres (2%) by 2050, while regularly flooded marsh may increase by 188 acres (413%). Estuarine beaches and tidal flats may increase by 240 acres (51%) in some locations [CIT. 2].
On Nantucket, estuarine open water may increase by 108 acres (2%) by 2050, while regularly flooded marsh may increase by 518.5 acres (1,066%). Estuarine beach and tidal flats may increase by 225 acres (91%) in some locations [CIT. 2].
ABOVE: Salt marshes depend on their ability to migrate with rising sea level. Some marshes may be trapped against steep landforms and drown over time, while others may have the space and time to migrate to new areas. Migration includes loss of low marsh, conversion of high marsh to low marsh, and migration of high marsh (given no barriers) as other low-lying areas experience increased or new tidal inundation. The current salt marsh system (blue) shown here in Madaket is projected to be fairly resilient to sea level rise through 2050, with some near-term losses of low marsh by 2030 (dark blue) offset by later expansion (light green). Eventually, however, accelerating sea level rise could result in a wholesale conversion of marsh to open water here in Madaket (light green), with only a modest potential for inland migration by 2070 (dark green).
LOOKING AHEAD
If we are to enhance the resiliency of salt marsh on the Islands, it requires thinking more holistically beyond specific town interests and making trade-offs to ensure that we focus on the places with the best chances for success. With limited time, resources, and funding, we need to act wisely and strategically together.
CONSERVING SALT MARSH MIGRATION PATHWAYS If new salt marsh is to migrate onto higher ground, land must be permanently and sustainably protected from development. Tools can include coastal buyouts, rolling easements, conservation and deed restrictions, and in-lieu fee programs once we identify what is protected and what is not. Examples include:
The Martha’s Vineyard Commission, Town of Oak Bluffs, and Martha’s Vineyard Land Bank are considering using a District of Critical Planning Concern to protect future salt marsh migration areas around Sengekontacket Pond.
Elsewhere in New England, Rhode Island’s Salt Marsh Conservation Project involves 1) reconciling maps showing where salt marshes will migrate landward against town data to identify the most important unprotected parcels that need to be conserved and 2) communicating these critical conservation opportunities to communities and land trusts. The project is led by the University of Rhode Island and funded using Volkswagen settlement funds awarded by the state Attorney General’s office.
RESTORE EXISTING MARSHES FOR RESILIENCY AT-SCALE This involves relocating structures, where possible, and removing or adapting tidal restrictions to improve tidal flow and help marsh keep pace with sea level rise. Low-risk, nature-based techniques include ditch remediation, runneling, assisted migration, and relocation of structures. Examples are:
Nantucket Conservation Foundation restored Medouie Creek, a high priority wetland restoration site. They installed a culvert under a lowered dike road and dredged an existing tidal creek and old ditch, allowing daily tides back into the wetland and restoring an historic salt marsh to its natural function.
Mass Audubon, Oak Bluffs, and Edgartown shellfish departments, the University of Rhode Island, and the EPA’s Atlantic Ecology Division created a living shoreline to restore a section of salt marsh habitat at Sengekontacket Pond in 2016. The project used 100% biodegradable materials including coir logs made of coconut fibers and bags of local oyster and quahog shells to dissipate wave action and reduce erosion, allowing the shoreline to establish.