Salt Marshes 

Salt marshes are one of the most productive ecosystems on the planet, and this region’s marshes are no exception. These wide expanses of natural open space provide more than a beautiful view, they are home to a diversity of wildlife, filter pollutants before they reach our bays, and provide storm protection to our coastal communities by buffering storm surge and absorbing excess rainwater. These landscapes, if healthy, also show incredible promise for slowing the effects of climate change by sequestering vast amounts of carbon — more than 10 times the amount sequestered by forests [CIT. 21].

But salt marshes are at risk due to rising seas. We must work together to ensure our marshes are resilient into the future.

Of the more than 250 miles of shoreline along Buzzards Bay and Narragansett Bay, approximately one third has salt marsh, totaling 4,900 acres [CIT. 6]. Marsh on the South Coast exists as narrow “fringe” areas along the edge of bay shorelines, as larger landscapes behind barrier beaches, and on islands. The marsh consists of both “low marsh,” areas that flood daily at a high tide, and “high marsh,” areas known for fine grasses and that flood only a few times each month. Most of the state’s existing marsh is high marsh and is critical for flood protection and habitat for at-risk species such as the salt marsh sparrow.

The survival of salt marshes depends upon their ability to flood and drain regularly, accumulate sediment over time, and migrate inland with sea level rise — natural processes that have been compromised by human alterations such as ditching for agricultural drainage and mosquito control, and roads and community infrastructure. Migration of marshes is also naturally constrained by topography, often being trapped against steep landforms or, as is the case with island marshes, simply having limited space to migrate.

South Coast salt marshes are especially vulnerable to rising sea levels. With a mean tidal range of 3.6 feet in New Bedford and 1.8 feet in Woods Hole (well below Boston at 9.5 feet which is representative of most areas north of Cape Cod) [CIT. 22], the fringe marsh that has developed in this area is much narrower than marsh in other places in the state such as the Great Marsh on the North Shore. As such, we see alarming projections for marsh loss as soon as 2050. We must act quickly to heal the hydrology of these degraded marshes and create pathways for them to migrate, or we may lose them.

WHAT THE DATA SHOWS

Data from the Sea Level Affecting Marshes Model (SLAMM) produced by the Commonwealth projects significant salt marsh transition and loss as soon as 2050 due to sea level rise [CIT. 6]. The rates of loss listed below assume marshes will be able to migrate into adjacent low-lying areas. Since this is not always possible, the projections for marsh loss could be even higher.

• Marsh is disappearing or converting much faster than other regions in Massachusetts. Projections as soon as 2050 indicate a 23% loss in total marsh (1,139 of 4,918 acres) and a nearly 80% transition of high marsh (3,537 of 4,562 acres) primarily to low marsh and estuarine beach/tidal flats [CIT. 6].

• Acute vulnerability of fringe and island marshes translates into projected large-scale transition of high marsh approximately 20 years earlier on the South Coast than is expected in the Great Marsh on the North Shore [CIT. 6].

LOOKING AHEAD

To ensure the resilience of our South Coast salt marshes, we need to think and act regionally. Prioritizing the health of these ecosystems across town borders is important not only for buffering the impacts of sea level rise and storm surge on our communities, but also for saving critical wildlife habitat.

Let’s find ways as a region to:

Conserve existing marsh and migration pathways: Strict enforcement of the Wetlands Protection Act by towns will be critical for the protection of existing marsh. Conservation of adjacent low-lying land to allow for salt marsh migration is also vital. To accomplish this, the state or individual towns could consider enacting protections on projected migration pathways. Other tools include coastal buyouts, rolling easements, and conservation restrictions.

Restore marsh hydrology: Healing ditches once used for agriculture or mosquito control and restoring natural drainage pathways will allow the marsh to build in elevation and increase resilience to sea level rise. Innovative nature-based healing techniques being implemented by The Trustees and partners include ditch remediation, a method of cutting marsh hay and securing it into ditches to trap sediment over time, and a technique called runneling that restores natural drainage pathways.

Implement innovative approaches to raise marsh elevation: While using the restoration techniques mentioned above will help salt marshes build in elevation, it is unlikely that natural accretion rates will be able to keep pace with sea level rise in the long-term [CIT. 24]. Innovative techniques can help the marsh build elevation faster including thin layer deposition and mud motors — both of which involve adding dredged sediment to the marsh. These techniques are being tested nearby in Rhode Island and neither are allowed under current regulations in Massachusetts.

Modernize the permitting process: Crossagency coordination, streamlining of restoration projects, and openness to new and innovative approaches to resilience will help partners implement these actions more effectively before salt marshes are lost.