In response to increasing sea-level rise, a group of nonprofits and public land owners are working to restore about 200 acres of vulnerable salt marsh on Chappaquiddick.

Alejandra Narvaez, Coastal Project Manager for the Trustees of Reservations, recently met with the Martha’s Vineyard Water Alliance to explain the project, which involves a partnership among the Trustees, Sheriff’s Meadow Foundation, Martha’s Vineyard Land Bank, and MA Division of Marine Fisheries – which together own about 80% of the salt marsh around Cape Pogue Bay and Pocha Pond – with additional support from BiodiversityWorks, the Martha’s Vineyard Shellfish Group, and the MVC.

The project area is one of the Island’s largest contiguous salt marshes, which provide key ecosystem services such as nesting and nursery habitat, water filtering, protection from storm surges, and carbon sequestration. In the past, salt marsh elevation has changed along with sea level as sediment and organic material are deposited, but this natural process will likely not keep pace with projected sea-level rise through 2100. Along with erosion, this threatens to transform the Island’s high-marsh habitat into tidal flats with fewer ecosystem services.

Many salt marshes in the region remain altered by agriculture in the 17th-19th centuries, when farmers would dig channels to lower the water level enough to harvest salt marsh hay. Additional ditches were later dug in some places to reduce mosquito breeding. One result has been increased waterlogging and subsidence around the ditches as they heal, preventing tidal water and sediment from traveling as far and evenly into the marsh as they once did.

The Trustees and others have been working on salt marsh pilot projects along the Massachusetts North Shore for about 10 years, with promising results. Adaptive strategies to restore the marsh hydrology on Chappy include gradually filling in the ditches, digging new ones where necessary, and other nature-based solutions.

One technique involves mowing hay from alongside the ditches, then securing it inside the ditches with stakes and twine. Over a period of 3-5 years, this can help them fill in as the hay catches the sediment. “That brings up the water table, increases the amount of vegetation that’s growing within the ditches and also allows for more optimal sediment transport evenly throughout the surface of the marsh,” Alejandra said, noting that the method has been highly successful on the North Shore.

Another technique is runneling, or carefully digging new ditches to reduce areas of waterlogging and subsidence where vegetation has died off. “What we see is that channelizing the flow alone allows native marsh vegetation to grow in and around those runnels really quickly, within two or fewer growing seasons,” Alejandra said.

Material excavated for the runnels can be used for marsh mounds, also known as nesting islands, which revegetate and provide potential nesting grounds for the salt marsh sparrow, which is listed as a species of concern in Massachusetts. 

Deeper runneling near the marsh edges where invasive phragmites have taken hold could be used to increase salinity from tidal water and reduce the phragmites, which prefer freshwater. “We haven’t seen this implemented in Massachusetts yet, so it will be a test as to whether we can permit it,” Alejandra said. However, she noted that it has reduced phragmites by almost 80% at sites in Maine and New Hampshire.

The Trustees recently received funding through the Southeast New England Program (SNEP) for final design and permitting, which it hopes to begin this spring and summer. Depending on additional funding, implementation could take place over the next 2-3 years. A potential second phase of the project would focus on protecting the marsh edges from erosion, including the possibility of oyster reefs, and living shoreline projects such as the one at Sengekontacket Pond in Oak Bluffs and Edgartown. The partnership also anticipates approaching private property owners for support restoring the parts of the salt marsh that are not on protected land.

MVC Water Resources Planner Sheri Caseau, who facilitates the monthly Water Alliance meetings, welcomed the partnership’s work, especially in light of the MVC salt marsh monitoring program, which began in 2016 and has shown that salt marsh elevations in Aquinnah and Edgartown will likely not keep pace with sea-level rise.

“There are a couple of our sites that are just disappearing,” she said. “We had to get longer stakes because the elevations were changing so much. So this is great work. I’d like to see it spread all over the Island.”

A video of the Water Alliance meeting is available here.
Recent coverage in the MV Times.
More on salt marsh migration in the Vineyard Climate Action Plan.
For more info, contact Sheri Caseau at caseau@mvcommission.org. 

Photo: Salt marsh at Pimpneymouse Farm, 2011, by David Foster.