A Bottom-up Boost for Coastal Habitat
Today we hear from Elizabeth Rogers, with the National Park Service at Fire Island National Seashore in New York State. Elizabeth spent some time this spring and summer working as a Public Affairs Specialist with the Fish and Wildlife Service, sharing the stories and science of resilient coastal communities and systems. On her days off, she can be found exploring the outdoors or dabbling in the kitchen.
The smell of the salt marsh can be overwhelming. Deep layers of mud dampened by the tides are rich in nutrients but low in oxygen. Bacteria within these layers feed on sulfate from seawater and produce the characteristic rotten-egg smell of the salt marsh. It is here, in the sometimes-smelly subsurface of the salt marsh, that Dr. Bart Wilson focuses his attention.
Salt marshes are considered “green” infrastructure. These natural habitats can help protect neighboring coastal communities by buffering against wind and waves and absorbing, then slowly releasing, floodwaters.
Salt marshes also provide habitat for fish, birds, and invertebrates and purify water by taking up nutrients that can be harmful in excess. According to Wilson, the U.S. Fish and Wildlife Service’s Northeast region coastal resiliency coordinator, marshes and the many benefits they provide are threatened by accelerated sea-level rise.
“Marsh elevation builds naturally from the bottom up,” says Wilson. “Organic material, like the roots of saltmarsh grasses, gives the surface a lift.”
To the plants and animals in tidal marshes, elevation is everything. Higher ground is home to plants and animals adapted to survive occasional flooding, and species that can withstand daily flooding by high tide live at lower elevations within the marsh. Smooth cordgrass (Spartina alterniflora) is salt-tolerant and typically grows where high marsh and low marsh meet, near the mean-high-water mark.
In addition to the bottom-up boost from below-ground plant roots, marsh elevation can slowly increase as sediments accumulate at the surface. Sediment may come from a nearby creek or river, or from the adjacent marine environment. Salt marshes tend to thrive in locations where these elevation gains are in a natural balance with elevation losses due to sea-level rise, erosion, and subsidence (the natural, gradual sinking of a land mass).
In many salt marshes, elevation loss has been outpacing elevation gain, threatening the survival of these systems. In response to this trend, nearly three decades ago, scientists began looking for a way to give salt marshes a lift.
According to Wilson, dredging sediment from nearby rivers or other waterways and spreading it onto high marsh free of vegetation builds “elevation capital.” The technique, called thin-layer deposition or TLD, helps salt marshes better withstand sea-level rise and subsidence. In some cases — where ditches or channels have been dug and have altered the way water naturally moves through the marsh, where upstream dams starve a salt marsh of sediment, or where infrastructure impedes the natural shoreward migration of marshes — restoration methods like TLD may be a way to maintain these ecosystems and preserve their many benefits.
The Service first tested TLD in the Northeast in 2003 at Blackwater National Wildlife Refuge in Maryland. This small demonstration project succeeded but left managers and scientists with more questions, including how to decrease costs while maximizing results.
In 2013, dredge material was used for the first time by the State of Delaware to restore a marsh at Pepper Creek. Not only did this effort help land managers better understand the amount of material needed to build maximum resiliency, it also demonstrated the practicality of reusing sediment from nearby dredge sites.
Following Hurricane Sandy in 2012, federal funding for recovery and resiliency offered the opportunity to begin to answer those questions. Sandy funds are supporting five TLD projects at four national wildlife refuges in the Northeast.
For Wilson, these restoration projects go beyond just testing the technique. TLD projects, he says, are about, “conserving bird, fish, invertebrate and mammal habitat; enhancing flood attenuation, carbon sequestration, and nutrient uptake; and, protecting roads, houses, and waterways.”
By restoring elevation, TLD is helping restore natural marsh function, and the multiple benefits of this gorgeous green infrastructure lining our coasts.
A small-scale study with big impact at Blackwater Refuge
For thousands of years, tidal marshes have “kept their heads above water by building up roots below the surface,” says Blackwater National Wildlife Refuge Supervisory Biologist Matt Whitbeck. Only recently has the natural ability of these systems to grow vertically not been enough. As a result, says Whitbeck, “the marsh is essentially sinking.”
The Blackwater Marsh is unique in that the causes of its decline have been well-studied. Until the Chesapeake Bay Nutria Eradication Project began in 2002, nutria, root-loving rodents introduced to the United States in the nineteenth century, hindered natural marsh build-up. Since then, erosion from storm waves and subsidence have been the primary causes of marsh loss.
This historic perspective, and information collected by scientists over the years, helped Whitbeck strategically select 40 of Blackwater Refuge’s 28,000 acres of salt marsh for a Sandy-funded TLD project managed by the Conservation Fund.
To the casual observer, the project site looks as lush and green as any other on the marsh. But Whitbeck, who has spent more than nine years monitoring the marsh, says this project was a much-needed boost for marsh threatened by rising seas and more frequent powerful storms.
“Because we knew this area was losing elevation faster than elsewhere on the marsh, we could work to protect it.” And safeguarding this relatively small site helps protect a swath of salt marsh that supports one of the densest populations of bald eagles on the East Coast.
Partnering for the future at Forsythe Refuge
Restoring marsh elevation requires resources. Securing funds, permits, and a sediment source can be a major challenge for land managers. At Edwin B. Forsythe National Wildlife Refuge in New Jersey, the Service is solving this problem by partnering with federal, state, and local agencies on three TLD projects.
Though the first project is still in the design phase, Edwin B. Forsythe National Wildlife Refuge stands to benefit from a New Jersey Department of Transportation channel-dredging project that will supply sediment for nearly 70 acres of degraded salt marsh. An environmental assessment of the project was completed in the fall of 2016 and will guide implementation, ensuring channel sediment is placed on the marsh when it is least likely to affect fish and wildlife.
The goal of the project is to “re-establish an optimum elevation for salt marsh plant growth,” says Amy Drohan, a Service biologist and lead on the project. As a bonus, Drohan says pulling the permits and plans together for this project will help speed the process in the future.
Recycling sediment to protect marshes at Rhode Island Refuges
The Service’s Hurricane Sandy resiliency coordinator, Dr. Jen White, has managed two TLD projects in Rhode Island since the storm hit in 2012. A focus of these projects has been to work with partners like The Nature Conservancy and Save the Bay to make use of available sediment to restore marsh habitat and function.
TLD, or “elevation enhancement,” as White prefers to call it, is considered a “beneficial reuse” of dredge or construction sediment. Material that would otherwise be disposed of at upland or offshore sites can instead be used to build up degraded salt marsh sites.
Elevation enhancement treatments were carried out at two Rhode Island refuges, where sediment placement helped restore saltmarsh sparrow habitat. “This is really new to Rhode Island,” said White, who is hopeful that understanding how these restoration projects perform will help the Service protect salt marshes for years to come.
Seeing how the marsh responds to restoration treatments will take time White says, but community interest was immediately apparent. Neighbors have been interested in the work and its benefits to coastal communities. Some have even offered to pitch in by planting vegetation at newly restored sites.
Though project sites can sometimes look like a sandy beach, many lie within marshes and require nimble footwork to avoid ditches and pools.
“Getting volunteers out to some of the elevation enhancement project sites can be an adventure,” says Wenley Ferguson, director of habitat restoration at Save the Bay. “But by the end of the day, they look forward to returning to see how their work helps protect this habitat.”
Nature has shaped salt marshes for thousands of years, building resilient natural systems that serve as an important stopover site for migratory shorebirds, critical nesting habitat for vulnerable species like the saltmarsh sparrow, and nursery grounds for valuable fish and shellfish. This productive habitat naturally provides additional benefits for people, like water purification, nutrient cycling, and flood protection.
Restoration techniques like TLD are giving salt marshes a fighting chance against sea-level rise and subsidence. And Hurricane Sandy Relief Aid gave the Service an opportunity to apply and study this technique.
With an improved understanding of TLD and help from partners, the Service can continue to work to ensure that salt marshes are healthy, sustaining species and making coastal communities more resilient to future storms.