Retreating From the Rising Sea: Where Will Salt Marshes Go?

Rhode Island is losing its salt marshes, along with vital fisheries and wildlife habitat, as well as water quality and erosion control services.

More than 4,000 acres, or more than half, of the state’s salt marshes, have been lost due to development since the early 19th century. Increasing pressures from sea level rise are further altering the landscape as marshes struggle to seek higher ground.

“We’re beginning to see a very rapid shift in zonation,” said Caitlin Chaffee, coastal policy analyst for the R.I. Coastal Resources Management Council (CRMC) at the Coastweeks Narrow River Tour, explaining vegetative shifts being observed along the Narrow River and elsewhere in the state. “We’re thinking that’s related to a few factors tied to accelerated sea level rise.”

marsh_zone

Tidal marsh zonation. Courtesy of USGS.

Salt marshes are the transition areas between land and sea, and are broken down into “high marsh” and “low marsh” zones to describe plant species that have adapted to varying levels of salinity, flooding from tides, temperature, and oxygen. In a healthy “high marsh” zone, bushes and shrub species grow, as well as different types of marsh grasses, such as salt hay grass (Spartina patens), spike grass (Distichlis spicata), black grass (Juncus geradii), and the short form of Spartina alterniflora. Closer to the water, the “low marsh” zone is dominated by the tall form of smooth cordgrass (Spartina alterniflora) because it can withstand higher salinity levels and daily flooding at every tide. “

We’re seeing a flip in zonation,” said Chaffee, explaining that plant species that would normally be near the water are now being found within the marsh interior and “high marsh” zones and vice versa.

“We’re seeing a lot of subsidence and ponded water in the interior of the marsh, and where there once was these meadows of Spartina patens, we’re getting short form alterniflora, which you would typically see in the daily flooded zone,” she said. “And on the marsh borders where you’d have a little higher elevation because the interior is subsiding, sometimes we’re seeing a very thin strip of Spartina patens. So this limited high marsh zone is right next to the eroding edge of the marsh, which is contrary to what we expect to find.”

These pools and salt pannes, which are typical in the interior of the marsh, are either expanding or multiplying, according to Chaffee, because they’re not draining off at low tide. This super saline environment kills off surrounding vegetation, creating a negative feedback loop that allows these pools to expand and further degrade vegetation.

Caitlin Chaffee, CRMC, talks about how standing ponds can hurt surrounding marsh vegetation if it can't be drained.

Caitlin Chaffee, CRMC, explains how lack of drainage can hurt marsh vegetation.

“What we think is happening, is that with the increase in sea level, areas are getting flooded more frequently, and water is not draining properly,” said Chaffee. “We’re seeing either existing pools expand, or new ones form, where vegetation has completely died off, and they’re not draining off at low tide … Water on the marsh is really the problem.”

The Sea Level Affecting Marshes Model (SLAMM), a statewide effort involving Rhode Island Sea Grant, has been developed to identify existing marsh conditions and vulnerable areas, and provide multiple future scenarios given 1-foot to 5-feet sea level rise projections. Maps developed by SLAMM for all of the 21 Rhode Island coastal communities have now been adopted by the state as a way to show how salt marshes will likely transition and migrate into adjacent upland areas. These SLAMM Maps are available at: http://www.beachsamp.org/tools/.

“There doesn’t appear to be a great deal of change with a 1-foot increase, but with 3 feet change we’ll see a dramatic change. Many of the tidal flats are gone,” said Chaffee, joking that with a sea level change over 3 feet, the Narrow River won’t be so narrow. “We’ll see the salt marsh turn to a tidal flat, which will turn into open water.”

So what can be done to stem these potential loses?

“As far as restoration goes, there are a couple of methods,” she said. “First, we need better drainage to get that standing water off the marsh to stop the cycle of subsidence and vegetation die-off.”

Chaffee noted that Save the Bay is helping with salt marsh assessments north of Middlebridge to identify areas that are stressed, and creating runnels, or “micro-creeks,” to drain pools and get standing water off the marsh. There have been some successes with revegetation in these areas, though what’s growing back might not have been there before.
“What these areas are revegetating to is variable. They’re not necessarily restoring to high marsh species, but at least we’re preventing that die-off,” she said.

But this doesn’t address the elevation issue. “We’re trying to add sediment to marsh surface, which is counterintuitive because we typically try to remove it,” she said. “But adding sediment layers, on the order of inches, will give marshes the elevation gain needed for vegetation to persist.”

“We’re starting to shift our thinking in terms of [sediment] being a resource for our marshes.”

In order to do this, dredging has become a recommended restoration action, and tidal flats within Narrow River are being looked at as possible resources. Not only will sediment material be provided to elevate vulnerable areas, but new habitat will be created that will support eelgrass growth, according to Chaffee. The amount of sediment that can be dredged is limited, so resources will be invested in areas within the marsh that aren’t functional anymore but have higher elevation so require less sediment.

“Sediment has a pretty bad rap,” said Chaffee. “We’re definitely concerned about sediment in our storm waters for water quality reasons, but we’re starting to shift our thinking in terms of it being a resource for our marshes.”

Another challenge for marshes, especially the Narrow River, is motorboat wakes that accelerate erosion along the marsh edge. To counter these impacts, “living shorelines” have been established. Three 100-foot sections along the river south of Middlebridge have been developed by The Nature Conservancy, and other partners, to test various techniques from coir logs made from coconut fibers to bagged oyster shells.

“Living shorelines started this spring, so it’s still too early to say what’s working,” said John Torgan, director of ocean and coastal conservation for the Nature Conservancy. “We’ll be monitoring in terms of accretion of sediment and looking for ecological benefits. We hope to help slow erosion and encourage re-colonization of native plants and animals.”

Monitoring and assessment efforts will continue for the next two growing seasons to fully understand the changing landscape of Rhode Island’s salt marshes,  in order to provide better management and protection of these valuable habitats.

 

– Meredith Haas | Rhode Island Sea Grant Research Communications Specialist