Coastal wetlands are considered key to mitigating climate change, because their plants absorb carbon from the atmosphere and store it in soil even faster than forests do. But research at a salt marsh near Dover raises questions about how much of a carbon “sink” tidal wetlands really are, and if that’s changing as the climate warms.
It’s a cloudy, breezy day at the St. Jones Reserve, and researchers from the University of Delaware gather on a boardwalk to look at a computer screen, where a line graph squiggles up and down, showing data in real time. The computer is hooked up to a tower that looms above the winding creeks and tall grasses still brown from the winter.
“This is a meteorological tower that measures the breathing of the estuary,” explains ecosystem ecologist and University of Delaware professor Rodrigo Vargas.
A few years ago, Vargas and his doctoral student, Alma Vázquez-Lule, set out to study how that “breathing”—or the movement of greenhouse gases in and out of the marsh—changes during different growth phases of the marsh grass.
Overall, the researchers expected to find the marsh sequestering carbon. But they got a surprise: the marsh instead acted as a net source to the atmosphere of carbon dioxide and methane—an even more potent greenhouse gas—over 3 years.
“I believe we are challenging a paradigm,” Vargas said. “What we expected is not as consistent as we thought.”
The key appeared to be the winter time, when Vargas’ team found the balance flipped. The marsh plants, which suck carbon dioxide out of the air, were dormant. But microbes, which eat buried plant matter and then release carbon dioxide and methane, kept on munching.
Vargas says some researchers tend to do field work only during the warmer months. Now it seems like that’s not the full picture.
“Are we losing carbon from this ecosystem or not? That's an open question,” Vargas said. “Are these ecosystems going to continue sequestering carbon?”
An uncertain future for coastal wetlands
The amount of carbon the St. Jones Reserve salt marsh emitted was small, and Vargas’ initial study didn’t account for carbon that may have entered or left the marsh by water. It’s too soon to say what’s behind Vargas’ unexpected results, but one possible factor is the warming climate.
“I think there are some places where coastal salt marshes will not be a sink, or they might be a sink now, but not maybe in 30 or 50 years from now,” said Tom O’Halloran, who studies wetlands and greenhouse gas fluxes at Clemson University.
O’Halloran says higher temperatures are expected to make both the wetland microbes that emit carbon and the plants that absorb it more active. That could throw things off. Then there’s the threat of rising waters.
“Coastal marshes, salt marshes like this, are experiencing different levels of sea level rise all around the world, depending on the region,” O’Halloran said. “But all marshes have to maintain their elevation relative to local sea level rise, or they will essentially drown.”
Coastal wetlands can adapt by migrating inland—as long as they don’t run into something like a road or a sea wall. They can also adapt by building elevation at least as fast as the sea rises. But O’Halloran says not all marshes have enough sediment available in the water that runs through them to do this.
Sea level rise is already apparent at the South Carolina salt marsh O’Halloran studies. Right around his monitoring equipment, the creeks are growing wider, and the marsh grass is thinning. He thinks he’ll probably find the area sequestering less carbon as a result. But he is also seeing signs that the marsh is migrating.
So far, the St. Jones Reserve salt marsh seems to be keeping up with the rising tides by building up sediment, says Kari St. Laurent, research coordinator at the Delaware National Estuarine Research Reserve there.
“Right now, it appears like our marshes are keeping pace with sea level rise, but projections of sea level rise are anticipated to get worse with time,” she said. “So while that's holding true right now, we don't know if that will hold true in the future.”
The Delaware Department of Natural Resources and Environmental Control (DNREC) predicts that sea level rise will lead to a loss of coastal wetland area in Delaware, adding to the significant wetland losses the state has already seen. One reason is that coastal marshes here don’t have a lot of room to move.
“We have some areas where there's really high slopes,” St. Laurent said. “We have some land barriers. And so that's an open question we have as well—where is marsh migration suitable, and where could there be barriers to it?”
According to DNREC, as of 2010, 30% of tidal wetlands in the St. Jones River watershed had shoreline obstructions, such as elevated roads, shoreline stabilization or residential buildings, preventing landward migration.
A new model for strengthening wetlands
Just as human activity threatens wetlands, human intervention may be able to help them survive.
A restoration project in Delaware has become a model for boosting coastal marshes’ ability to adapt to climate change—and sequester carbon.
“Resource managers on the coastline have a tendency in the past to kind of fight nature,” said Bart Wilson of the U.S. Fish and Wildlife Service. “Prime hook was a real departure from this.”
Wilson works on restoration projects in the region—and managed a $38 million project that transformed part of the Prime Hook National Wildlife Refuge near Lewes.
Prime Hook was originally a tidal salt marsh, similar to the one the University of Delaware team is studying near Dover. But in the 1980s, humans wanted to build better habitat for birds, Wilson says, so they built big dunes to stop the salty tides from coming in. This turned Prime Hook into what’s known as a freshwater “impoundment.”
It worked for a few decades, but in the mid-2000s, several storms breached the walls and brought saltwater in. Wilson says Hurricane Sandy was the “nail in the coffin.”
“[The impoundment] basically turned into a 4,000-acre swimming pool,” he said. “All the vegetation died. All that carbon was basically liberated and kind of moving around the system.”
This left managers with two choices: build bigger structures to keep out the rising seas and more intense storms associated with climate change—or embrace them.
“So we took out all the water control structures and excavated approximately 25 channels to restore that 4,000 acres back to a more natural marsh system that's evolving with the landscape,” Wilson said.
The tides push sand back onto the marsh, which helps it build elevation and migrate inland. The tides also help keep the carbon in the soil, by slowing down decomposition. And Wilson says the new saltwater vegetation stores more carbon as roots than the freshwater plants did.
“Some of the more freshwater areas, I always joke, you need a ladle to take a sample out, because it's all so soupy,” he said. “But when you go into some of these salt marshes, you can cut a plug out, and you see all these roots binding it all together. That's what we're talking about—sequestering carbon.”
The Prime Hook marsh restoration, finished in 2016, is thought to be the largest project of its kind on the east coast. Wilson says it’s already being used as a model for similar projects in Virginia and New Jersey. Several remaining freshwater impoundments in Delaware—like at Port Mahon, Little Creek and possibly even Bombay Hook—could someday follow its lead.
Wilson says even many salt marshes in Delaware had their original hydrology disrupted by straight channels that humans dug. He says restoring the way water flowed through the system before these interventions can help the system store more carbon.
A few miles up the coast, the University of Delaware research could help guide management of coastal wetlands to prevent them from releasing carbon.
“If there is something that we can do to mitigate these effects, such as agricultural runoff, or dredging, or something that will be specific for each ecosystem, then we can make informed decisions,” Rodrigo Vargas said.
Vargas and his team plan to keep studying the St. Jones Reserve to figure out why it’s been emitting more carbon than expected, and whether the trend continues.
“This could be important for forecasting what will happen about these ecosystems in the future,” he said.
The key will be figuring out whether the surprising emissions at the Dover marsh were temporary—or a sign that these crucial ecosystems could switch from helping offset climate change, to making it worse.