The essential barriers between land and sea are starting to drown. Salt marshes — coastal grasslands that are regularly inundated by seawater during high tide — are crucial elements of our ecosystem. They provide a habitat for 75 percent of fish species that are commercially harvested. And because they’re great at absorbing water, they protect coastal communities from flooding. But thanks to the human desire to build homes and cities along coastlines and the fact that sea-level rise is accelerating, coastal salt marshes are under threat.
Anne Giblin is the interim director of the Ecosystems Center at the Marine Biological Laboratory at Woods Hole, Massachusetts. She’s the principal investigator of research at Plum Island, a salt marsh in northern Massachusetts, where she studies nitrogen cycles in the marsh and how the rising sea level affects the overall chemical health of these coastal ecosystems. Though salt marshes aren’t found everywhere — mangroves, which are water-dwelling trees, perform similar functions in the tropics — they are a necessary ecosystem in middle and high latitudes around the world.
“Most of my work is in the mud,” Giblin says. Plum Island is the largest remaining salt marsh in New England. Understanding the chemical composition of the mud, Giblin says, will help scientists better predict how these areas are going to handle rising seas. And it’s especially important to prepare for their inevitable inundation, because the loss of salt marshes that has already occurred seems to be exacerbating flooding from storm surges.
“A lot of the salt-marsh areas in New England, something like 80 percent, have been destroyed.”
Salt marshes form at the mouths of rivers where they meet the sea. They perform important functions like reducing the force of waves during storms, Giblin says. “There have been some studies that suggest the flooding in Louisiana during Hurricane Katrina would have been much lower if marshes were still present.”
Plants in salt marshes, mostly grasses, grow in peat and trap sediment that comes in from the ocean or flows down nearby rivers. This includes dirt and any kind of small particulates. The grass grows and dies every year but doesn’t decompose. Instead, it creates peat that new grasses grow on each season. To survive sea-level rise, the marsh just has to grow at a rate faster than the ocean is rising. “Every year, the marsh grows a little higher. In a perfect world, the rate at which the marsh is growing and the rate of sea level rise are well matched,” Giblin says. “We know from the geologic record that’s been happening for thousands of years. But now sea-level rise is accelerating. In our area, over three, maybe as much as four millimeters a year.”
But the Plum Island marsh grows only at about 2.5 millimeters per year. There is concern that marshes won’t be able to keep up and stay above the rising ocean. Marshes are used to being inundated by water during high tide, but they also need time above water to survive.
The truth is that there aren’t enough salt marshes left, at least in the United States, for them to provide the essential flood resistance we will need as the ocean rises. “Because they are occurring on the coast, at least in the U.S., they’ve been tremendously modified and destroyed by dredging and filling,” Giblin says. “Most of Boston was originally a salt marsh. A lot of the salt-marsh areas in New England, something like 80 percent, have been destroyed.”
Most important, the peat — the sediment and organic matter — is also a carbon sink. “When the marshes are building themselves up, they’re storing carbon that the plants took up from the atmosphere,” Giblin says. “If we lose the marshes, all that carbon dioxide is going to go back up into the atmosphere.”
The remaining marshes also act as a nursery for fish. On Plum Island, Giblin says, juvenile striped bass use the marshes as sanctuaries where they can feed and reproduce while being protected from deep-ocean predators. “It’s also a good habitat for shellfish, oysters, and clams. They don’t actually grow on the grassy part—they grow on the tidal flats around the marsh and feed on organic matter produced by the grasses.”
Giblin says the marshes are also essential habitats for migratory birds, which stop to feed on smaller invertebrates that live in these regions during their seasonal migrations both north and south along the coast. The loss of these habitats could be devastating not only to humans who live around them, but also to the animals that rely on them for protection and food. But, Giblin says, based on her research into the chemical response of the peat to increased sea levels, there is both good and bad news to be had.
“From the work we’ve done at Plum Island and marshes around here, we’ve learned that we’re at a very critical time. Our measurements and measurements from other people — there are hundreds of people working on this — have found there’s a critical rate at which marshes can no longer keep up,” Giblin says. The rate varies depending on how quickly the marsh can grow and the supply of sediment to help the marsh increase its volume.
Some of this is also dependent on the tidal range, or how far the tide recedes when it goes out. Giblin says that sea-level rise doesn’t change the range of the tide, which means that when the tide comes in, it will come in higher, but it willstill go out at the same rate. In other words, in New York City, for example, the tide has and will always go down about three to four feet vertically at low tide. On Plum Island, the tidal range is about 10 feet.
That means areas with a larger tidal range will be better at handling sea-level rise. When the tide goes down, Giblin says, “the grasses have periods of time where the roots can dry out a little bit. Even though salt marshes grow in areas where they get flooded by the tides, they can’t be underwater all the time.” They will still have the chance to dry out, but as the sea rises, they’ll be wet for longer periods.
It turns out that marshes are, for now, showing some ability to survive rising sea levels. But that may change as conservative projections put the rise at onecentimeter per year. “Can the marshes survive that? We don’t exactly know, but if we look back to after the last ice age, there were no salt marshes until that rate slowed down to about one centimeter per year,” Giblin says. “That’s probably the upper limit [of survivability]. It’s not looking good now.”
All that said, people seem to be recognizing the importance of protecting and even resurrecting marshes — especially as there is an increasing realization that they can be an important barrier in protecting coastal cities from flooding. Projects have been popping up around the country to, for example, remove dams that prevent sediments from flowing downstream. And there are even some experiments with manually spraying sediments onto marshes, though this can be tricky, Giblin says, as it’s possible to add too much sediment too quickly.
And marshes seem to be able to retreat up shore, essentially saving themselves from being drowned by sea-level rise. As they lose ground on the ocean side, Giblin says, it has been shown that they can grow bigger on the land side. “That’s more difficult now because marshes back up onto sea walls or parking lots,” she says. But in areas where there isn’t nearby human activity, their retreat onto land is a form of protection.
Giblin is also studying how to fertilize the marshes to increase their growth. Adding nitrogen, for example, will boost plant growth within a given marsh. But nitrogen can also cause bacteria to metabolize carbon, which decomposes it and releases it into the atmosphere. “There’s a double-edged sword in marshes receiving runoff from houses and sewage,” Giblin says, adding that human waste is nitrogen-rich and could negatively affect carbon capturing in peat. “We have big debates in our own group as to how important this is. I come down somewhere in the middle. It’s another way that human activity next to marshes might have negative impacts.”
“Once people are disconnected by more than a generation, they don’t care at all.”
As to what people can do to help protect and restore marshes in their communities? Giblin says the best thing to do is vote. People have had some success in convincing their local officials to prioritize salt marshes. Giblin says that in coastal towns with big birding communities, there tends to be a bigger social push because of salt marshes’ importance as bird habitats. That also goes for communities with a traditional relationship to marshes, like where people have grown up clamming or fishing.
“My personal theory is that once people are disconnected by more than a generation, they don’t care at all. In Cape Cod, it’s been 20 to 30 years since there’s been eelgrass, and people don’t scallop. All they really care about is the dock,” Giblin says. “I think if you maintain a healthy ecosystem and educate people about what it’s doing for them, they’ll help protect it.”
This article is part of the series, Climate Changed. Read the previous installment here: