The Place in Between

How a marsh holds life together

by Erik Vance

A typical marsh on the Eastern Shore.
A typical marsh on the Eastern Shore. Photo, Eudora Miao

There is only one way to properly experience a marsh on the Chesapeake Bay. You need to get in a boat and see it, from the bottom up.

Sliding a kayak into the water at dusk in Blackwater National Wildlife Refuge — a complex of brackish marshes, uplands, and open-water ponds that cover 45 square miles of Maryland’s Eastern Shore — seems, at first, like an intrusion. The blackbirds grumble and warn each other of a potential threat. The cordgrass, which normally sways gently like harvest wheat in the failing light, occasionally explodes with an outraged mallard. A great blue heron looks up and eyes you suspiciously.

But after a minute or so the marsh goes on about its business. Above the water’s surface, Spartina grasses compete with taller Phragmites. A bald eagle lazily lifts off from a dead tree, while a kingfisher drops into the water and flies off, seemingly laughing. Tidal channels are mazes of hidden passageways, and you quickly realize how complex the marsh is. The loss of even an inch of water could get you stuck out here all night.

“That’s a no man’s land,” says the guy who rents me the boat. “A few years ago, a guy got lost out there for four days.”

I believe it. But his warning illustrates the dual role of marshes in the human imagination. On one hand, they are boggy, impassable, often smelly places filled with spiders, pasty mud, and sharp reeds that poke you in a thousand different places. On the other, they are serene landscapes and among the most important ecosystems on Earth — not just for the animals that inhabit them but also the humans that observe them from shore.

But what does that duality mean? At this time, when so many crucial ecosystems are threatened, why should anyone care about something as seemingly expendable as a smelly little bog by the Bay?

Anatomy of a saltwater marsh

A marsh is a dynamic ecosystem driven by flooding or inundation from tidal waters. Many habitat characteristics — including the plants that grow there and the animals that live there — are defined by how often and how much part of a marsh floods. The tidal flat and the low marsh get inundated twice daily due to regular tides. Tidal flats are often fertile areas for submerged aquatic vegetation, while plants like Spartina alterniflora live in the low marsh. High marsh is inundated during high tides and supports a variety of salt marsh grasses. Upland areas beyond the flooding zone are habitat for poplars, pines, and other tree species. Phragmites, which readily adapts to different conditions, often spans the high marsh into the upland. A transition zone, such as a forested wetland or a swamp, often occurs between the high marsh and upland, and it can be inundated during high spring tides or storm surges.
Side-view graphic of saltwater marsh.
Graphic, Nicole Lehming (original graphic by Virginia Institute of Marine Science)

A Dynamic Place

On the surface, a marsh is just a transition zone. Too dry to be bay, too wet to be forest, it’s the habitat that can’t make up its mind.

But it’s that ambivalence that engenders the ecosystem that’s indispensable to both. A marsh forms a bridge between aquatic and terrestrial ecosystems, sustaining life forms that can’t live in either.

As the Chesapeake Bay loses much of its land to erosion and sea level rise, marshes reinvent themselves. They lose and gain ground, as sediments ebb and flow. High areas become wet only occasionally, such as during big storms or very high tides, while low areas are inundated with seawater more frequently. But marshes are dynamic and respond to changes in water level by shifting inland. High marsh can become low marsh if water intrudes more often. Plants assist in marsh stability by trapping mud. Under favorable conditions, the ecosystem can maintain a dynamic equilibrium with increasing water levels.

A recent study published in the journal Nature Climate Change suggests that scientists might have overestimated the vulnerability of these wetlands, including some in the Chesapeake Bay. The research team, led by Matt Kirwan at the Virginia Institute of Marine Science, used models that may be more robust than previous ones. They consider the ability of marshes to migrate inland and build elevation by capturing suspended sediment and also the contribution of organic debris from plants to the marsh surface. If marshes can migrate and are not deprived of sediment from bays and rivers, Kirwan and his team believe that marshes can adapt, even with increasing rates of sea level rise.

Another study — by Leah H. Beckett of the Northwest Indian Fisheries Commission and University of Maryland scientists Andrew Baldwin and Michael Kearney — proposes that sea level rise can help marshes increase elevation with more sediment input that stimulates plant growth and reduces decomposition rates — but only up to a point.

We need marshes to protect the land from the sea, the sea from the land, and the inhabitants from everything else. Perhaps a bigger threat than climate change, say Kirwan and his team, is hardened shoreline and the barriers that humans create to protect themselves. An engineered environment does not let marshes take advantage of natural ebb and flow. With 20 percent of the Chesapeake shoreline hardened, marshes can’t be their dynamic selves.

How do marshes protect the land from the sea? Along the Chesapeake Bay, they act like massive sponges absorbing storm energy. Or to put it another way, marshes do to waves what waves do to people who run through them: they trip them up and drag them to a standstill.

When a powerful hurricane or tropical storm, like Sandy or Harvey, slams into the coast, we hear a lot about the ability of marshes to dull the storm’s impact. Essentially, they absorb the water and the energy of the waves that nibble at their edges.

Then they replenish themselves with the constant flow of silt from bays and rivers and dead plant material from the wetland itself. With the continual die-off and return of grasses, they produce more than twice as much biomass as a pine forest or a farm field. And all that material sinks back into the soil, decaying into a rich, black clay. In this way, the marsh can be self-correcting, always sitting just above sea level.

These days, though, some experts wonder if marshes may be losing their battle for survival. As sea levels rise at faster rates, ocean water increasingly washes over these wetlands, sometimes covering them altogether — and it only takes a couple of inches of water to do that.

“It’s not about the height of the water, it’s about the time of inundation,” says Keryn Gedan, a biologist at George Washington University who specializes in marshes and coastal ecosystems. “Especially here in the Chesapeake, where sea level rise is happening faster than nearly anywhere in the world.”

The rate of sea level rise between Cape Cod and Cape Hatteras is three times the global average. About a third of that rise is caused by a form of subsidence, called glacial isostatic adjustment, where land was shoved up by glaciers. Now the land is reverting to its original shape. Mostly, though, the rise is due to increasing ocean temperatures.

In the mid-Atlantic, sea level rise is exacerbated by other conditions, including melting of polar ice sheets and changes in the flow of the Gulf Stream. Together they are causing ocean levels to rise at a rate of more than two feet per century, according to the 2013 Maryland Commission on Climate Change report.

The effects are already visible in Blackwater, where Gedan does research. The rising water and saltwater intrusion has been so severe here that it’s covered marshes and killed entire stands of pine trees, creating so-called ghost forests. Trees here appear as clusters of nude sticks protruding from the water or invading marsh, while 50 feet away the forest flourishes.

“The trees are dying faster than they are falling,” Gedan says.

Overhead view of marsh with graphic lines showing previous forest lines.
Marshes at Blackwater National Wildlife Refuge
Marshes gain and lose elevation as they move. The aerial photo shows a marsh and its environs in Delaware Bay. The edge of the forest has shifted as tidal marsh has pushed landward over seven decades (1930–2007). Marshes at Blackwater National Wildlife Refuge have converted to open water.Graphic, Joseph Smith; Photo, Daniel Strain / MDSG

Land and Sea

It’s not just the land that needs to hold back the sea: the sea also needs to hold back the land. That is the second service that tidal wetlands offer. They take in the runoff of modern civilization and send it out clean. It’s no wonder they’re often called the kidneys of the Chesapeake.

“You can dump raw sewage into a wetland for a while and it’ll work,” says the University of Maryland’s Baldwin, a marsh ecologist. He quickly points out that this would be a very bad idea for the ecosystem, but it demonstrates the extraordinary filtering power of tidal marshes. In fact, the town of Mayo, south of Annapolis, runs nearly one million gallons of wastewater per day through several types of constructed wetlands, mimicking natural processes to clean the water.

On the Patuxent River, according to a 2014 report produced by the Chesapeake Bay Program Office and the Center for Watershed Protection, marshes pulled out 35 percent of the nitrogen pollution and 81 percent of the phosphorus. Most of the water wasn’t even running directly through the marshes. Yet the pollutants found their way there anyway.

Paradise for Birds, Dicey for Humans

Marshes protect the land from the sea and the sea from the land. But they have one more job: they protect wildlife from the sea, from the land, and from us.

Photo of Blue Heron standing in a marsh
Blue heron at Blackwater. Many birds rely on marshes for habitat, food, and shelter from predators. Photo, Nicole Lehming / MDSG

“All of those other things are really valuable, and I would not dispute that for a second,” says  Lorie Staver, a plant biologist at the University of Maryland Center for Environmental Science. “But in my mind, the wildlife value trumps almost everything else.”

Staver has spent years studying marshes and trying to bring them back from decline. Her specialty is island marshes, which are especially valuable to wildlife because predators can’t access them without wings or a very good breaststroke. Currently she is a part of a vast project to design new marshes on Poplar Island, near the border of Maryland and Virginia. The island is being rebuilt using dredged sediment from approach channels to the Port of Baltimore. It’s a difficult and precise process in which just a few millimeters of water could mean the difference between success and failure.

But it’s worth it. Birds like rails, stilts, and saltmarsh sparrows might live in the wetlands year-round. Egrets, pelicans, and cranes might visit for hours or days and then leave. Teal, godwits, and geese might rely on them as a rest stop during migration. Terns, sandpipers, and ducks might lay eggs and raise their young on the marsh.

Hundreds of fish and invertebrate species might use the marsh as well. Tidal channels and marshes are crucial spawning, nursery, and feeding habitat for economically important Chesapeake Bay species, such as menhaden, blue crab, and oysters. Whether you have feathers, scales, or a hard outer shell, the marsh is the perfect place to raise a family.

But what’s good for a bird or a fish is not always ideal for an ecologist. Marshes are not easy places to work in, as I learn on a hot day in early October when I ditch my kayak to wade in the water with Jessica MacGregor and Man Qi, researchers from Gedan’s lab.

When I arrive, they’re in the process of collecting seed heads to study how the wetlands are moving and what that means for the services they provide to society and nature alike. The seeds will help identify what plants are present, how healthy they are, how much inundation they can withstand, and how diverse the marsh is.

The migrating wetlands have begun to take over farm fields and backyards. Gedan would like to see some of these new marshes protected and allowed to grow.

“Watch your step,” MacGregor warns me as she investigates a ratty flower on a cordgrass strand. “It looks like we are in the low marsh now,” she says, referring to an area that floods daily with shallow, silty water.

I am struck by how uniform the grasses are, almost like fields of wheat or sorghum. But the scientists tell me to look closer: it’s a knee-high, dense forest of dozens of species, like black needlerush, bulrush, and invasive Phragmites, speckled with purplish saltmarsh aster.

MacGregor and Qi are eager to find samples of smooth cordgrass, a dominant Chesapeake marsh species, so we head deeper into the low marsh.

Water moves around marshes through dizzyingly complex channels that look like blood vessels or nerves from the sky. They’re hard to see underfoot. While carefully picking my way along, I hear a yelp and a splash, as MacGregor steps into an especially deep rivulet.

“That was a bad one. Now I’m in the marsh — up to my hip,” she laughs.

It takes some effort to pull her out of the sticky black mud, but it’s not the first time MacGregor has been stuck.

Looking out across the flat marsh, I can see why so many birds live here. Any predator larger than a fox would be visible for miles and would get hopelessly mired in minutes. Raptors have nothing to perch on. Even humans, with our infinite capacity for clever new ways to travel, haven’t found a good way to cross a marsh.

Heading back toward the cars, I notice a line of dead loblolly pines. The marsh here isn’t in retreat, it’s moving uphill. I suddenly remember something Baldwin said earlier: although wetlands conjure a doom and gloom scenario, they are actually very resilient.

The key to the survival of the Chesapeake tidal marsh — and all the services it provides — is helping it to adapt in time. For Baldwin, the marsh doesn’t need protecting so much as room to migrate. Likewise, Staver believes that animals that depend on the marsh can survive if they have the time and space for adaptation.

“They can adjust to change at a certain pace, up to a certain threshold, and beyond that it’s difficult to adjust quickly enough,” she says.

That’s the fundamental question, not only for Chesapeake marshes but for all ecosystems in the line of fire for climate change. Can they adapt in time? Baldwin says the marshes seem to be pliant. Gedan agrees, but worries that humans might not be.

Another issue: Where will the marshes go when they move, and what will become of those places?

The best place for marshes to spread is farms — some a mile inland — abandoned because of the encroaching salt. But it’s not clear when, if ever, those pioneer plants will turn fields into habitat. Gedan is studying this process and researching transition crops to protect the farmers, just as the marsh protects the shore.

Perhaps the radical changes in Chesapeake Bay marshes will be too much, and our grandchildren will never see sandpipers flitting in front of their kayaks. But perhaps the future won’t be so bleak, and the marshes will change form, move, and find ways to survive. Perhaps, with just a little nudge from us, these soggy but crucial in-between places will persist for a while longer.

Erik Vance, a biologist, is a longtime science writer and the author, most recently, of Suggestible You.


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