What’s killing off our salt marshes?

September 15, 2008 at 1:05 am Leave a comment

Up and down the Eastern Seaboard, the coastal wetlands are dying, and no one knows for sure why this is happening.

First observed in the Florida panhandle in 1990, the shoreline degradation, called sudden wetland dieback, has been observed in hundreds of locations from Louisiana to Maine. Scientists say that while it’s normal for coastal marsh vegetation to have its bad years, they have never seen marsh grass die and not recover, until now.

The identified dieback sites include a number in Connecticut, mostly east of New Haven. “I would expect that it’ll be found along Fairfield County, too,” said Ron Rozsa, a scientist with the state Department of Environmental Protection.

“The unusual aspect of this is that it’s affecting many different locations at the same time,” Rozsa said, who is one of the leading researchers on the subject.

There are a number of suspected causes, with a species of fungus — Fusarium — as one of the prime suspects.

But there are other suspects in the lineup, experts say. These include drought, rising sea levels, rising soil acidity, the purple marsh crab, tiny nematode worms and the heating of the Earth’s atmosphere.

Rozsa said that his hypothesis is that the species of Fusarium that’s causing the trouble originated in Africa’s Sudan and was transported to the Eastern Seaboard by piggybacking on the dust that blows here from the Sahara.

“In the last eight or so years, the incidence of African dust making landfall in the U.S. has increased, both in terms of frequency and amount,” he said, adding that Fusarium and other plant pathogens have been found on African airborne dust.

 

But researchers note that Fusarium is ubiquitous in the plant kingdom — most plants have at least one strain of the fungus, and usually it’s benign. So researchers wonder whether stress to the marsh grass is causing it to be more susceptible to the fungus.

“Things have been happening to the marshes since the beginning of time, but this, we think, is new,” said Wade Elmer of the Connecticut Agricultural Experiment Station in New Haven.

In his laboratory, Elmer tends to dozens of plastic flowerpots where he grows Spartina alterniflora, the marsh grass that grows on the very edge of Long Island Sound, and the species that’s mysteriously dying. He’s trying to determine which of the various species of Fusarium that’s infecting them.

“Here are all of the species of Fusarium,” said Elmer, hefting a textbook as big as a telephone directory, each page describing a different species.

“One idea is that there is slightly more nitrogen in the soil — and that, with rising sea levels and maybe a drought, is causing a tipping point,” Elmer said. “There’s evidence that as the lower marsh becomes wetter and wetter — and maybe the increasing temperature has something to do with — the grass doesn’t do as well.” It’s possible that the dieback might be a poorly understood natural phenomenon — perhaps something that occurs every several hundred years. But the Earth’s climate is clearly getting hotter from human activity, and this has stressed ecosystems around the globe, marsh grass included, experts say.

“Our tidal marshes formed about 3,000 years ago when sea levels were low, and for a long time the seas were rising at a rate of only 1.5 millimeters per year,” Rozsa. “Now we’re looking at forecasted rates on the order of three or four millimeters per year. With that, we don’t think that the marshes can sustain themselves as these big, flat expanses like we see today — they’ll only exist in narrow bands along the shore.” Elmer agreed. “We’ll see just a thin ribbon of marsh in New England,” he said.

S. alterniflora is the only species in North America that can tolerate high concentrations of salt combined with oxygen-poor mud and peat along the shoreline.

It can be damaged by ice in winter and “wrack lines” in summer — the rows of dead seaweed and other dendrites that’s deposited by waves and tides. But it will usually recover from this after a few months, Elmer said.

In many places along the shoreline, the marsh is hemmed in by homes, roads and other structures, so, once damaged, it can’t migrate further inland. Elsewhere, rock outcrops keep it in its place.

Scientists say that sudden wetland dieback is apparently an Eastern Seaboard problem — it hasn’t been observed on the West Coast, nor on the shores of the other continents.

The onslaught of the invasive Phragmites — the tall wetland grass with its characteristic feather-like fronds — is not part of the die-off equation. “Phragmites can’t tolerate the salt levels that Spartina can — it’s found a little farther inland,” Elmer said. “In fact where Phragmites has died, we’ll find that the Spartina has died before it.”

Some researches have suspected that the purple marsh crab, Sesarma reticulatum, has a role in the dieback equation. The tiny nocturnal crab has been known to have a “lawnmower” effect on Spartina. But Elmer sees the crab and its damage as more of an effect, not the cause, of an unhealthy marsh. The purple marsh crab is a omnivore, consuming both plants and small animals.

Stephen Smith, a scientist with the Cape Cod National Seashore, said that there is a clear correlation between Spartina loss and crab density. He notes that the population of purple marsh crabs in dieback areas is 10 times what it should be.

“You can plant Spartina alterniflora anywhere on these marshes, and it will grow — so long as you protect them for these crabs,” he said.

“In places where they’re not protected, the grass is chewed down to a nub. We don’t see a situation where the grass turns brown and dies — it’s there one day and gone the next.” The question for future research, Smith said, is finding out why purple marsh crab populations are exploding and why they’re choosing Spartina over other items their diet.

Researchers agree that solving the marsh dieback puzzle is important — not only for the Sound, but for the Earth as well.

“The salt marsh is the second most productive ecosystem on the planet — only the tropical rainforest will produce more biomass per square kilometer,” Elmer said. “It also serves as a home for many organisms.

“A lot of what’s harvested for seafood in Long Island Sound lived out their juvenile stages in the salt marsh. So if we didn’t have the salt marsh, these harvest numbers would drop considerably.” Marshes also absorb nitrogen and toxins that are present in rainwater run-off that would otherwise wreak havoc on the Sound’s water quality, he said.

“And salt mashes provide a buffer for storm surges,” Elmer said. “In New Orleans, if their marshes were intact, the storm surge of Katrina would not have reached the levees. They really provide a tremendous resource.”

HERE TODAY, GONE TOMORROW NAME: Sudden Wetland Dieback WHAT IT IS: The rapid loss of marsh grass along shoreline wetlands. SPECIES AFFECTED: A type of saltwater-tolerant marsh grass known as Spartina alterniflora, the front line species of grass that populates the water’s edge in saltwater marshes. WHY IT’S IMPORTANT: Marshes are a vital habitat for thousands of marine animal species that form the bottom of the ocean food chain. The die-off results in shore erosion — and a smaller marsh. POSSIBLE CAUSES: Scientists are looking at such causes as fungus, a tiny worm called a nematode, the purple marsh crab, the heating of the Earth’s atmosphere and rising sea levels. EXTENT OF PROBLEM: Dieback has been observed in hundreds of locations from Louisiana to Maine. It was first seen in the 1990s.
By JOHN BURGESON
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Entry filed under: Dive In, Natural Waterfront, Region. Tags: , , .

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