PLANT
PATHOLOGIST PROBES DEMISE OF COASTAL WETLANDS
Originally published July 29, 2007
MADISON
- On the banks of a shallow, winding creek in Hammonasset State Park,
something is wrong.
Wade
Elmer, a plant pathologist with the state Agricultural Experiment
Station, crouches beside a few golden-green clumps of marsh grass,
his duck boots pressing into deep mud that covers a large part of
the surrounding creek bank. He snaps a few photos and inspects the
grass.
A
few feet away - where there's no sign of the grass clumps - the creekbank
resembles a reef: It's pockmarked with softball-size holes. A stream
of tiny fiddler crabs scurries through the mud. Running the length
of this section of barren, reef-like embankment is a jagged fissure
- the kind you expect to see in a dry desert, not in a coastal wetland.
"It
looks like it's cracking off," Elmer said. "This whole shelf
could go right into the water."
It's
likely this creekbank once looked like the rest of Hammonasset's wetlands.
It was probably lush with marsh grass that would die off in the winter
and grow back in the spring. But in the past several years, that grass,
called spartina alterniflora, has disappeared - victim to a condition
scientists call "sudden wetland dieback."
There
are a few telltale signs of the dieback, said Ron Rozsa, a coastal
ecologist with the state Department of Environmental Protection.
First,
it affects low marsh grass in coastal wetlands, or marshes that flood
twice a day. And unlike wetlands around the state that Rozsa said
have shrunk because of a decades-long rise in sea levels, the grass
dies quickly, sometimes in months. In Connecticut, it has failed to
grow back.
Scientists
are wrestling with several theories to explain these dead wetlands
- theories that might sound like a science-fiction movie plot: one
hypothesis involves a disease-causing fungus that may have migrated
to New England from Africa.
Another
says herds of crabs devour the marsh grass in the night. But what
exactly is behind the transformation of Hammonasset's creekbanks is
still a mystery.
Tracking
dead wetlands
Connecticut's
dieback dates back several years, when, by accident, Rozsa noticed
grassless mudbanks in a photo of a proposed dock in Madison. Shortly
before, he said, a Connecticut College professor discovered large
swathes of dead marshland at the Cape Cod National Seashore. Rozsa
was stunned. In his decades-long career as a wetland ecologist, he
had never seen anything like it in the area.
Since
that discovery, Rozsa has cataloged dead wetlands around Connecticut,
and estimates most marshes between the Housatonic River and East Lyme
have been hit by the dieback - though the dieback probably stretches
into western Long Island Sound and Westchester County, N.Y., he said.
Similar
diebacks have turned up along the East Coast and into the Gulf of
Mexico, Rozsa said. In some areas of the national seashore at Cape
Cod, the condition has claimed as much as 80 percent of wetland shoreline,
said Mark Bertness, a biology professor at Brown University.
And
in Louisiana, more than 100,000 acres of its wetlands were wiped out
by a similar phenomenon dubbed "brown marsh" from 1999 to
2001, said Greg Grandy, a senior project manager with the Louisiana
Department of Natural Resources.
Though
must of Louisiana's wetlands recovered, about 15 percent have not,
Grandy said.
Wetlands
are the planet's second most productive ecological system after tropical
rain forests. They trap sediment and absorb nitrogen that flows downriver
from cities and towns, cleaning the water; they provide habitats for
countless invertebrates; and they are feeding areas for waterfowl,
marshbirds and shorebirds.
Since
the dieback, Rozsa has seen a few changes in how this ecosystem at
Hammonasset works.
"The
decaying remains of plant material is gone, so there's no cover for
marsh invertebrates - small animals like snails, ferry shrimp and
isopodes, or potato bugs. They're absent. When the low marsh would
flood, killifish would come into the marsh," Rozsa said. "The
killifish come into the marsh to feed on those organisms. The fish
are there, but there's nothing for them to forage on, so they have
to go elsewhere."
Elmer's
role is fairly narrow - though he said he's often tempted to explore
titillating theories.
Every
week, he tends to the clumps of marsh grass he planted earlier this
year in the dieback areas to determine whether the fungus fusarium
is killing the grass. Though Louisiana's dieback was attributed to
several different factors - including drought and increases in the
soil's salinity, acidity, toxicity and oxygen levels - scientists
also discovered fusarium amid the dead marsh grass.
Grandy
said it's unclear what role the fungus played in the dieback.
"It
may have even been present before," he said. "But when a
plant becomes stressed, the pathogen can become toxic."
Elmer,
a fusarium expert, was called in to examine what role the fungus might
be playing in Connecticut's dieback.
He
said there's a lot to learn - such as where the species of fusarium
he has found at Hammonasset originated. In Louisiana, the fungus drifted
over with African dust, Rozsa said. Though he's not sure when this
migration occurred - it could have happened thousands of years ago
or a few years ago - he said it's plausible that recent changes in
Africa have increased the amount of dust heading to the Northeast.
"Deserts
in Africa have increased in size, and dust carried to the Caribbean
has grown," he said. "In the spring, wind currents are such
that we get African dust in New England. It's not an unlikely source."
Elmer
said he has not seen evidence that the species of fusarium at Hammonasset
is African. He is just trying to determine where the fungus is in
the park. If its black lesions appear in healthy and dying marshland,
it's likely the fungus is one of a combination of "stressers"
- similar to what happened in Louisiana - that keeps the marsh grass
from growing, he said.
"My
theory is that they might just be epiphytes, meaning they live on
the top of a tissue without causing disease," Elmer said. "If
something stresses the plant - like drought, salinity, flooding, crabs,
whatever - if there's something in there stressing the plant, then
these species can become aggressive, and actually cause disease. They
may just be the straw that breaks the camel's back."
The
crab theory
When
Elmer sees fist-size holes in the mud and recently planted marsh grass
that looks likes it's been run over by a lawnmower, he believes it
is nocturnal marsh crabs.
"They're
herbivores, like cows," he said. "They go out there and
graze on the young grass."
In
Cape Cod, scientists have photographed sesarma crabs in the middle
of the night doing exactly what Elmer described. The scientists have
experimented with marsh grass transplants in dieback areas, caging
some and leaving others exposed. The caged grass stays in good shape,
said Bertness of Brown. The exposed grass disappears, however.
"There
are lots of crabs out there," Bertness said. "Dozens of
them per meter square."
He
said a similar wetland dieback caused by a burrowing crab occurred
in Brazil and Argentina and by snow geese in the Canadian Arctic.
Both of these diebacks were triggered not by "bottom up"
factors - as Bertness calls the fungus theory - but by people, he
said.
Along
the South American coast, the burrowing crab's predators were overfished,
causing a population surge. The large crab populations then devoured
the region's marsh grass.
In
North America, after snow geese in the Hudson Bay lost their wetland
habitat to development their populations tripled, Bertness said. These
overpopulated flocks then caused a large dieback by consuming the
marsh grass at their summer breeding grounds in the Arctic.
Around
Cape Cod, a similar scenario could be playing out, where predators
of the sesarma crab are disappearing, Bertness said. He isn't sure
whether Connecticut's wetlands are being eaten by the same crab, but
he does not think the fungus Elmer is researching plays much of a
role.
"If
you go look at a salt marsh, you're going to find fungus," he
said. "Fungus could have some role, but I don't think it's killing
them."
But
Rozsa said the opposite - that crabs could have some part in destroying
the area's wetlands, but the blame likely lies with fusarium. Yet
he also throws another factor in the mix: a microscopic worm that
invades plant roots and keeps them from growing. That worm, called
a root knot nematode, is usually only found on land, Rozsa said. But
in Hammonasset, he said it's possible the worm burrowed into plant
roots as larvae while the soil was aerated.
"In
watermelons and tomatoes and cotton there's all these examples of
fusarium and nematodes working together - a synergism - and the plant
dies much quicker," Elmer said. "And that may or may not
be occurring here."
Another
possibility, Elmer said, are rising sea levels.
Though
elevated waters are possibly part of the gradual, long-term stunting
of the state's wetlands mentioned by Rozsa, Elmer is investigating
how sudden surges in sea levels might contribute to the sudden dieback.
He pointed to two years with peak water levels recorded at Boston
Harbor - one in 1999 and another in 2002 - that could have affected
the Sound and its wetlands.
But
that idea, like all the rest, is still just a theory.
"Too
many bizarre things are happening all at once," Elmer said. "This
could be part of a 200-year cycle. There are still so many things
we don't know."
