But what the zebra mussel lacks in style, it makes
up for in destructive ability: Tiny but prolific, they
can quickly take over a body of water, clogging power
plant intake pipes, stealing food and oxygen from other
species and even suffocating the native mussels that
they attach themselves to and eventually encrust.
In the Mississippi, they've pushed a species of native
mussel closer to extinction. In Lake St. Clair on the
Michigan-Ontario border, they might be poisoning a kind
of duck known to eat them. And in Lake Erie, they are
suspected of contributing to an oxygen-deprived "dead
zone" where no fish can survive.
Native to the Caspian Sea, zebra mussels were detected
in North America in 1988, apparently having made their
way over in the ballast tanks of oceangoing freighters.
Within a few years, the species had established itself
throughout the Great Lakes and the Mississippi River
and its tributaries, and now is found in waters from
Minnesota to Louisiana. Zebra mussels have been found
in the upper reaches of the Chesapeake Bay watershed,
in parts of the Susquehanna River in New York, raising
fears that they could pose a threat to municipal water
systems and power facilities in Maryland such as the
The St. Croix River, whose waters are glassily still
as it passes through this small town, is the latest
battleground in the war against the invading mussel,
and where those fighting its spread hope to draw the
"At this point, it's impossible to eradicate
them," said Byron Karns, a biological science technician
with the National Park Service. "All we can hope
to do is contain them."
In the St. Croix, zebra mussels are largely found
in the southernmost stretch, close to where it meets
the Mississippi River, which has long been plagued by
the invasive species.
Wildlife agencies are desperate to protect the St.
Croix - the first to be named a Wild and Scenic River
under a 1968 congressional act designed to preserve
the nation's most beautiful waterways - from a similar
fate. The river is the last stand of an endangered native
mussel, the winged mapleleaf, that once thrived in the
waters of a dozen states but now has vanished everywhere
but in the St. Croix.
"What makes the St. Croix special is there are
42 species of native mussels here, and those 42 are
what have been here since European settlement,"
Karns said. "So, we have an obligation to maintain
Karns was among a group of divers from the park service
and the U.S. Fish and Wildlife Service who spent a week
gauging the spread of the zebra mussel in the St. Croix.
In two boats, they cruised down the river to look for
the tiny mussels.
They found that the mussels are spreading slowly northward.
The divers found the invasive mussels in a bay about
five miles south of Stillwater, the furthest north that
they have been detected in the three years since the
monitoring program began. The zebra mussel density in
the St. Croix, however, remains substantially below
that of the Mississippi, where some parts have tens
of thousands of zebra mussels per square meter.
"Sometimes, in the Mississippi River, you'll
have four or five layers of zebra mussels," said
Scott Yess, a biologist with the Fish and Wildlife Service.
While the divers are concerned when they find any
zebra mussels, they are particularly worried when they
find tiny, young mussels. Finding only one or two adult
mussels could mean that they simply fell off a passing
boat, but finding younger mussels as well could be a
sign that a colony is establishing itself in the area.
"The main thing we're looking for are signs
of reproduction," said Nick Rowse, a biologist
with the Fish and Wildlife Service. "They produce
so many offspring, they'll blanket everything. They'll
suffocate the native mussels."
Once a colony begins, it can grow rapidly - one female
zebra mussel can lay more than a million eggs in a single
spawning season. Power plants around the world have
spent millions of dollars unclogging pipes where the
zebra mussels have settled.
Unlike native freshwater species, zebra mussels have
Velcro-like threads that enable them to attach themselves
to seemingly anything hard in a river - rocks, boats,
pipes, mussels. This ability has allowed the zebra mussels
to spread far and wide, hitching barnacle-like onto
boats and moving greater distances than they could on
For two years, the park service has restricted boats
from traveling up the St. Croix beyond a point near
Stillwater to prevent them from transporting zebra mussels.
While the restriction has angered boaters, the park
service says it has been necessary to contain the spread
of the mussels.
Containment is crucial, because once zebra mussels
get established in a river or lake, their effect on
the ecosystem can be devastating. They compete with
native aquatic species for food, upset the food chain
and even change oxygen levels.
Zebra mussels, and a related species, quagga mussels,
are possible culprits in a mysterious "dead zone"
of depleted oxygen that has formed in Lake Erie. The
dead zone is caused by excessive phosphorus, which contributes
to the growth of algae that in turn drains oxygen from
the water. Researchers suspect the mussels because,
as part of their feeding process, they take in organic
material and expel phosphorus. A team of scientists
is studying the problem.
While zebra mussels have no major predators in North
America, some fish and ducks are known to eat them.
A 50 percent decline in one species of duck, the blue
bill, may be due to its consumption of zebra mussels.
The mussels are filter feeders, meaning they take in
and retain toxins from the water, which are then passed
on to ducks that eat them, scientists say.
Researchers are looking into natural predators that
could help control the zebra mussel population. Federal
and state wildlife agencies in Wisconsin are looking
at whether fish in the Lake Winnebago system, particularly
the freshwater drum, will eat enough zebra mussels to
serve as a natural control. Previous studies have indicated
that smaller fish don't eat enough zebra mussels
to have an effect.
While zebra mussels can be vulnerable to predators
- their tiny size makes them a more manageable meal
than larger native mussels - their capacity to breed
often more than makes up the difference.
"They reproduce in such great numbers, it's
hard to make a dent in the population," Rowse said.
"The zebra mussel has brought home how devastating
an invasive species can be," said Dan Stinnett,
a field supervisor with the Fish and Wildlife Service.
"Everything is connected in a river. That's
what makes the river dynamic. That's the beauty
of the river."