Page 94 - Environment: The Science Behind the Stories

Central CaSE StUdy

Black and White, and Spread all Over:

Zebra Mussels Invade the Great Lakes

CANADA

Lake Superior
“We are seeing changes in the Great Lakes
Lake Huron that are more rapid and more destructive
than any time in [their] history.”
Lake Ontario
—andy Buchsbaum, National Wildlife Federation
Lake Michigan
“When you tear away the bottom of the food
Lake Erie
chain, everything that is above it is going to
UNITED Great Lakes be disrupted.”
STATES
— tom Nalepa, National Oceanic and atmospheric
administration
Atlantic
Ocean
Things had been looking up for the Great Lakes. The pollution- ships may be transported from one place to another, even to
fouled waters of Lake Erie and the other Great Lakes shared isolated lakes and ponds well away from major rivers. More-
by Canada and the United States had become cleaner in the over, in North America the mussels encountered none of the
years following the Clean Water Act of 1970 and a binational particular species of predators, competitors, and parasites that
agreement in 1972. As government regulation brought indus- had evolved with them in the Old World and limited their popu-
trial discharges under control, people once again began to use lation growth there.
the lakes for recreation, and populations of fish rebounded. Zebra mussels clog water intake pipes at factories, power
Then the zebra mussel arrived. Black-and-white-striped plants, municipal water supplies, and wastewater treatment
shellfish the size of a dime, zebra mussels attach to hard sur- facilities (Figure 4.1a). At one Michigan power plant, workers
faces and feed on algae by filtering water through their gills. counted 700,000 mussels per square meter of pipe. Great
This mollusk, given the scientific name Dreissena polymor- densities of these organisms can damage boat engines,
pha, is native to the Caspian Sea, Black Sea, and Azov Sea in degrade docks, foul fishing gear, and sink buoys that ships
western Asia and eastern Europe. In 1988, it was discovered use for navigation. Through such impacts, zebra mussels cost
in North American waters at Lake St. Clair, which connects Great Lakes economies an estimated $5 billion in the first dec-
Lake Erie with Lake Huron. People had brought it to this conti- ade of the invasion, and they continue to impose costs of hun-
nent by accident when ships arriving from Europe discharged dreds of millions of dollars each year. CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
ballast water containing the mussels or their larvae into the Zebra mussels also have severe impacts on ecological
Great Lakes. systems. They eat primarily phytoplankton: microscopic pho-
Within just two years of their discovery, zebra mussels had tosynthetic algae, protists, and cyanobacteria that drift in open
multiplied and reached all five of the Great Lakes. The next water. Because each mussel filters a liter or more of water every
year, these invaders entered New York’s Hudson River to the day, zebra mussels consume so much phytoplankton that they
east, and the Illinois River at Chicago to the west. From the can deplete populations. Phytoplankton is the foundation of
Illinois River and its canals, they soon reached the Mississippi the Great Lakes food web, so its depletion is bad news for
River, giving them access to a vast watershed covering 40% of zooplankton, the tiny aquatic animals that eat phytoplank-
the United States. In just three more years, they spread to 19 ton—and for the fish that eat both. Researchers are finding that
U.S. states and two Canadian provinces. By 2010, they had water bodies with zebra mussels contain fewer zooplankton
colonized waters in 30 U.S. states. and open-water fish than water bodies without them. Zebra
How could a mussel spread so quickly? The zebra mus- mussels also suffocate native mollusks by attaching to their
sel’s larval stage is well adapted for long-distance dispersal. Its shells (Figure 4.1b).
tiny larvae drift freely for several weeks, traveling as far as the However, zebra mussels benefit many bottom-feeding
currents take them. Adults that attach themselves to boats and invertebrates and fish. By filtering algae and organic matter 93

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