One of the reasons that invasive species in ballast water came to be an issue of such global concern is the conquest of the Great Lakes by the zebra mussel, Dreissena polymorpha. Native to the Caspian and Black Seas, zebra mussels were most likely transported in the ballast water of transatlantic ships and were first discovered in the U.S. in 1988. Scientists had been predicting such an introduction since the 1920s, but poor water quality prevented zebra mussels from colonizing. It was successful efforts to improve the Great Lakes environment that eventually allowed their introduction. Since then, the mussels have spread throughout the eastern United States and down the Mississippi River basin, reaching 20 states and causing both devastating ecological damage and economic loss.
Zebra mussels are highly fertile, with females releasing up to 5 million eggs per year, which allows populations to reach large sizes quickly. This trait has assisted the mussel in its quick colonization of lake and river ecosystems. Zebra mussels attach themselves to any submerged hard surface, including the shells of native mussels. By cementing to native mussels in great numbers, zebra mussels interfere with the natives’ growth, feeding, movement, respiration, and reproduction. Researchers have observed native mollusk populations crash within four years of zebra mussel colonization. It is predicted that zebra mussel invasions will reduce native mussel species by as much as 50 percent in the next decade, causing the extinction of up to 140 species. Zebra mussels affect other aspects of the host ecosystem as well. They feed voraciously on phytoplankton, outcompeting zooplankton for this food source and disrupting pre-existing food webs.
The zebra mussel’s penchant for suctioning to hard surfaces has caused technical problems for the American power industry. Water intake pipes are often encrusted with thousands of zebra mussels which increase sedimentation and corrosion of the pipes, as well as restricting or even stopping water flow. Maintenance of pipes clogged with zebra mussels costs the power industry up to $60 million per year and temporary shutdowns due to insufficient water flow can cost over $5,000 per hour. The total cost to the United States of the zebra mussel invasion is estimated at $3.1 billion over the next ten years.
Many methods of zebra mussel control and eradication are now being tested. Manual scraping and abrasive blast cleaning has been successful, but are expensive and time-consuming. Oxidizing and non-oxidizing biocides have been used as well as ultraviolet radiation, with mixed results. Biological control seems promising, utilizing the blue crab, Callinectes sapidus, and mullucicidal strains of Bacillus, but the effects of introducing these nonindigenous control agents are still under examination. Equally as important as effective control, however, is prevention. The U.S. Coast Guard has established "Voluntary Guidelines on Recreational Activities To Control the Spread of Zebra Mussels and Other Aquatic Nuisance Species," which alert divers, hunters, anglers, and other recreational users of water resources to the potential danger of zebra mussel transport. The guidelines contain suggestions for zebra mussel removal from boat hulls and equipment.
-- Stein, B.A. and S.R. Flack (Eds.) 1996 America’s Least Wanted: Alien Species Invasions of U.S. Ecosystems
(Arlington, VA: The Nature Conservancy) p. 10
-- U.S. Congress. Office of Technology Assessment, Harmful Non-Indigenous Species in the United States, OTA-F-565
(Washington, DC: U.S. Government Printing Office, September 1993). p. 68
-- Bright, C., Life Out of Bounds: Bioinvasion in a Borderless World , Worldwatch Environmental Alert Series
(New York, NY: Norton, 1998 pp. 89-92, 182)
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