Clifford E. Kraft is a biologist in the Sea Grant Institute at the University of Wisconsin, Green Bay, and an adjunct professor at the University. He provides this overview:

"Zebra mussels were first discovered in the Great Lakes in 1988. Since then, they have become widely recognized as a pest with great potential for changing aquatic ecosystems and interfering with commercial activities. Native to eastern Europe, this small mollusk has two unique features that enhance its nuisance potential. First, zebra mussels have abundant, free-floating (planktonic) larvae that allow them to spread quickly and settle in large colonies. Second, upon settling, they produce 'byssal threads' that enable them to attach to hard surfaces. This latter feature provides a capacity for fouling hard surfaces, including man-made structures.

"Zebra mussels have spread rapidly from the Great Lakes to most major inland rivers in the Mississippi River watershed. This dispersal was aided by the extensive inland navigation system that connects the Great Lakes and the Mississippi. Commercial barges that transport cargo throughout this system also transported zebra mussels, inadvertently bringing the mussels to new areas.

"In lakes not connected to commercial waterways, the overland spread of zebra mussels has occurred at a much slower rate. By early 1997, less than 50 inland North American lakes that are not connected to commercial waterways were infested with zebra mussels.

"Once zebra mussels have colonized a lake or river system, there are no known 'miracle cures' to eliminate them or control their abundance. Most control efforts have focused upon dealing with zebra mussels in water intakes and water systems of power plants, industrial facilities and water utilities. Although the economic costs for control efforts have not reached the widely-reported $4 billion level predicted by government officials in the early 1990s, control costs of $120 million have been documented.

"Ecosystem changes brought about by zebra mussels have been dramatic. In areas of great abundance--such as Saginaw Bay, western Lake Erie and Green Bay--these 'filter feeders' have removed most small edible algae from the water column. This feeding activity has cleared the water, allowing light to reach previously shaded sediments, which can now sustain extensive beds of underwater vegetation and substrate-dwelling algae. Numerous bottom-dwelling invertebrates now live among massive zebra mussel colonies, feeding on their waste materials and uneaten food. Surface blue-green algae blooms have become common in zebra mussel-dominated waters, because these algae frequently produce toxins, float high in the water above mussel colonies or are large enough to resist feeding by zebra mussels. Zooplankton communities have also changed due to zebra mussel-induced shifts in the algae food supply for these microscopic animals.

"The most clear and dramatic impact of the zebra mussel on other animals has been the elimination of native clams from heavily infested areas of the Great Lakes. A unique and diverse fauna of Unionid clams is native to North America; they are every bit as fascinating and endangered as the fashionable animals of the rain forest. Zebra mussels attach to and compete with native clams, usually causing their death. In this manner, zebra mussels are accelerating the disappearance of Unionid clams--a decline initiated by other human changes to their environment.

"Much attention has been focused upon the potential impact of zebra mussels on fish populations, yet researchers have identified few instances of associated changes fish abundance. Such impacts will probably be demonstrated at some future date, but two factors make this kind of detective work difficult. First, fish populations are highly variable, so it is difficult to associate any changes with a particular cause. Second, other non-native fish--such as Eurasian ruffe, round goby and white perch--entered and expanded throughout the Great Lakes at the same time as zebra mussels, making it difficult to determine which particular change in fish abundance is due to which invader.

"Prior to the invasion of zebra mussels, Great Lakes and inland-river system habitats already had been extensively modified by human activities. Zebra mussel impacts are only one more in a series of major ecological transformations brought about by humans. People find some zebra mussel-induced changes to be favorable--for instance, they bring about clearer water. Other effects, such as more frequent and prolonged blue-green algae blooms, are often viewed with displeasure. But some of the most dramatic ecological alterations, such as changes in bottom-dwelling invertebrates or the loss of native clams, go unnoticed by most people.

"One lesson from the European experience with zebra mussels is that the invasion of inland lakes will continue for hundreds of years. New lakes in the Republic of Belarus are continuing to become colonized two hundred years after zebra mussels first invaded nearby rivers. Zebra mussel populations might eventually decline in some lakes, but they are unlikely to go away. We'll just have to learn to live with our new neighbors."

Donald A. Schloesser studies zebra mussels at the Great Lakes Science Center in Ann Arbor, Michigan, which is part of the Biological Resources Division of the U.S. Geological Survey. He adds some additional material:

"Zebra mussels and quagga mussels, a related non-native species are found throughout the Great Lakes wherever food is abundant enough to support them--that is, in warmer waters of bays and shoreline areas. Western Lake Erie has been the focus of the 'invasion' and provided an example of what on organism unchecked by environmental constraints can do. The populations of mussels are now relatively steady, though declining. The quagga mussel is believed to be displacing zebra mussels in parts of the Great Lakes, but this process is only beginning to be studied.

"The zebra and quagga mussels (collectively called dreissenids, because they belong to the genus Dreissena) are here to stay. Their numbers and occurrence will change depending on environmental constraints. The impacts of mussels on water withdrawal systems can be controlled and even stopped with proper maintenance and engineering.

"The clogging of water pipes is now being prevented by appropriate maintenance and engineering designs. Generally, these mussels tend to collect at water intake points where flow is already constricted by pipes or other devices. Of particular concern to boaters, the mussels can and will foul water intakes that supply engine-cooling water in certain boat designs. Unchecked, these cooling problems can damage or even destroy the motor through overheating. It is also believed that recreational boats with 'attached' mussel passengers have often been responsible for introducing the invaders to new sites and new water systems.

"The most noticeable change brought about by dreissenid mussels is increased visibility through the water (mussels filter out large particulate particles). Mussels have caused high mortality in native Unionid clams (though some clams seem able to coexist with zebra mussels), altered the makeup of populations living at the bottom of the waterways and reduced plankton communities. Through their filtering of the water, mussels seem to be creating 'cascading' effects that change the nutritional value of the water for other species. For example, in western Lake Erie, mayfly nymphs are returning to the sediments after an absence of 40 years. This event was not expected to occur for decades; because of the high numbers mussels present, however, this return is taking place in less than a decade."

Tiao (Terry) J. Chang, an environmental water engineer and a professor in the civil engineering department at Ohio University, has worked for several years on controlling zebra mussels. Here is his reply:

"The invasion of zebra mussels to the Great Lakes came as a surprise to scientists at first. Since 1988, the debate has been over how to keep them from the problem areas by chemical or mechanical means. While the debate goes on, water agencies and industries are busy dealing with the problems brought about by zebra mussels, namely the fouling of water intakes and piping systems. It is believed that the problem is far from over. In fact, zebra mussels have been found in the entire Mississippi River system, including the Ohio River and the Arkansas River.

"It is also believed that the damage of using chemicals to deal with the problems may do more ecological harm than zebra mussels themselves. Hence, researchers have emphasized the use of non-chemical means of control, with a hope that Mother Nature will kindly accept these exotic species as a part of the ecological system. In a similar sense, nature has been forced to accept 'exotic structures' such as highways and dams.

"A control method that uses the reduction of dissolved oxygen to control zebra mussel activities was granted a U.S. Patent (#5,376,282) in December 1994. A mechanical device using the vacuum system, which incorporates the reduced-oxygen method, has been successfully tested on zebra mussels in the laboratory; in November 1996, it too was granted a U.S. Patent (#5,578,116)."

Christine M. Custer at the Upper Mississippi Science Center in La Crosse, Wisconsin, another part of the Biological Resources Division of the U.S. Geological Survey, offers another perspective:

"The positive side of zebra mussels is often overlooked. Zebra mussels, although injurious to parts of the Great Lakes ecosystem, have actually benefited some species. Diving ducks, such as lesser and greater scaup, and common goldeneye, now eat prodigious quantifies of zebra mussels during migration through the Great Lakes. Nearly 99 percent of lesser scaup diet is zebra mussels in Lake Erie and 79 percent of common goldeneye diet is zebra mussels. Some of the other diving ducks that migrate through the Great Lakes--such as redheads, buffleheads, and canvasbacks--also eat zebra mussels, but to a lesser degree. These data are available in the following publication: Custer, C. M., and T. W. Custer. "Food habits of diving ducks in the Great Lakes after the zebra mussel invasion." in Journal of Field Ornithology, Vol. 67 (1996), pp. 86-99.

"There is some indication that the presence of zebra mussels in the Great Lakes is modifying the migration and winter patterns of some of these diving duck species. More ducks are migrating through and staying longer as they migrate through the Great Lakes because of this now-abundant food source.