Invasion of the Zebra Mussels

A tiny but troublesome mollusk, accidentally introduced into a North American lake a few years ago, may soon infest most of the continent

AN OCEANGOING SHIP TEEMS WITH ALIEN LIFE. There is no manifest for this cargo, although its constituents may number in the hundreds of thousands. They come aboard in ballast water, pumped from harbors into ships. There are a number of reasons why a ship will “ballast up.” One is stability: a lightly loaded vessel may ride too high, and is thus more liable to capsize. Another reason is propulsion: the propeller of an underloaded ship may rise half out of the water. To avoid these problems a ship may distribute ballast water through a network of tanks inside the hull.

In 1986 or late 1985 a ship leaving a freshwater European port for North America began to take on ballast. It opened the sea cocks and started the pumps. A thriving population inhabited the harbor from which the ship drew the ballast, and members of that population came aboard along with the water. Alas for North America, one of them was a hardy Asian bivalve named Dreissena polymorpha, the zebra mussel, which over a 150-year period had spread from the region of the Caspian Sea into much of Europe, plugging water pipes and clamping onto the hulls of boats as it went. The unknown ship that carried D. polymorpha across the Atlantic was headed for the St. Lawrence Seaway. Past Quebec City it sailed, past Montreal, past Toronto, and on through Lake Erie. When the ship finally reached its destination and flushed its ballast, somewhere above Detroit, a founding population of zebra mussels tumbled into Lake St. Clair, and a new continent.

Ballast-flushing is by no means a new procedure, so the question of why zebra mussels didn’t arrive earlier arises. The answer, biologists surmise, has to do with pollution. For years, they suggest, Europe’s major freshwater harbors were so dirty that mussels simply could not survive in them. Environmental cleanups eventually changed all that. The zebra mussel began colonizing the harbors—and being taken aboard with ballast. Many of North America’s freshwater harbors had once been little cleaner than Europe’s, but by the 1970s they were subject to environmental controls that would soon ensure a habitable home for the zebra mussels.

Biologists are hastening to quantify the effects of this “invasion,”as the introduction of any new species is called, but already the costs have been staggering. Propelled by relentless fecundity and a talent for spreading themselves abroad, zebra mussels are clogging the intakes of power stations and water-treatment plants; colonizing navigation buoys in such numbers that they drag them under; fouling fishing nets, marine engines, and the hulls of boats; and usurping spawning grounds that are the mainstay of a sports and commercial fishery valued in the billions. Europeans have adjusted to the zebra mussel over many decades. In European water systems strategies for combating mussels—using pipes of wider diameter to reduce the chance of occlusion and to facilitate cleaning, burying intakes in sand to filter out larval mussels—are now routine. But the mussels’ spread through Europe was slow. In North America they seem to have found an especially hospitable environment, They have already swept down the length of Lake Erie and penetrated Lake Ontario. It is only a matter of time, biologists insist, before the mussels will have extended their range from the Great Lakes into the Ohio and Mississippi river drainage systems, and thence into the waters of some two thirds of the United States.

An Order of Magnitude a Year

THE FIRST ZEBRA MUSSEL TO PRESENT ITSELF to the puzzled gaze of North American science was pulled from Lake St. Clair in a Ponargrab, a scoop used to take samples from the bottom of a lake or river. The grab was deployed from an open boat on June 1, 1988, by a research technician and two summer interns at the University of Windsor’s Great Lakes Institute. They were conducting a routine sampling of the invertebrate population on the lake bottom as part of a program to monitor water quality. They did not recognize the strange mollusk that they found clinging to a rock. But they were curious.

“They were looking for native mussels and they found this other bivalve,” recalls Gerry Mackie, a professor of zoology at the University of Guelph, near Toronto, and a freshwater-mussel expert. “They described it to me over the phone. I had a pretty good idea what it was, but it’s not the kind of thing where you want to jump to conclusions. I knew what kind of biofoulers these things were in Europe. So I asked them to bring it in.”Mackie’s suspicion—that the mussels were Dreissena polymorpha— proved correct. The zebra mussel had officially arrived. As word spread, biologists began to count mussels.

Over a four-day period at the beginning of August, 1988, the Great Lakes Institute measured zebra-mussel densities at ten sites in Lake St. Clair. They found densities as high as 200 mussels per square meter. Because new zebra-mussel populations tend to increase by an order of magnitude, and sometimes two, every year, the biologists realized that they had detected the animals at a very early stage of their North American career. Where they were now finding 200 zebra mussels, the biologists reasoned, there might very well be 20,000 the next time they counted. It was by working backwards from the populations they found in 1988 that biologists were able to establish 1986 or late 1985 as the probable time of the mussels’ arrival.

Downstream from Lake St. Clair lies Lake Erie. Shallow, warm, and fed by chemical nutrients that pour into its tributaries from the agricultural lands that surround it, Lake Erie is the largest freshwater fishery in the world. The sports fishery operated from the Ohio shore alone is worth $900 million a year. Joseph Leach, a limnologist, or specialist in lakes, at Ontario’s Lake Erie Fisheries Station, was immediately interested in the news that an unfamiliar competitor had arrived in the food chain upstream. When something hungry enters the water, something else may have to leave.

“I already had some plankton stations in the western basin,”Leach says, “so it was a simple matter for me to add another species to what we were already looking for.” There were good reasons to look. Lake Erie’s western basin is studded with rocky shoals, which zebra mussels love. In addition to the tongue-like “feet” that mussels use to push themselves along the bottom, zebra mussels, alone among freshwater mussels, possess byssuses—clumps of threads that allow the mussel to attach itself to hard surfaces, like rocks or steel or other mussels. Mussels can attach to mussels that attach to mussels. Populations of 10,000 to the square meter are common in Europe. By comparison the Lake St. Clair mussels seemed remarkably sparse. Possibly they would be denser downstream.

Habitat was only one reason to think so. Unlike native mussel larvae, which disperse themselves by hitching rides on fish, zebra-mussel larvae are veligers, meaning that each possesses a velum—a clump of cilia, similar to tiny hairs—that enables it to suspend itself in water. Zebra-mussel larvae in a current can spread with remarkable swiftness. As it happens, there is a considerable current in Lake St. Clair. Every nine days an amount of water equivalent to the entire volume of the lake empties into the Detroit River, from which it empties into Lake Erie. The river pours 5,400 cubic meters per second into the choice habitat of Lake Erie’s western basin. If zebra mussels had arrived in Lake St. Chair in 1986, then Leach’s plankton stations in western Lake Erie, owing to their downstream location, might very well have intercepted populations of drifting larvae already. “I found them right away,” Leach says. “I found them every time I went out, and everywhere I checked. By the end of the summer I was finding them halfway down the lake.”

DREISSENA POLYMORPHA IS A FETCHING CREATURE. Often no more than half an inch long, it comes in a handsome shell marked by alternating bands of light and dark. These stripes come in many patterns. Some stripes are broad and smooth, others zigzagged. Whatever their appearance, zebra mussels require a certain minimum level of calcium in their habitat, a level not available everywhere in North America. Extremes of acidity or alkalinity are also inimical to the mollusk. It is unlikely, for example, that zebra mussels could colonize the acidified lakes of the Adirondacks. Nevertheless, they can survive almost anywhere in a range that covers about two thirds of the United States and the southern part of Canada. In terms of temperature, only Canada’s cold northern lakes and the warmer waters of the American South will nor sustain them.

There seems to be no doubt that the zebra mussel will colonize much of this vast continent. Its feats of propagation are prodigious. A female zebra mussel can produce 40,000 eggs a year, and the male a like amount of sperm. Even if only a small percentage of these eggs are fertilized and advance to maturity, the rate of proliferation will be impressive. By last year the zebra mussel had spread through most of Lake Erie. Biologists studying the western basin in the spring and fall were shocked by the progress it had made. In May zebra mussels were found on Little Chicken Island Reef—a prime walleye spawning ground—at a density of 1,200 to the square meter; by October the density was 9,000 to the square meter. On Hen Island Reef the May count was 3,500 to the square meter; five months later the count was 23,000.

“Some of those reefs in the western basin are pitted

limestone,” Leach observed late last winter, “an ideal spawning ground for walleye, lake whitefish, and white bass. The pits give the eggs a place to settle. At the same time, the reefs are swept by currents. This supplies the spawn with oxygen. The zebra mussels will compete for that oxygen. Organic debris and the mussels’ own feces will catch in the spaces between their shells. The decomposition of that material will also draw oxygen from the water. I wonder whether there will be enough oxygen left to support the spawning process.” The zebra mussel also threatens native fish with its insatiable appetite for phytoplankton, the microscopic green plants at the very bottom of the aquatic food chain. Because many fish feed on the zooplankton that eat phytoplankton, tinkering with phytoplankton is tinkering with one of nature’s building blocks. If the mussels suck too much phytoplankton out of the water, the population of zooplankton could crash. That would have disastrous consequences for certain fish, for the fish that prey on those fish, and for the ducks that eat them and the people who catch them. In this way the zebra mussel, in what amounts to a nanosecond of evolutionary time, could alter the ecosystem of the entire Great Lakes and, when it gets around to it, most of the United States. The question is, How much alteration will there be?

David Garton, a professor of zoology at Ohio State University, who has done field work at Ohio State’s F. T. Stone Laboratory, on South Bass Island, in Lake Erie’s western basin, believes that fish will not necessarily be prevented from hatching by the colonization of the spawning reefs by zebra mussels—at least not because of oxygen starvation. “The water is cold when the walleye spawn,” he points out, “and there is more oxygen in the water at cold temperatures. Not only that, the mussels are always jostling around. They let go of their byssal attachments in order to rise to the top of the heap. They’re always playing king of the mountain, because whoever is on top has a better crack at the food. This jostling would tend to promote circulation.”

Leach believes that wave action is a more important supplier of oxygen than the jostling of mussels. This spring divers found wave action on the reefs to be unimpaired by the mussels, and this has somewhat dampened Leach’s earlier concern. “We went out and made measurements in those areas,” he now says, “and found the oxygen to be okay. That’s not to say it might not become a problem later—for example, when the mussels are thicker on the reefs.”

Leach lives in Kingsville, Ontario, beside Lake Erie, and he works on its shore and watches what emerges from the nets. When dead zebra mussels piled up a foot deep on the west beach of Point Pelee, the event had a personal impact. “When you have something like the zebra mussel, it disrupts the system,” Leach says flatly.

“When you introduce a new biomass and there is no change in the productivity of the system, then something’s got to give.”

One of Leach’s worries, that zebra mussels are sucking the water clean of nutrients, is not an idle one. Last year he lowered a secchi disc—a flat circle painted in alternating quadrants of black and white—into Lake Erie. Low - ering a secchi disc into the water until it disappears is a way to assess the water’s clarity, a property that is of interest because zebra mussels tend to clarify water. In Russia scientists have observed that the mussels clean algae pollution from canals. Clarity might be aesthetically pleasing to people, but walleye, a large commercial crop and Lake Erie’s most popular game fish, loathe it. Walleye are dark-adapted fish. They like to hide. Research suggests that an increase in clarity will drive the walleye population into deeper water. When Leach hauled up his secchi disc and checked the records, he found that the Water was twice as clear as it had been the year before.

Coupled with a plunge in mean chlorophyll—a measure of the prevalence of phytoplankton—this indicated that a lot of food was being cleaned out of the water.

“Yes, We’ve Got Them”

IT IS STILL TOO EARLY TO TELL IF ZEBRA MUSSELS will drive the walleye out and damage the fishery; invaders have colonized the Great Lakes before. Alewives, at first considered a plague, ultimately turned into a useful forage species for other fish. The predatory sea lamprey, in contrast, preys upon native species and is still the target of a relentless abatement program decades after it penetrated the lakes. But whatever the zebra mussel’s impact on North America’s ecology, for man the newcomer is bad news.

In the bright pink of a clear dawn sky the smokestack plume of Detroit Edison’s Monroe generating station is visible from twenty miles away. Detroit Edison supplies electricity to half the people in Michigan, and the 3,000megawatt coal-fired facility in the city of Monroe, on Lake Erie, is its largest plant. Last year Detroit Edison checked the Monroe plant’s forebay—the area that draws in water for coolant. The canal is lined with steel-sheet piling. There are steel grates. There is concrete. It is, in short, a great place to put down byssuses. “You have a unidirectional flow,” says Don Schloesser, a fisheries biologist with the U.S. Fish and Wildlife Service in Ann Arbor, Michigan. “That brings food to the organism. It continually gets new food, plus dissolved oxygen, and its waste gets carried away. It’s an environment they like.” In February, Detroit Edison found densities of zebra mussels there of up to 1,000 mussels per square meter. “We went back in the fall,” says William Kovalak, the utility’s senior biologist. “Because of the European experience, we expected to find an increase of seventy times the spring population. Instead, we found seven hundred times the density. They were so thick on the trash bars that you couldn’t see the individual bars—and those bars are three inches apart. They passed right through threeeighths-of-an-inch mesh inside the plant. Our message to other utilities is, Don’t think you have two or three years. When they show up, they’re right onto you.”

Janiece Romstadt, who runs the water-treatment plant at Oregon, Ohio, a Lake Erie city of 30,000, says, “We put in a brand-new intake structure in the fall of 1988. We looked at it again in September of 1989 and it was completely covered. It’s a wood-and-steel structure, and the diver couldn’t even tell where the wood ended and the steel began.”

Ontario Hydro is one of the largest utilities in North America. It operates eighty generating stations—fossilfuel, hydroelectric, and nuclear. It is a net exporter of power, mostly to the United States. In 1988 alone it sold five billion kilowatt hours outside the province, netting $38 million. “Yes, we’ve got them,” says Renata Claudi, a senior scientist in Ontario Hydro’s Environmental Protection Department. “We found them in the fore bay at Nanticoke at a hundred thousand to the square meter. That was last fall. We’ll have more this year.”

Don Lewis is a biologist with Aquatic Sciences, a company in St. Catharines, Ontario, that specializes in underwater construction and environmental assessment. In 1988 none of the company’s business came from mussel abatement. One year later this new sector accounted for 60 percent of the environmental division’s activity. “It’s full-time for me now, and I’m swamped,” Lewis says.

NOTHING WILL STOP THE ZEBRA MUSSEL’S SPREAD. The extension of its range is part of what some biologists describe as the homogenization of the planet—a process both inevitable and inexorable, as species after species casts itself abroad. It can be a process of breathtaking rapidity. “Some of these new species reproduce so fast that they develop new morphs—genetic alterations,” says Edward Mills, a Cornell University biologist. “Sometimes they go into hyperjump,” agrees James Carlton, the director of the Williams College-Mystic Seaport’Maritime Studies Program and a veteran student of biological invasions. “It took the zebra mussel a single year to get from one end of Lake Erie to the other,” Mills says. “Now it’s in Lake Ontario. The Erie Canal admits Lake Ontario traffic into New York State. There are ninety-three thousand boats a year on that canal.”

A bill now before Congress, the Non-Indigenous Aquatic Nuisance Act of 1990, would force incoming ships to discharge freshwater ballast at sea and replace it with saltwater ballast; out would go the stowaways, into the inhospitable deep ocean. In addition, the bill would secure funds for research into ways to control the spread of exotic species already here. But what of the present? Detroit Edison is trying to control mussels be scraping and hydroblasting during regular maintenance. Janiece Romstadt has received federal permission to use a commercial mollucicide. Ontario Hydro is treating some of its coolant with hypochlorite, an oxidant that chews away at the soft parts of the organism and is the active ingredient in household bleach; the utility admits, however, that this short-term solution is offensive to a public anxious about the environment. One alternative is ozonation. Like hypochlorite, ozone is an oxidant; it is also environmentally benign. But it is extremely expensive. Ontario Hydro estimates that ozonation would cost it $9 million per plant. John Stanley, of the U.S. Fish and Wildlife Service, puts the bill for re-engineering, maintenance, and other forms of mussel abatement at almost half a billion dollars a year. But none of the emergency measures, though they may alleviate specific problems here and there, will do anything to halt the overall proliferation of zebra mussels.

It would be nice if something were to come along and eat the zebra mussel out of our lives, but this is a slender straw and no one is grasping at it with much conviction. A fish called the drum, a bottom feeder, does seem to like the zebra mussel. Sadly, this benefits only the lowly drum itself. “There’s not much demand for them,” the limnologist Joseph Leach says. “One Ontario fish plant experimented with fish sticks made of drum, but no one liked them. Someone in Ohio tried to market pieces of drum fried in oil. They called them ‘drum drops.’ Nobody liked those either.”At one site on Lake Erie a population of lesser scaup, a kind of diving duck, has also begun to prey on zebra mussels. In 1987 there were a mere twenty scaup at the site. In 1988 the scaup discovered Dreissena, and the visiting scaup population shot up to 800. Last year it soared to 13,500. “I don’t think they’ll have much impact on mussel control, though,” Leach says. “The European experience is that only overwintering waterfowl have an effect. Our populations are migratory.”

Temporary relief—a breathing spell, really—could come in the form of a population crash. A standard pattern of biological invasion is spectacular growth followed by a crash. The crash might come about through predation, as native species learn to eat the newcomer, or by disease, or by starvation, once the invader has looted the cupboard bare. However it happens, the crash is usually followed by another spurt of growth, then another crash, and so on, until at last the animal finds a population level it can sustain. Will the zebra-mussel population crash? No one knows.

Next Stop: The Mississippi

“I RATE THIS AS A MAJOR ECOLOGICAL DISASTER,” says Douglas Dodge, of the Ontario Ministry of Natural Resources. “There are two things that will affect the total cost. One is the range of the animal. The other is, What will its final level of population be? In Sweden its numbers have not come down. They’ve stayed up for ten years. That’s one of the things about an invading species. The checks and balances aren’t there. Its numbers go sky-high. As to its range, my gut feeling is there is no conceivable way of preventing its spread. Even if ninety-nine percent of the boats leaving the Lakes scraped their hulls, all you’d need would be one boat with mature mussels on the hull and it’s game over. At Cleveland there’s a very narrow barrier between the Great Lakes and the Ohio drainage system.”

Tom Nalepa, a biologist with the National Oceanic and Atmospheric Administration, agrees that the mussel’s spread is inevitable. “The general opinion now is that this summer it will spread throughout Lake Ontario. From there, through the Erie Canal, it can enter the Hudson River system. As for the Great Lakes, I don’t think it’ll be as abundant in the other lakes as it is in Lake Erie. Lake Erie is perfect for it. It’ll do best in the shallow water and in bays.”

The way into the American heartland lies through Lake Michigan. A scries of channels known as the Chicago diversion pumps 3,200 cubic feet of water per second from Lake Michigan into the Des Plaines River. From the Des Plaines, where it dilutes the effluent from Chicago’s sewage-treatment plant, the water passes into the Illinois River. The Illinois drains into the Mississippi. “The Mississippi would be a good environment for them,” Nalepa says. Spreading through the waterways that lace the continent, the mussels’ progeny will drift in their hundreds of thousands on the rich warm currents. Carried on boats and the feet of water birds, the invaders will make their way up streams and over land.

In a survey of the European experience with Dreissena polymorpha prepared by the Ontario environment ministry, the authors’ forecast for North America is bleak.

The mussels are very strongly byssate and they will attach to insides and occlude the openings ot industrial and domestic pipelines, clog underground irrigation systems of farms, greenhouses and any other facility that draws water directly from the Great Lakes, encrust navigation buoys to the point of submerging them, and encrust hulls of boats and other types of sailing craft that remain in the water over the summer and fall. . . . The mussels may also become a significant vector of parasites that are lethal to game species of waterfowl and fish.

Last spring zebra mussels were reported by observers in Green Bay, Wisconsin, and Duluth, Minnesota. They had entered the upper lakes.