Overtaken waters: A primer on the zebra mussel’s invasion of the Great Lakes

By Heidie (Raine) Senseman

The zebra mussel. Dreissena polymorpha. Rounded triangle shells, banded with dark brown stripes that mimic the mammal for which they’re named. The size of a pistachio or fingernail. Often clustered together on top of rocks, buoys, crustaceans or water intake pipes. Glossy. Inedible. Sharp enough to slice your feet.

In its natural habitats — Black Sea lagoons and Caspian Sea drainage basins — the zebra mussel is a fitting creature. Its high capacity for water filtration equips it to endure the seas’ salinity. A single zebra mussel can filter a liter of water daily, pulling out phytoplankton, zooplankton and detritus from already salty and sediment-laden waters. Likewise, the mussel’s resilience helps it survive the seas’ low oxygen levels. It seems like a creature made to endure the harsh conditions of its European seas.

But take a zebra mussel out of its saline-sea home, and it will wreak havoc. It will out-compete native species for resources, deplete waters of microorganisms that fend off bacteria, destroy aqueous infrastructures and crowd the ecosystem due to its lack of predators. Put simply, it will turn an environment upside down.

That’s exactly what the zebra mussel did to the Great Lakes.

Dan Egan, author of the Pulitzer Prize finalist “The Death and Life of the Great Lakes,” describes zebra mussels as cancer cells. In his chapter titled “Noxious Cargo,” he explains the mussels’ destructive history. They infested Hungary’s waterways in 1794 and London’s in 1824.   By the late 1800s, scientists recognized zebra mussels as invasive species.

“We define invasive species according to a few characteristics,” Dr. Mark Gathany, professor of biology at Cedarville University, explained. “If it’s a nonnative species that can outcompete native species, overpopulates and has no known predators, we’d call that invasive.”

The mussels embodied this set of characteristics as they continued invading ecosystems. They worked their way into Swiss, Italian and Finnish waterways by the 1970s.

Then, zebra mussels jumped the Atlantic. In 1988, novice biologist Sonya Santavy discovered a zebra mussel in Lake St. Clair, a break in the river system connecting Lake Huron to Lake Erie. The Great Lakes, though distinctly named, make up one continuous waterway inter-connected by nearly 5,000 tributaries. Thus, a zebra mussel in any portion of the lake system compromises the whole.

Sure enough, by 1992, zebra mussels were not only present in Lake Michigan, but they accounted for 98% of the lake’s invasive mussels. The Detroit Free Press estimates today, as many as 750 trillion zebra mussels live throughout the Great Lakes.

Biologists believe the mussels first arrived in the Great Lakes in the 1960s via ship-steadying ballast water. As Great Lake commerce developed throughout the 20th century, Army engineers carved a shallow navigation groove in the center of Lake St. Clair in 1959 to allow foreign ships to pass through and deliver their cargo to American ports.

St. Clair’s shallowness often required ships to lighten their load by emptying their ballast tanks — tanks swimming with exotic species from their native waters. Despite nearly 200 years of scientists pleading with politicians to regulate the dumping of these ballast tank “floating ecosystems,” satisfactory policies weren’t passed until 2008. And thus, the zebra mussel found its way to the Great Lakes before the turn of the 21st century.

The mussels dispersed rapidly. Female zebra mussels produce one million eggs a year, each covered in small “hairs.” The hairs help eggs catch food, ride currents to new locations and form shells. As the shells develop, their increasing weight forces the mussels downward. A mussel finally settles when it binds itself to a hard surface with its leathery-tough plaque, as strong as the epoxies sold by hardware stores.

Those binding fibers are just one effect of zebra mussels that have led them to, in Egan’s words, “suck … the life out of the waters they invade.”

When zebra mussel eggs began infiltrating the whole of the Great Lake system 20 years before Santavy’s discovery, they established themselves at the expense of the lakes.

They fused to boat motors and hulls and ruined the equipment that supports sportfishing industry on the lakes, especially Lake Michigan. They fused to rocks, ladders and docks, which made the surfaces dangerously sharp. They fused to water intakes and compromised water supplies.

In December of 1989, 50,000 people living in and around Monroe, Michigan, lost their Lake Erie water supply for two days because zebra mussels and ice formed a blockage — nearly three feet in diameter — in their water intake pipe.

However, below the surface, zebra mussels were waging another type of war: ecological. As people felt the mussels inconvenience their recreation, economy, equipment and amenities, native species were being wiped out by this intruder.

The zebra mussel’s high capacity for water filtration over-filtered the Great Lakes.

“This nearly vodka-clear water is not the sign of a healthy lake,” Egan explained. “It’s the sign of one in which the bottom of the food web is collapsing.”

In Lake Michigan, phytoplankton levels have dropped by 90% since the mussels took over.

Many native mussels and prey fish — also filter feeders — starved as zebra mussels took all the microscopic nutrients for themselves.

The mussels also cause destructive chain reactions. Hyper-clear lake waters make way for Cladophora, a seaweed-like and oxygen-consuming plant, to thrive. As the plant steals oxygen from the water, botulism-causing bacteria seep into the water and are filtered out by zebra mussels, which are then eaten (at a rate not commensurate to the mussels’ growth) by another invasive species called the goby. Gobies, poisoned by botulism, die and float to the surface of the lake where they become easy food for and poison the birds that eat them.

Thus, the Great Lakes region that was once filled with healthy native species are now, as Egan calls them, “exotic mussel beds.”

The extent to which zebra mussels have overtaken the Great Lakes has forced ecologists to change their approach to the problem. After decades of belated policies regulating ballast water, failed attempts to prevent further mussel growth, and unsuccessful initiatives to eliminate mussel populations, ecologists are accepting that zebra mussels will forever be a part of the Great Lakes.

“We aren’t hopeless about the mussels, but we have resigned in a certain sense of the word,” Gathany said. “We refer to invasive species that have been here a while as naturalized. And with naturalized species, we focus on maintenance.”

Maybe this shift toward maintenance indicates that the ecological community is coming to terms with reality. Scientists have been fighting a losing battle with zebra mussels since 1988. It’s likely time to accept that zebra mussels are the “new normal” of the Great Lakes.

Harvey Bootsma, an ecologist at the University of Wisconsin-Milwaukee’s School of Freshwater Sciences, agrees that zebra mussels are here to stay — but he also longingly remembers what Lake Michigan was like before invasive species overtook it. He grieves that his children will never know the lake that he did.

“I snorkeled some of the same areas that I did as a kid,” Bootsma told Egan in a personal interview. “It was just devastating to see all the mussels. … This isn’t the lake it was 25 years ago.”

But perhaps the most troubling aspect of the zebra mussel predicament is how it could have been avoided had cargo ships handled their foreign ballast water responsibly. Two years after Santavy’s 1988 zebra mussel discovery, Congress told the Coast Guard to begin regulating ballast water from overseas ships, but the Coast Guard only asked freighter ships to flush their tanks instead of requiring them to. Scientists discovered 27 new exotic species in the Great Lakes between 1990 and 2008 as a result. 

Even so, the Great Lakes will survive the zebra mussel invasion. They have survived other significant tragedies: oil spills, runoff and rubble from shoreline fires, and industrial pollution.

Bootsma, however, cares about more than mere survival. He seems to say that even though the lakes survive, they have lost their original shape, character and identity because of human carelessness.

“Water in our minds has a purifying ability,” Gathany adds. “We think, ‘I wash my hands in the sink, so it’s probably fine to wash industry’s hands in the sink, too.’”

That presupposition likely underpinned the freight industry’s belief that ballast water from Europe’s saline seas would pose no threat to America’s lakes.

“But we overestimate that ability,” Gathany said. “And the lakes are bearing the consequences of our miscalculation.”