Zebra mussels invaded the Great Lakes in 1988 and since then have coated boat hulls and clogged the water-intake pipes of power and sewage- treatment plants. Navigational buoys have sunk under their weight, and their razor-sharp shells have befouled beaches. The pesky mollusks hitchhiked across the Atlantic from their native European waters, probably in the ballast tanks of ships. But for all their faults, it now seems that they have at least one redeeming quality: they may be helping to restore the populations of some bottom-dwelling aquatic plants to the Great Lakes.
David Moore, a botanist at Utica College of Syracuse University, recently surveyed the aquatic plants at Put-in-Bay Harbor on the southwestern end of Lake Erie. Put-in-Bay has been the site of several such surveys over the past hundred years, including an exhaustive one done in the late 1960s by botanist Ronald Stuckey of Ohio State. With the help of some students from Ohio State, Moore collected plants from the harbor bottom. He found that eight native species had returned to the bay for the first time in 30 to 50 years.
Moore and his crew also identified five species that have dwindled since Stuckey’s survey. Most of them are nonnative--such as the European water milfoil, which in its own way has been as much a scourge of American waters as the zebra mussel. And all of them are plants that can tolerate turbid waters. When Stuckey did his survey, says Moore, Put-in-Bay was quite murky--due to runoff from local vineyards, exhaust from boats, and the absence of a sewage system in the town. Many of the native plants-- annuals whose seeds must settle to the bottom to germinate--were at a disadvantage: their seeds weren’t getting the light they needed, because it couldn’t penetrate the murk.
Today the waters of Put-in-Bay are clear again--and Moore and Stuckey credit the arrival of the zebra mussel. Zebra mussels are famous for their ability to filter water, nearly stripping it clean of plankton and algae. Even after the town installed sewage treatment in 1983, Stuckey says, the harbor had remained turbid--but in the late 1980s the water began to clear up. Now we can actually see down to the bottom of the harbor, perhaps 18 feet. While that’s bad for some fish and other animals that live off plankton, it has been good for native species of bottom- dwelling plants. And on the whole it has even been good for fishing: both bass and catfish feed on tiny animals that attach themselves to the plants.