Lake Erie's ecology is shifting. And those intimately associated with the lake agree that two factors, phosphorous and zebra mussels, have dramatically influenced its ecosystem. But there ends any consensus.
Because of Lake Erie's size and diverse uses, understanding its ecological and economic complexities compares to understanding the ocean, scientists say. For instance, a clean, clear lake benefits those swimming, boating, and drinking the water. In contrast, a more opaque lake may aid in growing more fish, helping the lake's fishing operations.
The issue for lake watchers is what is causing this lake to change and what are the long-term implications. They disagree about why the numbers of fish caught by commercial and sporting fishermen have gone down since the late 1980s. Fishermen say the lake doesn't have enough nutrients, namely phosphorous. But scientist Joseph Koonce of Case Western University in Cleveland says computer models show current fish populations probably developed from predator-prey cycles and Lake Erie's regulatory policies.
In 1969, mercury contamination closed the fisheries. In the following years, overfished walleye recovered and then the population exploded. In the late 1980s, "even really bad fishermen like me could go out and catch fish," Dr. Koonce says.
Scientists also disagree about the extent and implications of the 1988 zebra mussel invasion. A native of eastern Europe, zebra mussels probably arrived through ships emptying their ballast water into the lake. With no natural predators in the lake, the zebra mussel population expanded.
Feeding on algae, zebra mussels may leave little food for small invertebrates called zooplankton, says Tim Johnson of the Ministry of Natural Resources in Wheatley, Ontario. He points out that because young fish eat zooplankton, but not zebra mussels, this may disrupt the flow of energy from larger to smaller organisms. "The zebra mussels are taking what production is available and moving it from the top of the water column down to the bottom," Dr. Johnson says.
Dave Culver of Ohio State University in Columbus disagrees. Bottom-dwelling zebra mussels have increased biological activity in the lowest regions of the lake, he says. But zebra mussels can't get to algae more than three feet above them, he adds. That suggests zebra mussels may affect the types of fish populating the lake's bottom, but shouldn't affect those circulating in higher waters.
If the fish population is declining, contention has arisen over how best to deal with the issue. Some fishing organizations suggest adding more phosphorous to grow more algae and, thus, more fish. However, that idea stirs up images of a dirtier, less recreational lake as when the lake was an ecological bellwether in the 1960s.
Back then few fish could grow in Lake Erie. Algal blooms from excess phosphorous gave the lake a slimy, murky appearance. The blue-green algae made the lake smell. And bacteria digesting dead algae depleted much of the lake's oxygen, leaving little for other organisms..
The Clean Water Act of 1972 signed by the US and Canada helped make Lake Erie into an environmental success story. Both countries reduced phosphorous levels by upgrading sewage-treatment plants, lowering the phosphorous content in laundry detergents, and encouraging agricultural practices aimed at keeping fertilizers in the ground, says Paul Bertrum of the Environmental Protection Agency in Chicago. The Clean Water Act set target levels of phosphorous for Lake Erie at 11,000 metric tons a year.
In 1972, about 28,000 metric tons a year of phosphorous entered Lake Erie. Today, scientists estimate the level may have fallen below 8,000 metric tons. The decreases combined with the zebra mussel invasion have caused some to worry that phosphorous levels are now too low.
"It's the $10-million question," says Bill Culligan of the Lake Erie Fisheries Unit in New York, commenting on the phosphorous strategy. "But if we add more phosphorous, we might grow more zebra mussels than fish."
Although overall amounts of phosphorous have declined, laboratory experiments show zebra mussels digest and excrete concentrated phosphorous, says Bob Heath of Kent State University in Ohio. In laboratory experiments that simulate the conditions in Lake Erie, zebra mussels effectively raise phosphorous levels at the lake's bottom.
Other changes in the lake may also stem from zebra mussels and the reduced phosphorous. Phil Ryan of the Ministry of Natural Resources in Port Dover, Ontario, says the composition of fish has changed in recent years. Other scientists note that amphipods, shrimp-like animals living in close proximity to zebra mussels, have increased. And a species of mayfly, thought lost from the lake, now flourishes.