The U.S. and Canada took steps to reduce the release of phosphates, and the water quality had been improving-until recently. In the center of Lake Erie, the largest dead zone in decades is forming.

Case Western Reserve University Geologist Gerald Matisoff is a lead researcher on a two-year exploratory project. He says what goes on in the lake can have serious consequences for life on the shore.

"We have a major fishing industry, and not only sports fish, but there are some commercial fisheries," says Matisoff. "We drink the water, so water quality is important. We use it for recreational purposes, so undesirable toxins that would be in the water would certainly be a health risk."

Matisoff says a leading theory is that zebra mussels, first introduced in the region in 1989, have significantly changed the ecosystem.

"It's a native of the Black Sea region of Europe, and it has spread throughout much of Europe," Matisoff explains. "And we think that ship traffic has brought those organisms into the Great Lakes."

Matisoff says the bottom-dwelling zebra mussels have so successfully adapted to the Great Lakes that they're disturbing the natural food chain, and bringing much more activity to the bottom of the lakes.

Just like people need to breathe air, fish would suffocate without oxygen in the water. During the summer, the cold water on Lake Erie's bottom doesn't mix with the warmer surface water, which has oxygen. If the mussels and other organisms use up all of the oxygen on the lake floor, it could wreak havoc on the ecosystem until the entire lake cools off in the fall allowing the waters to mix again.

Matisoff says another theory on the dead zone looks at the impact of climate change. As part of a two-year project to find the causes and solutions for the oxygen depletion, researchers are spending the summer on the lake gathering data.