UNIVERSITY PARK, Pa. -- Hunter Carrick is a scientist, not a detective, but he is involved in an intense effort to unravel a mystery involving Lake Erie -- the shallowest and once most polluted of the Great Lakes.

Something is causing a large region of depleted oxygen -- sometimes called the "dead zone" -- to form each summer deep in the central basin of the lake, stretching from Sandusky, Ohio, to Erie, Pa. Nobody is quite sure what is responsible for oxygen levels sagging so low that almost nothing can live in Lake Erie's depths by August each year. But the assistant professor of aquatic ecology in Penn State's College of Agricultural Sciences and his colleagues are closing in on suspects.

Some researchers believe the recent invasion of millions of diminutive shellfish -- unwanted intruders from Asia deposited in the Great Lakes by the bilges of ocean-going ships -- are playing a key role in the formation of the dead zone.

In less than two decades, scientists theorize, the prolific zebra and quagga mussels -- each no bigger than an adult's thumbnail -- have become so numerous that their constant water-filtering feeding activity has changed Lake Erie's aquatic environment.

"It appears that exotic species may have altered the food web and the associated nutrient transfers and cycles," Carrick explains. "One idea to explain the expansion of the low-oxygen area is that the mussels filter large volumes of water, straining out material in the water column and depositing it onto the bottom near them. The organic material deposited on the bottom by the mussels is creating a greater demand for oxygen as it decomposes.

"Having said this, we cannot rule out the importance of the physical factors that may play a role in lowering lake oxygen levels," Carrick adds. "For instance, lower water temperatures and water levels in the Great Lakes could reduce lake oxygen content."

As an ecologist at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich., in the late 1980s, he saw oxygen depletion decline markedly as pollution in Lake Erie was curtailed.

The cleanup of Lake Erie -- once plagued by chemical- and detergent-laced discharges from heavy industry and poorly treated sewage from large municipalities such as Cleveland, Ohio, and Buffalo, N.Y. -- was one of the world's greatest pollution-recovery stories. By the end of the 1960s, the central basin of the lake was so fouled by sewage and industrial effluent that fish populations plummeted or disappeared, and boaters and swimmers avoided the filthy water.

But thanks mostly to clean water regulations and improvements in pollution-fighting technology, the lake has made a miraculous comeback. In recent years, Lake Erie has been considered one of the world's best walleye and smallmouth bass fisheries. That's why current levels of oxygen depletion and the summer dead zone concern Carrick and his peers.

"We all have contributed to a major turnaround," Carrick says. "To see interruptions in these trends is discouraging. When oxygen-depletion rates were at their worst in the '60s, Lake Erie experienced the extinction of local fish populations. We lost other important aquatic species. Pollution was the source of major food-web changes. Lake Erie is a real success story in terms of pollution abatement, ecosystem management and habitat restoration."

Zebra and quagga mussels -- which are so similar that they are hard to distinguish, both sporting black stripes on tan bodies -- are established throughout the Great Lakes chain. They are triggering changes in ecosystems to varying degrees, but nowhere is their impact felt so greatly as in Lake Erie, according to Carrick. "Lake Erie suffers from a double whammy," he says.

"Zebras colonize on harder surfaces in shallow water; quaggas tend to colonize on soft sediments in deeper water," Carrick explains. "So there is almost no habitat in Lake Erie that cannot support some species of exotic mussels. The deeper Great Lakes don't provide optimal habitat for them. Erie is shallow, with hard substrate in shallower water and soft bottom in many deeper areas."

Another exotic species that could somehow figure into the summer oxygen-depletion problem in Lake Erie, according to Carrick, is the round goby. The strange-looking Asian fish also presumably was introduced by ship bilges. Because it eats zebra and quagga mussels, its population is exploding in Lake Erie. Scientists just don't know yet how or if it might fit into the oxygen-depletion puzzle.

The dead zone and oxygen-depletion problem in central Lake Erie's depths attracted the attention of the U.S. Environmental Protection Agency last year. EPA officials moved quickly to deal with the problem, dispersing a $500,000 research grant to a group comprised of 17 educational and government institutions that banded together to study the phenomenon and recommend how to resolve it. Among the institutions are Penn State, the University of Windsor, Case Western Reserve University, University of Waterloo, Michigan State, Ohio State, the National Oceanic and Atmospheric Administration's Great Lakes Laboratory and the Canadian Center for Inland Waters.

Carrick spent most of the summer of 2002 as chief scientist on the EPA's Lake Guardian, a research vessel investigating the Lake Erie dead zone. The ship had 15 to 20 scientific staff, with three Penn State School of Forest Resources students onboard. They were graduate students Barrett Gaylord and Daniel Gillenwater and Morgan Johnston, an undergraduate student majoring in environmental resource management.

"Since there is little that can be done to remove the mussels," Carrick says, "governmental agencies may have to consider tougher pollution controls on shore to counteract the recent changes. We still are trying to figure out what the problem with Lake Erie is. That could be the easy part. Solving it may be the real challenge."

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EDITORS: Contact Hunter Carrick at 814-865-9219 or hjc11@psu.edu.

Contact: Jeff Mulhollem jjm29@psu.edu 814-863-2719 814-863-9877 fax #285