Zebra mussels have turned up in a northern Pennsylvania reservoir, marking the first time the exotic species has been found within the state's portion of the Susquehanna basin.

Biologists discovered the thumbnail-sized mussels May 17 during routine monitoring at Cowanesque Lake, a reservoir used to supplement downstream water supplies during droughts, near the New York state line. The lake is on the Cowanesque River, which eventually flows into the Susquehanna.

The discovery was later verified by the Department of Environmental Protection and Pennsylvania Sea Grant.

It's not the first discovery of zebra mussels in the Susquehanna basin. Biologists from the State University of New York in 2000 discovered zebra mussels in the Eaton Brook Reservoir of Madison County, at the northern tip of the watershed. They have since seen the mussels move slowly downstream.

The new discovery was on a different branch of the Susquehanna, and biologists said it may have stemmed from a separate introduction, as Cowanesque Lake-like Eaton Brook-is used by boaters. The mussels frequently attach themselves to boats and are accidentally transported to new areas.

"The introduction of invasive species like zebra mussels can have a substantial and lasting impact on the balance of aquatic life in a waterway," said Doug Austen, executive director of the Pennsylvania Fish and Boat Commission. "Anglers and boaters should exercise great care when moving from one waterway to another that they don't inadvertently move an aquatic nuisance species with them."

Since the discovery, Pennsylvania Sea Grant has worked with the U.S. Army Corps of Engineers, which owns the reservoir, to alert visitors about the presence of mussels in the lake.

Zebra mussels, named for the striped pattern of their shells, are thought to have arrived in the Great Lakes in 1988 in the ballast water holds of ships from Europe. Since then, they have spread through much of the country.

They threaten aquatic ecosystems because of their ability to filter about a quart of water a day. While that improves water clarity, it disrupts food chains by removing plankton that support native mussels and fish.

They also have the potential to foul industrial facilities and plug public water supply intakes that use infested waters. In addition, they can also interfere with the operation of locks and dams on rivers and damage boat hulls and engines.

While scientists say high salinities would make most of the Bay off-limits for the freshwater species, zebra mussels may find suitable habitat in many rivers, potentially altering food webs and increasing nutrient flows down the tributaries.