There's a remote possibility the Chesapeake Bay could soon be transformed into the cleanest estuary in the world. Rivers now turbid and dingy and laden with silt and nutrients may become crystal-clear tributaries free of municipal waste and industrial pollutants.
Local and federal agencies have spent nearly $1 billion to achieve this goal, but the actual cleanup of the Chesapeake may eventually be credited to millions of tiny bivalves known as "zebra mussels."
Unfortunately, these highly prolific, efficient filter feeders could also bring about the demise of the world's largest estuary.
Five years ago, only a handful of U.S. marine scientists knew of the existence of zebra mussels. The tiny mollusks thrived in northern European freshwater lakes, and their range, until recently, was restricted to the Baltic States.
In mid-1985, scientists discovered small colonies of zebras in Lake St. Clair. It's theorized that the mollusks were transported in a ship's ballast tank and deposited in the lake when ballast waters were discharged. It didn't take long for zebras to quickly colonize freshwater lakes throughout the region and, in some instances, dominate all forms of aquatic life.
In the Canadian town of Lincoln, the depth of a reservoir dropped from 13 feet to a scant 6 inches. Public works officials quickly discovered 12-foot-thick zebra colonies. The reservoir's intake water, which flows from Lake Ontario, decreased by more than 36 percent as mussels clogged pipes more than 6 feet wide.
The effect these tiny bivalves will have on the overall health of the Chesapeake Bay is unknown. Although zebras are considered freshwater mollusks, scientists recently discovered they'll tolerate salinity levels of 5 to 10 parts per 1,000. Depending on the time of year, this could allow them to extend their range down the Chesapeake as far as Virginia.
"We're really not sure how far zebra mussels will travel down the bay," said Ron Klauda, a marine biologist at the state Department of Natural Resources.
Klauda, who researches zebra mussels, says DNR officials worry about the effects they'll have on clams, oysters and other indigenous forms of Chesapeake Bay marine life competing for available food sources. Klauda says it's believed that blue crabs will eat adult mussels, but that zebra mussels could multiply faster than crustaceans could consume them.
"We don't know when they'll arrive and what damage they'll do, but one thing's certain: They're coming to the Chesapeake, and they're bound to have a significant impact on the overall health of the bay," Klauda said.
In response to the zebra mussel threat, the city of Baltimore closed all three major reservoirs to recreational fishing from privately owned and operated boats.
"We know the reservoirs will eventually become infested with zebra mussels; it's just a matter of time," said Eugene Scatula, director of Baltimore's zebra mussel control program. "By closing the reservoirs to privately owned boats, we hope to delay the invasion long enough to prepare for their arrival."
Zebra mussels were reported in the upper reaches of the Susquehanna and Hudson rivers last fall, Scatula said, adding, "We could see them as soon as this fall or early next summer."
Zebra mussel monitoring devices have been placed at several key locations in the Susquehanna River, upper Chesapeake Bay, Conowingo Lake and Holtwood Pool.
If the mollusks invade Maryland's portion of the Chesapeake, they could have devastating effects on Conowingo Dam's hydro-electric system, Peach Bottom Nuclear Power Plant and the water supply for Baltimore and Havre de Grace, as well as any industry using water from the upper reaches of the Chesapeake Bay and its tributaries.
A full-grown zebra mussel is about the size of a thumbnail and has a dark-gray shell marked with light-colored stripes.
People sighting these tiny bivalves should report the location to the Maryland Department of Natural Resources by calling 974-3782.