When dams went up on the Susquehanna nearly a century ago, their impact may have reached far beyond the migratory fish whose movements were blocked.

Recent studies by the U.S. Geological Survey suggest the structures may have also affected the reproduction of a normally abundant species of freshwater mussels—often located far upstream of the dam. The loss of those mussels, and their water-filtering capability, might be affecting water quality both in the river, and the Bay.

“It has potentially enormous significance to the health of the Bay and the river system if this really turns out to be the way I think it is,” said William Lellis, who heads the USGS Northern Appalachian Research Lab in Wellsboro, PA.

The developing story, if correct, suggests that when power companies built a series of large hydroelectric dams in the lower Susquehanna during the 1900s, they disrupted a delicate ecological linkage no one imagined at the time.

Biologists at the USGS lab, who research rare and endangered mussels, began examining the issue after they completed a large-scale, two-year survey of freshwater mussels in a 125-mile stretch of the upper Delaware River.

The survey showed that 98 percent of the mussels in the river consisted a single species: Elliptio complanata—not a huge surprise as it’s the most common mussel in the Northeast.

But what intrigued the scientists was the scarcity of that species in the Susquehanna basin, where their lab is located. The Susquehanna, Lellis said, has “mile after mile of nothing when it comes to mussels.”

Lellis has not completed as comprehensive a survey on the Susquehanna as he heas on the Delaware, but sampling to date shows that while Elliptio is present, it is nowhere near as abundant as in the Delaware. Further, the populations are old—with little evidence of juveniles being produced.

“Anecdotal evidence tells us that there were huge populations of mussels there,” Lellis said. “Old timers tell us that all the time.”

So he and his colleagues turned their attention to the laboratory. Mussels reproduction requires that their larvae attach as parasites to an intermediate “host” species before they transform into small mussels. Many mussels rely on a few specific host species for that life stage.

Elliptio broadcasts its larvae, called glochidia, into the water along a spiderweb like mucus. In the lab, the biologists found Elliptio glochidia attached to only a few species of fish, and only at relatively low numbers. Some of those, such as brook trout, lived only in restricted areas of the river—not the mainstem where the bulk of the mussel populations would be expected.

But the scientists found two species to which Elliptio attached in huge numbers—lake trout and American eel. Lake trout are not found in the Susquehanna.

American eel are native to the Susquehanna, but they were largely removed from the river when dams were built in the early 1900s.

As a result, Lellis hypothesizes that the lack of a large Elliptio population in the Susquehanna—and the apparent lack of reproduction—may stem from the closure of the river to eels.

Supporting evidence comes from surveys near the lab in Pine Creek, a tributary to the West Branch of the Susquehanna. “We have seven mussel species in Pine Creek, and we found evidence of reproduction in most of the species except Elliptio, which is even more compelling because the other species are much rarer,” Lellis said.

Elliptio still persists because mussels can potentially live for more than a century. Also, although the Susquehanna was closed to eel migration with the completion of the Conowingo dam in 1928, eels remained in the rivers for years before dying out. In addition, the Pennsylvania Fish and Boat Commission conducted a stocking program in the 1970s and 1980s.

The mussel may also persist because they might use other, less effective hosts. But building the huge population seen in the Delaware may require large eel populations, Lellis said.

If the connection between eels and Elliptio is true, the impact goes beyond those species.

Lellis calculated the Delaware River had about 2 million mussels per mile, mainly Elliptio, and those mussels could filter 0.5–1 gallon of water per hour. That extrapolates to the potential of filtering between 2 billion and 4 billion gallons of water per day—or six times the average daily summer flow.

“It shows to me that there is potentially enormous filtration capacity in a functioning healthy system,” Lellis said.

If a larger mussel population once had a similar impact on the Susquehanna, their loss could have been a blow to both the river and the Bay. The mussels pull particles out of the water and—like oysters in the Bay—deposit material they don’t consume as “psuedofeces” on the river bottom, where they remain.

That removes sediment and nutrients from the water. Bacteria quickly consume any nutrients associated with the psuedofeces so they don’t flow downstream, Lellis said. “The amount of psuedofeces they produce is enormous,” he said.

While Lellis has suggested an explanation for the mysterious demise of mussels in the Susquehanna, he cautioned that the case it not yet closed. “We have some intriguing pieces of information that could be put together to make a particular story,” he said, “but an additional piece of the puzzle, or a rearrangement of the puzzle, could tell us a different story.”

If the story holds up under more study, Lellis said it may be a reason to find a way to give eels a helping hand up the river. “If the upper Susquehanna has lost a significant freshwater population due to a human activity, then the ecological services that they once provided have also been lost, and that may have been huge,” he said. “And the river, and ultimately the Bay, may be really suffering for it.”