Natural Filters for Polluted Waterways

In the Delaware River and other watersheds in the U.S. and Europe, threatened populations of mussels are being re-established to help filter out pollution generated by agriculture, industry, and development

One of the U.S. East Coast's major watersheds, the Delaware River and tidal estuary, has its share of charismatic species — striped bass, American shad,Atlantic sturgeon. And yet, when the Partnership for the Delaware Estuary went looking for a species that might focus their own and the public's attention on the issues of ecosystem restoration and improved water quality, Danielle Kreeger, the organization's chief scientist, says the clear choice was the freshwater mussel.

When it comes to ecosystem services, freshwater mussels have much to recommend them: They are voracious filter feeders able to cycle gallons of water per hour. In streams and in freshwater tidal habitat they capture phytoplankton and small particles — living and non-living — and leave behind clearer water and a nutrient-rich detritus that fertilizes the stream and its sediments. They are, as David Strayer, a freshwater ecologist at theCary Institute for Ecosystem Studies, puts it, "nutrient capacitors — capturing nutrients at one time, storing them and releasing them at a later time."

Comprised of roughly 900 known species, freshwater mussels live on every continent but Antarctica. And everywhere they are facing declines in diversity, abundance, and range. As U.S. Forest Service biologist Wendell Haag put it, freshwater mussels have "among the highest extinction and imperilment rates of any group of organisms on the planet."

In Europe, where there are some dozen species, the freshwater pearl mussel is already a cause célèbre. A once-prized source of freshwater pearls harvested by the millions is now red-listed as threatened across most of its range, from northern Europe and Scandinavia to the U.K. (Its range also extends across the Atlantic to Canada and the northeastern U.S.) China has some 60 species, and Africa about 100 species.

But North America, with more than 300 species, is the center of freshwater mussel diversity. In the upper Midwest, mussel beds once supported a mussel shell button industry worth tens of millions of dollars. The Mississippi River basin alone, according to Haag, supports three to four times as many mussel species as the Amazon-Orinoco or Congo River basins. The Tennessee River has some 100 species. In some parts of the eastern United States, says Haag "more mussel species can be found in a square meter of river bottom than occur on the entire continent of Europe."

Everywhere, however, numbers of mussels are in peril. Over the last 100 years, 30 North American mussel taxa have gone extinct and 65 percent of those remaining are considered endangered, threatened, or vulnerable. Half of these are not likely to survive the century.

Everywhere the causes of decline are the same: alteration and damming of streams and rivers, and human-induced runoff of silt and nutrients.

A stream with a healthy population of mussels indicates a pretty pristine habitat and good water quality, as well as a population of fish that's able to serve as hosts to the mussels' parasitic larvae. Once established, mussels stick around. Among the earth's longest living invertebrates, some species survive 100 years and more. This means that if even a small part of a population can endure hard times they might recoup their numbers as conditions improve.

"We know their potential is significant," says Kreeger, whose organization is based in Wilmington, Delaware. Significant enough that water companies with intake pipes along the Delaware River have expressed interest in seeing whether restored populations of freshwater mussels might eventually relieve the companies of some of the burden and expense of mechanical water filtration.

"What if we rebuilt the mussel beds in front of the Philadelphia Water Department's intakes?" says Kreeger.

What makes them so endangered are the very things that make them so attractive an umbrella species to Kreeger and other scientists. Freshwater mussels' long lives mean that they face long-term exposures to environmental assaults. When their fish hosts are lost to dams or impoundments they have no way to grow their populations. Most critically, being filter feeders, which means they feed on particles and small organisms in the water, they are the first victims of the increases in silt and loads of excess nutrients that come with runoff from developed urban and agricultural land.

Mussels need a substrate of clean, coarse sand in which to burrow and a supply of well-oxygenated water. Silt clogs the substrate. Excess nutrients wreck the chemistry and deplete the oxygen. Broken down by bacteria, excess nutrients produce ammonia that, new studies indicate, may be deadly to mussels even at low levels. When floods of storm water runoff finally scour the streambeds down to the bedrock, mussels have nowhere to dig in. Strayer believes that pristine rivers once averaged anywhere from one to ten mussels per square meter and perhaps ten or more times that in the most densely populated beds. But nutrient loads have doubled and more since these Edenic times.

The Delaware River once had roughly a dozen native freshwater mussel species. While several still survive in isolated populations, the majority of the river's mussels—and the species Kreeger'sFreshwater Mussel Recovery Programhas, since 2007, focused on — is the Eastern elliptio, a dark shelled mussel up to five inches long with a pearly pink interior. While rare in the tidal estuary, elliptio is common in the river above the reach of the tides at Trenton, New Jersey.

William Lellis of the U.S. Geological Survey (USGS), who surveyed the upper Delaware mussels, estimates there might be as many as two million per river mile. With no dam along the Delaware's main stem, the American eel, which hosts the mussel's developing larvae on its gills, can repopulate mussels into beds far upriver, where, as aWild and Scenic River, the runoff is freer of silt and nutrients than in the highly developed and industrialized estuary. There, the nutrient loads can be very high. The U.S. Environmental Protection Agency's lastNational Coastal Condition Reportfound that the Delaware estuary had more high-nitrogen sites than any other Northeast coastal estuary. Surveys have shown that of 70 Pennsylvania streams, for instance, only four contained any freshwater mussels.

Kreeger realizes it's critical to find out where and how many mussel beds remain in the estuary, determine where they might still survive if given the chance, and then try to restore those streams to the point where their mussel populations could be rebuilt. (To reach more streams and involve the public in the project, Kreeger has enlisted and trained volunteers to survey mussels in their local streams.) In 2011 and 2012, Kreeger reintroduced mussels into three southeastern Pennsylvania streams and, despite severe flooding, most of the animals survived. The idea, of course, is that, once established, mussels will begin to clean both the water and sediments in their new habitat. Can they do it?

Until now, most mussel research focused on trying to preserve and protect highly imperiled populations. "We're finally looking at the ecology," says Lellis, as well as getting crude estimates of just how capable mussels might be at filtering water. "Let's say we had a half billion mussels and each could filter a gallon an hour," says Lellis. "You can come up with a number that excites the imagination."

Kreeger hopes her math will do just that: In four sites surveyed in the tidal freshwater portion of the Delaware River, Kreeger estimated that some 670,000 mussels could be filtering 6.3 million gallons of water per day — an amount equal to 1.6 percent of the drinking water Philadelphia withdraws from the Delaware. The idea, says Kreeger, is to gradually scale up by expanding restoration of habitat, hatchery production of mussels, and reintroduction of mussels into streams. Even if there's no natural reproduction, Kreeger says, "if you believe in an ecosystem cost/benefit, reseeding every ten years at a cost of $50,000 might be worth it if you can save the cost of mechanically providing water quality."

Her counterparts in Europe see things the same way.

"It makes sense to focus on mussels," says Juergen Geist, chair of Aquatic Systems Biology at Technical University of Munich. "But it also needs to be a watershed-wide effort."

Source: Resilience

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