By Richard A. Lovett
Efforts to clean up the Potomac River, which forms part of the border between Maryland and Washington, D.C., have markedly improved conditions for fish and waterfowl, according to a study published September 6 in the Proceedings of the National Academy of Sciences. The finding raises hopes for success in restoring degraded tidelands and bays worldwide--including Chesapeake Bay, into which the Potomac flows.
Henry Ruhl, an ocean scientist at the National Oceanography Centre at the University of Southampton, UK, and Nancy Rybicki, a hydrologist and environmental scientist with the US Geological Survey in Reston, Virginia, found a correlation between changes in the abundance of submerged aquatic vegetation and improvements in sewage treatment procedures at the Blue Plains Advanced Wastewater Treatment Plant in Washington, D.C. They found that a two-thirds reduction in organic nitrogen entering the river from the plant, mostly in the form of nitrates--a drop from nearly 20 tonnes per day in 1990 to about 6 tonnes per day in 2007--was associated with a dramatic increase in the amount of submerged vegetation in the river.
Such vegetation serves as food for waterfowl, fish and shellfish. "It is the base of the food chain," says Rybicki. It also improves water quality by impeding currents, thus allowing suspended particles to settle to the bottom. "The number of fish, particularly young fish, and the number of invertebrates increase dramatically within the plant beds," she adds.
Submerged plants were once common in the tidal sections of the Potomac, but disappeared when agricultural run off and urban waste water led to too many nutrients entering the water in the 1930s. The nutrients fuelled blooms of plankton, blocking access to sunlight for plants just a metre or two below the surface.
"They were bright green," Rybicki says of the plankton. "They looked like paint on the river."
Rybicki has been studying the river since the late 1970s, when its bottom was virtually barren. It wasn't until the early 1980s, she says, that plants first began reappearing. But for years, the dominant species was Hydrilla (Hydrilla verticillata), a European aquatic weed that had invaded Florida in the 1960s and moved steadily northwards. "People were saying it would take over," she says.
Instead, it is native species that have taken the most advantage of improving conditions. Since 1990, Ruhl and Rybicki have found, on the basis of their own studies and data provided by regional agencies, that native vegetation has expanded from about a mere 1.2 square kilometers to about 12.5 square kilometers. By 2007, Hydrilla was no longer even the most common species.
The finding is a boon for restoration efforts in Chesapeake Bay, for which the Potomac River is a major tributary.
Chesapeake Bay is America's largest estuary, sprawling across 166,000 square kilometres of mostly shallow water and collecting run off from seven states. Once famous for its crabs and oysters much of the bay now suffers from algal blooms, sediment, and low oxygen levels that have hit fish and shellfish populations, according to a 2006 report by the inter-agency Chesapeake Bay Program.
In 2009, recognizing that past restoration efforts in much of the bay had not succeeded, US President Barack Obama signed an executive order recognizing the bay as a "national treasure" and calling on the federal government to lead a renewed effort to restore it.
"The results are exciting because the study clearly shows that restoration efforts are making a difference," says Margaret Palmer, director of the University of Maryland's Chesapeake Biological Laboratory in Solomons. "There is great passion for restoring the bay but we have had a hard time keeping public morale up because the reports for many parts of it have not been good."
Rybicki notes that other freshwater sections of the bay, where pollution originates from other sewage treatment plants that have been cleaned up, also seem to be improving. Parts of the bay, however, are more strongly affected by agricultural runoff, which has been harder to control, she says. But that doesn't mean there isn't hope. "There's a lag effect," she says.
Also significant, she says, is the fact that the study found that despite improvements in water quality, submerged vegetation still suffered in years in which major tropical storms hit. "Given that we are expected to have more intense storms in this region under future climates, it is particularly important to reduce stressors that we have more control over," she says.
William Ball, an environmental engineer at Johns Hopkins University in Baltimore, Maryland, agrees that the findings are encouraging. "Because much of the nitrogen decrease in the Potomac has been due to advanced wastewater treatment, this study is a strong validation of the utility and importance of applying this technology worldwide," he says.
"If we can clean things up in the bay," Palmer adds, "it offers great hope for other ecosystems, such as the Black Sea, the Baltic and many others."