Nearly one-third of the nitrogen entering the Chesapeake Bay arrives through the air, and half of that loading originated as nitrous oxides rising from sources like coal-burning power plants and factories along the Ohio River valley (top) and other urban and industrial sites located far from the Bay. While the Bay's watershed covers 64,000 square miles, the Bay's airshed covers nine times as much territory, stretching over 570,000 square miles and extending into 12 states. According to the U.S. Environmental Protection Agency, the Clean Air Act in 2010 saved 165,000 lives and prevented 130,000 heart attacks and 1.7 million asthma attacks. Cutbacks in air pollution are also helping clean the waters of the Chesapeake Bay. Photograph by Alfred T. Palmer (1944), Library of Congress Collection; airshed and watershed maps, Chesapeake Bay Program.

THE WATER IN THE BAY MAY BE GETTING CLEANER, largely because the air is getting cleaner. That's an unexpected and somewhat ironic success story that is emerging from recent research on the upper reaches of some of the Bay's tributaries.

In 1997, the U.S. Environmental Protection Agency began enforcing the Clean Air Act more aggressively, trying to clamp down harder on the release of airborne nitrous oxides. And shortly thereafter, hydrologist Keith Eshleman began seeing drops in the amount of nitrogen washing into the rivers of Western Maryland and southern Pennsylvania, rivers that run into the Bay.

Nitrogen inflow into the Bay is one of the primary causes of many of the Bay's contemporary ills. It overfertilizes phytoplankton and algae blooms, causing cloudy waters, dieoffs in bay grasses, dead zones of no oxygen, and frequent fish kills. And nearly a quarter of the nitrogen that ends up in the Bay begins as nitrous oxides pouring into the air from the exhaust stacks of factories and power plants that burn coal and oil and from the exhaust pipes of cars and trucks and buses that burn gas.

A lot of those power plants and factories and cars are located hundreds of miles away, well outside the Bay watershed. When those nitrous oxides land in the watershed, they can be washed into rivers that lead down to the Bay. And their arrivals have been measured by Eshleman, a professor at the UMCES Appalachian Lab in Frostburg, Maryland. His data come from river systems with stream gauges that have been recording nitrogen inputs for 25 years. His findings are unambiguous. "On average, we are seeing about a 50 percent reduction in nitrates," says Eshleman, who is getting ready to publish his results. "It's a really clear-cut effect."

More evidence of the effect comes from the scientists running the watershed model for the Environmental Protection Agency's Chesapeake Bay Program. According to estimates by Gary Shenk, a modeler for the program, total nitrogen loads to the Chesapeake have been cut by 20 percent since 1985, thanks in part to controls on farm runoff, urban runoff, and wastewater discharges. But nearly half that reduction in nitrogen, 46 percent, has come from cutbacks in atmospheric deposition of nitrous oxides.

These reductions, according to Eshleman, can be traced in part to changes in the much-amended Clean Air Act. Originally launched in 1963, the Act was first given teeth in 1970, with strong amendments added in 1977 and 1990. Under President Clinton, the EPA in 1997 added tougher rules on nitrous oxide emissions, a decision that was controversial and historic and expensive because the new rules required many older factories and power plants in the Midwest to switch to cleaner-burning fuels or install advanced scrubbers similar to those already in use in the Northeast states. In addition, many states had to move more aggressively to reduce automobile emissions and encourage mass transit options. Within a few years the stream gauges in Mid-Atlantic rivers were showing declines in nitrates. "From watersheds with thousands of acres down to small watersheds, we are seeing robust reductions," says Eshleman. "This is a huge success story."

It's also an unexpected success story. Protecting human health was always the primary goal of the Clean Air Act, but protecting the ecological health of the Chesapeake Bay has, ironically enough, been a surprising payoff. According to ecologist Michael Kemp, an expert on nutrients in the Chesapeake, these nitrogen cutbacks are helping both to revive bay grasses along some shallow areas and to slowly reduce the dead zone along the deep mainstem of the Bay. These "secondary" benefits from clean air legislation, in his opinion, outweigh the benefits from clean water legislation or any other effort to improve the water quality of the Chesapeake Bay. "The Clean Air Act," Kemp says, "has provided us with a gift."

— Michael W. Fincham