The study suggests that the nitrogen pollution emitted in fossil-fuel combustion from vehicles and electric power plants into sensitive coastal rivers and bays could be twice as great as previous estimates for the northeastern United States. Previous studies focused on the nitrogen in acid rain that falls well away from urban and suburban sources, but the new study shows substantially more nitrogen -- largely in gaseous form -- being deposited near highways and other urban sources.
Indeed, says the study's lead author, Robert Howarth, a Cornell biogeochemist and aquatic ecosystem scientist, "Although we've known that the nitrogen pollution in the atmosphere is also a source of watershed pollution, it's significantly bigger than we thought, yet very few efforts are being focused on reducing the emissions."
Howarth will present his findings at 1:45 p.m. on February 20 at the annual meeting of the American Association for the Advancement of Science in the Marriott Wardman Park hotel, Washington, D.C. He also will discuss his findings at a news briefing at 4 p.m. today in the Marriott Wardman Park Hotel, Lobby Level, Virginia Suite C.
The study also finds that climate significantly influences nitrogen pollution of watersheds: Much more nitrogen flows into watersheds in wet climates than in dry climates throughout the Northeast into systems such as the Chesapeake Bay, Delaware Bay and Long Island Sound.
Nitrogen is the biggest pollution problem in the nation's waters, with two-thirds of the coastal rivers and bays moderately or severely polluted, says Howarth. The excess nitrogen promotes algal growth that blocks sunlight to bottom-dwelling plants and sucks oxygen from the water when the algae die. This causes oxygen-depleted "dead zones," where fish, oysters and crabs cannot survive.
"The situation is particularly acute in Chesapeake Bay, which is one of the most nutrient-sensitive coastal marine ecosystems in the country," Howarth says.
Previous models indicated that about 20 to 25 percent of the nitrogen that humans introduce into the landscape -- through agriculture, use of vehicles, burning of fuels to produce electricity and sewage -- flow down rivers to coastal waters. Howarth's new study, however, shows that 35 to 40 percent of this nitrogen pollution gets washed into watersheds with wet climates in the northeastern United States, compared with only 10 to 15 percent of the nitrogen from watersheds in dry climates.
And most climate change models predict that global warming will lead to wetter watersheds in the northeastern United States in the future. "If so, this may undercut efforts to reduce coastal nitrogen pollution in areas such as Chesapeake Bay," says Howarth.
Howarth, who heads the North American Nitrogen Center, which is part of an international effort to focus attention on nitrogen pollution, is the David R. Atkinson Professor in Ecology and Environmental Biology at Cornell. He also chaired the National Academy of Sciences Committee on Causes and Management of Coastal Nutrient Pollution from 1998 to 2000 and served as a consultant to the Pew Oceans Commission on nutrient pollution from 2001 to 2002.
Until now, scientists believed that the primary sources of nitrogen pollution in the Chesapeake Bay were agricultural -- runoff from fields and feedlots -- and sewage-treatment plants. As a result, most efforts to reduce nitrogen have targeted these sources. But, Howarth says, "Controlling nitrogen deposition should be a higher priority in the Chesapeake Bay restoration program. Our findings indicate that climate variability and change can have a big influence on the inputs of nitrogen to the bay and, therefore, should be a major consideration in efforts to restore the bay."
He says that managers need to take heed of the magnitude of nitrogen going into the atmosphere from vehicles and power plants and to recognize that climate change can substantially undermine their efforts to reduce nitrogen in the Chesapeake Bay and other coastal rivers and bays in the northeastern United States.
Howarth and his colleagues analyzed 16 major watersheds in the northeastern United States and found that not only is more nitrogen flushed into rivers in wetter years, but that wetter climates also have a long-term, steady-state influence on nitrogen pollution into rivers. The new climate models also forecast that nitrogen flows into the Chesapeake could increase by as much as 17 percent by 2030 and by up to 65 percent by 2095 because of increased human activity that contributes to nitrogen pollution.
Co-authors of the AAAS presentation are research scientists Dennis Swaney, Roxanne Marino and Neil Bettez from Cornell and Elizabeth Boyer, assistant professor in the Department of Environmental Science, Policy and Management at the University of California-Berkeley.
A broader study on the influence of climate on nitrogen pollution in northeastern watersheds will be published later this year in the journal Biogeochemistry .
The research is supported in part by the U.S. Environmental Protection Agency and the Woods Hole Sea Grant Program.