Robert Howarth: Nitrogen pollution in coastal water is double previous estimates

Biogeochemist Robert Howarth, shown here in his office in Corson Hall. At the AAAS annual meeting in Washington, D.C. on Feb. 19, he described a Cornell-led study of the Chesapeake Bay that shows that nitrogen pollution released into the atmosphere is twice as great than previously thought. Frank DiMeo/University Photography

By Susan Lang

Much of the nitrogen spewing from vehicle exhausts appears to be contaminating coastal systems, such as Chesapeake Bay, to a much greater extent than previously thought, a Cornell researcher told a news briefing at the American Association for the Advancement of Science (AAAS) in Washington, D.C., on Feb. 19.

Robert Howarth, a biogeochemist and aquatic ecosystem scientist, was reporting on a study suggesting 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, said Howarth, "Although we have 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 also presented his findings at the AAAS on Feb. 20 in a talk, "Impacts on Nitrogen Delivery to Chesapeake Bay."

Howarth, who heads the North American Nitrogen Center, which is part of an international effort to focus attention on nitrogen pollution, holds the David R. Atkinson Professorship 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.

The study, said Howarth, 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, said 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 said.

Previous models indicated that about 20 to 25 percent of the nitrogen that humans introduce into the landscape -- through agriculture, use of vehicles and 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.

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," said Howarth.

Until now, Howarth noted at the press briefing, 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 said: "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."