Increasing Nitrogen Pollution in U.S. Coastal Waters

Much of the nitrogen spewing from vehicle exhausts seems to be contaminating coastal systems, such as Chesapeake Bay, to a much greater extent than previously thought, according to a study by researchers at Cornell University (Ithaca, N.Y).

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.

“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,” said the study’s lead author, Robert Howarth, a Cornell biogeochemist and aquatic ecosystem scientist.

Howarth presented his findings Feb. 20 in Washington, D.C., at the annual meeting of the American Association for the Advancement of Science (Washington, D.C.). The study also shows 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 Chesapeake Bay, Delaware Bay, and Long Island Sound.

Bumper-to-bumper traffic in the Northeast could be generating more nitrogen pollution than previously thought.

Nitrogen is the biggest pollution problem in U.S. waters, with two-thirds of the coastal rivers and bays moderately or severely polluted, Howarth said. 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% of the nitrogen that humans introduce into the landscape – through agriculture, use of vehicles, burning of fuels to produce electricity, and wastewater – flows down rivers to coastal waters. Howarth’s new study, however, shows that 35% to 40% of this nitrogen pollution gets washed into watersheds with wet climates in the Northeast, compared with only 10% to 15% 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,” Howarth said.

Howarth heads Cornell’s North American Nitrogen Center, part of an international effort to focus attention on nitrogen pollution, and is the David R. Atkinson professor of ecology and environmental biology at the university. 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 Chesapeake Bay were agricultural – in the form of runoff from fields and feedlots – and wastewater treatment plants. As a result, most efforts to reduce nitrogen have targeted these sources. However, 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.”

Managers should take heed of the magnitude of nitrogen going into the atmosphere from vehicles and power plants, Howarth said, and 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 Northeast and found that not only is more nitrogen flushed into rivers in wetter years, but 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 as much as 17% by 2030 and up to 65% by 2095 because of increased human activity that contributes to nitrogen pollution.

For more information, contact Howarth at rwh2@cornell.edu

Copyright Water Environment Federation Sep 2005