Sulfur, Nitrogen Emissions Play a Role in Changing Chemistry Near the Coast

Newswise — The release of sulfur and nitrogen into the atmosphere by power plants and agricultural activities plays a minor role in making the ocean slightly more acidic on a global scale, but the impact is greatly amplified in the shallower waters of the coastal ocean, according to new research by atmospheric and marine chemists.

Ocean "acidification" occurs when chemical compounds such as carbon dioxide, sulfur, or nitrogen mix with seawater, a process which lowers the pH and reduces the storage of carbon. Ocean acidification hampers the ability of marine organisms—such as sea urchins, corals, and certain types of plankton—to make hard outer shells or "exoskeletons." These organisms provide essential food and habitat to others, so their demise could affect entire ocean ecosystems.

"Acid rain isn't just a problem of the land; it's also affecting the ocean," said Scott Doney, lead author of the study and a senior scientist in the Department of Marine Chemistry and Geochemistry at the Woods Hole Oceanographic Institution (WHOI). "That effect is most pronounced near the coasts, which are already some of the most heavily affected and vulnerable parts of the ocean."

The excess nitrogen inputs into coastal waters might also contribute to the overgrowth of phytoplankton and other marine plants. Nitrogen promotes plant growth, but too much plant activity can lead to harmful algal blooms and eutrophication, the creation of oxygen-depleted "dead zones" in the water.

The findings were published this week in the online "early edition" of the Proceedings of the National Academy of Sciences; a printed version will be issued later this month.

Doney collaborated on the project with Natalie Mahowald, Jean-Francois Lamarque, and Phil Rasch of the National Center for Atmospheric Research, Richard Feely of the Pacific Marine Environmental Laboratory, Fred Mackenzie of the University of Hawaii, and Ivan Lima of the WHOI Marine Chemistry and Geochemistry Department.

"Most studies of ocean acidification have traditionally focused on fossil fuel emissions and carbon dioxide, which is certainly the dominant issue," Doney said. "But no one has really addressed the role of nitrogen and sulfur deposition in the ocean."

Farming practices and the combustion of fossil fuels cause excess sulfur and nitrogen to be released into the atmosphere, and much of it is deposited in the coastal and open ocean. This rain of chemicals into the ocean changes the chemistry of seawater, with the increase in acidic compounds lowering the pH of the water while reducing the capacity of the upper ocean to store carbon.

The research team compiled and analyzed many sources of data on fossil fuel emissions, agricultural runoff, and other atmospheric emissions. They then built theoretical and computational models of the ocean and atmosphere to approximate where nitrogen and sulfur emissions would have the most impact.

The most heavily affected areas tend to be downwind of power plants, (particularly coal-fired plants); geographically, the effects are most pronounced along the coasts of North America, Europe, and south and east of Asia.

Seawater is slightly basic (pH usually between 7.5 and 8.4), but the ocean surface is already 0.1 pH units lower than it was before the Industrial Revolution. Previous research by Doney and others has suggested that the ocean will become another 0.3 to 0.4 pH units lower by the end of the century, which translates to a 100 to 150 percent increase in acidity.

Funding for this research was granted by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration.

Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment.