The release of sulfur and nitrogen into the atmosphere by power plants and agricultural activities is making seawater more acidic, especially in coastal waters, according to a study published September 2007 in the Proceedings of the National Academy of Sciences.
“Acid rain isn’t just a problem of the land; it’s also affecting the ocean,” said Scott Doney, a marine chemist at Woods Hole Oceanographic Institution (WHOI) and lead author of the study. “That effect is most pronounced near the coasts, which are already some of the most heavily affected and vulnerable parts of the ocean due to pollution, overfishing, and climate change.”
Farming, livestock husbandry, and the combustion of fossil fuels releases excess sulfur dioxide, ammonia, and nitrogen oxides to the atmos-phere, where they are transformed into nitric acid and sulfuric acid. A portion of these compounds is blown offshore, where they enter the ocean and alter its chemistry.
The acids lower seawater’s pH and strip it of carbonate ions. (Ultimately, so does ammonia, a base, which is converted to nitrates and nitric acid.) That hampers the ability of marine organisms—such as sea urchins, shellfish, corals, and certain types of plankton—to harness calcium carbonate to make hard outer shells or “exoskeletons.” These organisms provide essential food and habitat to other species, so their demise could affect entire ocean ecosystems.
Ocean acidification is already a concern because excess carbon dioxide from fossil fuels produces the same effects. Though carbon dioxide remains the dominating factor, “no one has really addressed the role of acid rain and nitrogen,” Doney said.
Doney collaborated on the project with Natalie Mahowald, Jean-FranÇois Lamarque, and Phil Rasch of the National Center for Atmos-pheric Research, Richard Feely of the Pacific Marine Environmental Laboratory, Fred Mackenzie of the University of Hawaii, and Ivan Lima of WHOI.
The research team analyzed data on agricultural, fossil fuel and other atmospheric emissions. They built theoretical and computational models of the ocean and atmosphere to simulate where the nitrogen and sulfur emissions were likely to have the most impact. Then they ground-truthed their model’s results with field observations in coastal waters by other scientists. The most heavily affected areas tend to be downwind of eastern North America, western Europe, and east and south Asia.
In January 2008, the editors of Discover magazine recognized the research as one of its “100 Top Science Stories of 2007.”
Funding for this research was provided by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration.