The research was conducted by scientists from the Woods Hole Oceanographic Institution (WHOI), the National Center for Atmospheric Research, the Pacific Marine Environmental Laboratory, and the University of Hawaii, and funded by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration.

Ocean acidification occurs when chemical compounds such as carbon dioxide, sulfur or nitrogen mix with seawater, a process that changes the chemistry of the water, lowers the pH and reduces the capacity of the upper ocean to store carbon.

The acidification hampers the ability of marine organisms such as sea urchins, corals and certain types of plankton to harness calcium carbonate for making hard outer shells–exoskeletons. Because these organisms provide essential food and habitat to other species, their demise could affect entire ocean ecosystems.

In addition to acidification, excess nitrogen inputs from the atmosphere promote increased growth of phytoplankton and other marine plants which, in turn, may cause more frequent harmful algal blooms and the creation of oxygen-depleted dead zones in some parts of the ocean.

According to the study, the most heavily affected areas tend to be downwind of coal-fired power plants and predominantly on the eastern edges of North America, Europe and south and east of Asia.

“Acid rain isn’t just a problem of the land; it’s also affecting the ocean,” says Scott Doney, lead author of the study and a senior scientist in the WHOI Department of Marine Chemistry and Geochemistry. “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, over-fishing and climate change.”

Doney collaborated on the research 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.

The team compiled and analyzed publicly available data sets on fossil fuel emissions, agricultural 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. They also compared their model results with field observations made by other scientists in the coastal waters around the United States.

Farming, livestock husbandry and the combustion of fossil fuels cause excess sulfur dioxide, ammonia and nitrogen oxides to be released to the atmosphere, where they are transformed into nitric acid and sulfuric acid. Though much of that acid is deposited on land, some of it can be carried in the air to the coastal ocean.

When nitrogen and sulfur compounds from the atmosphere are mixed into coastal waters, the researchers found, the change in water chemistry was as much as 10 to 50 percent of the total changes caused by acidification from carbon dioxide.

Previous studies by Doney and others have predicted a 100 to 150 percent increase in ocean acidity by the end of this century.