Acid rain has a disproportionate impact on coastal waters

In a new study, atmospheric and marine chemists report that the release of sulfur and nitrogen into the atmosphere by power plants and agriculture plays a minor role in making the ocean more acidic on a global scale, but the impact is greatly amplified in the shallower waters of the coastal ocean.

The findings are important for the bioenergy community, because, compared to coal, the production of power from biomass substantially reduces all major emissions that lead to ocean acidification: sulfur dioxide (by up to 80%), nitrogen oxide (by up to 50%), and of course carbon dioxide. Even taking into account the emissions produced during the production of energy crops, the benefits compared to coal remain large (overview of data on lifecycle emissions of biomass for power generation at the U.S. Department of Energy - Energy Efficiency and Renewable Energy, Biomass Program).

Maps depicting the model-estimated atmospheric deposition rates of carbon, nitrogen, and sulfur; alkalinity; and potential alkalinity to the ocean caused by human activity relative to conditions before the Industrial Age began. Source: Scott Doney et al, from Proceedings of the National Academy of Sciences.

Ocean acidification occurs when these chemical compounds 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 harness calcium carbonate for making hard outer shells or 'exoskeletons'. These organisms provide essential food and habitat to other species, so their demise could affect entire ocean ecosystems.

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

Acid rain isn’t just a problem of the land; it’s also affecting the ocean. 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. - Scott Doney, lead author

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 eutrophication (the creation of oxygen-depleted 'dead zones') in some parts of the ocean.

Most studies have traditionally focused only on fossil fuel emissions and the role of carbon dioxide in ocean acidification, which is certainly the dominant issue. But no one has really addressed the role of acid rain and nitrogen:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: fossil fuels :: sulfur :: nitrogen :: coal :: acid rain :: acidification :: carbon cycle ::

Scott Doney, senior scientist in the Department of Marine Chemistry and Geochemistry at the Woods Hole Oceanographic Institution (WHOI), collaborated to analyse these effects together 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 research team compiled and analyzed many 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 (since it does not remain in the air for long), some of it can be carried in the air all the way to the coastal ocean.

Perturbation maps of simulated surface water pH, dissolved inorganic carbon, and total alkalinity trends and air–sea CO2 flux due to anthropogenic atmospheric nitrogen and sulfur deposition. Source: Scott Doney et al, Proceedings of the National Academy of Sciences.

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 (map, click to enlarge).

This rain of chemicals 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 most heavily affected areas tend to be downwind of power plants (particularly coal-fired plants) and predominantly on the eastern edges 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.

Ultimately, acidification leads to a reduced capacity of oceans to store carbon. Together with plants, marine organisms play the key role in nature's way of cycling carbon dioxide. If this mechanism comes under strain, ecosystems risk to get out of balance and may reach a tipping point after which more carbon emissions result in ever stronger negative effects. This is why it is time to act now on reducing the amount of greenhouse gases we put into the atmosphere, while reducing sulfur and nitrogen emissions as well.

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

References:
Scott C. Doney, Natalie Mahowald, Ivan Lima, Richard A. Feely, Fred T. Mackenzie, Jean-Francois Lamarque, and Phil J. Rasch, "Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system", Proc. Natl. Acad. Sci., Published online before print September 5, 2007, DOI: 10.1073/pnas.0702218104

Woods Hole Oceanographic Institution: Acid Rain Has a Disproportionate Impact on Coastal Waters: Research Suggests Sulfur, Nitrogen Emissions Play a Role in Changing Chemistry Near the Coast - September 7, 2007.