Excess carbon dioxide is not the only thing from fossil fuel burning that makes Earth’s oceans more acidic. Sulfur and nitrogen from fossil fuel burning, agricultural runoff and other human sources are also changing ocean chemistry — and that impact is especially pronounced along the coasts, new research suggests.
When sulfur dioxide and nitrogen oxides from burning fossil fuels mix with water and oxygen in the atmosphere, they form acidic compounds that eventually fall to Earth as acid rain, which damages soils, waters and forests. But acid rain is not only a land problem, according to marine geochemist Scott Doney of the Woods Hole Oceanographic Institution in Massachusetts. For the past few years, Doney has been working on a biogeochemical model that examines the effects of rising carbon dioxide levels on the acidity of the ocean surface. He decided to expand that study and look at what the sulfur and nitrogen compounds in acid rain would do to oceans as well.
To determine how such emissions impact the oceans on a global scale, Doney and his team combined existing studies detailing patterns of acid rain deposition with comprehensive models of how such inputs would change the surface water chemistry of the oceans. “A lot of it is modulated by the lifetime of these compounds in the atmosphere,” he says. Sulfur dioxide emissions, for example, can be oxidized and transformed into sulfuric acid, which attaches to aerosol particles in the atmosphere and rains out quickly. Because the lifetime of such particles in the atmosphere is fairly short, on the order of days to a week, the deposition patterns of such particles are well-constrained. Bands of sulfate aerosol depositions extend out from major emissions regions and into the North Atlantic, but “tend to drop off quite rapidly once you’re a few kilometers offshore,” he says.
How these depositions affect the seawater, particularly in coastal regions that are already impacted by nutrient fertilization, pollution and overfishing, can be complicated. In addition to altering an ecosystem’s productivity, depositions of sulfur and nitrogen compounds can not only increase the acidity of the waters, but also reduce its alkalinity — a measure of the water’s ability to neutralize acids that is closely linked to its carbonate chemistry. Such acidification and the loss of that carbonate buffer present a big threat to ecosystems. “A lot of shell-forming plants and animals depend on the availability of carbonate” in coastal waters, Doney says, and lab studies show that a decrease in carbonate ion concentration leads to reduced shell growth and lower reproduction rates for many creatures. Globally, the impact of acid rain on the chemistry of the deep ocean is small relative to the impact on the ocean of changes in atmospheric carbon dioxide — but in coastal waters, acid rain can have a significant impact, the team reported Sept. 11 in Proceedings of the National Academy of Sciences.
“We’ve known that nitrogen deposition to estuaries and other coastal areas [can cause] eutrophication and hypoxia,” conditions arising when an overabundance of nutrients promotes algal blooms and leads to very low-oxygen conditions in the water, says Jim Galloway, an environmental scientist at the University of Virginia in Charlottesville who was not involved in the research. “But what’s new [in Doney’s research] is that now we have another dimension: These systems could be affecting the acidity of the water.” The assessment of the atmosphere as a potentially important source for nitrogen and sulfur to coastal waters, in addition to inputs from rivers and groundwater, lays important groundwork for future, more region-specific studies, Galloway says. “If you look at the regions that will receive large increases in nitrogen deposition in the future, they are not along the coast of North America and Europe,” he says. “Southeast Asia has a large region of shallow water, and already has high nitrogen deposition, which is going to increase in the next few decades” as the population and development increase, he says.