It is hardly a revelation that burning fossil fuels is adding increasing amounts of carbon dioxide to the atmosphere. Less well known, however, is that approximately 30 percent of the CO2 that has been emitted as a result of human activity has wound up in the ocean.
One consequence of this has been a change in the pH of the ocean, known popularly as ocean acidification. A 2005 report by Britain's Royal Society noted that the average pH of surface seawater had fallen by approximately 0.1 units since pre-industrial times and could fall by 0.5 units by the year 2100. "This pH is probably lower than has been experienced for hundreds of millennia and, critically, at a rate of change probably 100 times greater than at any time over this period," the report underlined.
One of the particular concerns that researchers have raised about ocean acidification has been its potential impact on calcifying organisms - species that build calcium carbonate shells, skeletons or plates, species like corals, shellfish or coccolithophore algae.
A recent paper in the Proceedings of the National Academy of Sciences suggests that ocean acidification might have an additional impact: altering the nitrogen cycle in the ocean.
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Nitrogen is one of the most important nutrients in the ocean. All organisms use it to make proteins, for example. Some microbes can also use different chemical forms of nitrogen as a source of energy; one subset of these microbes, the ammonia oxidizers, plays a pivotal role in determining which forms of nitrogen are present in the ocean. By oxidizing ammonia – essentially, combining ammonia and oxygen – they produce nitrous oxide, which is released into the atmosphere, and make nitrogen available in the form of nitrate in the water column, a process known as nitrification.
However, in experiments using water samples from multiple sites in the Atlantic and Pacific, Michael Beman of the University of California at Merced and colleagues found that, as levels of ocean acidity increased, rates of ammonia oxidation decreased – in some cases, by as much as 38 percent.
"What we saw is almost uniform across the ocean, or at least in all the experiments we conducted, which seems to suggest this is fairly consistent effect," said Beman. Additionally, the extent of the decrease suggests that ocean acidification "could have a pretty substantial effect on how nitrogen is cycled in the ocean."
One silver lining could be that reduced nitrification would lead to a decrease in the amount of nitrous oxide released into the atmosphere, off-setting the emissions of this greenhouse gas as a result of human activities.
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The broader consequences for the marine environment are less clear, but seem likely to be profound, given that nitrification in sunlit waters is responsible for one-third of all organic material in the ocean.
Reduced nitrification would lead to decreased amounts of nitrate in the water column and higher levels of ammonia, and while the authors conclude that it is hard to be certain of the consequences of such a change, they predict that it would likely lead to a fundamental shift in the makeup of the species at the base of the marine food web. Some phytoplankton would succeed at the expense of others, profoundly altering primary production in the ocean, with cascading effects throughout marine ecosystems.
“Some of these nutrient cycling processes ultimately affect the entire food web," noted Beman. "So I would argue it is worth examining them in more detail, to try to figure out what sorts of effects we might expect to see."
Tags: Biodiversity, Carbon Emissions, Food Chain, Oceanography, Water
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