Ocean acidity increased by human-induced carbon dioxide, finds study

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DURHAM, NC, Jan. 2, 2014 -- Numerous studies have shown that increasing amounts of atmospheric human-induced carbon dioxide are permeating into the world's oceans and potentially causing acidification. 

Ocean acidification is a result of the dissolvability of carbon dioxide, which reduces the water's pH and the ability of organisms to form calcium carbonate minerals -- the building blocks of many species' shells and skeletons. If current trends continue, experts predict that the mean ocean pH will decrease by about 0.2 units over the next 50 years, which could have far-reaching effects on ocean ecosystems and organisms.

One example of this occurrence is exemplified in a new Duke University-led study, which documents dramatic, natural short-term increases in the acidity of a North Carolina estuary driven by changes in temperature, water flow, biological activity and other natural factors. Likewise, they are occurring in addition to the long-term acidification taking place in Earth's oceans as a result of human-caused climate change.

"The natural short-term variability in acidity we observed over the course of one year exceeds 100-year global predictions for the ocean as a whole and may already be exerting added pressure on some of the estuary's organisms, particularly shelled organisms that are especially susceptible to changes in pH," said Zackary I. Johnson, Arthur P. Kaupe assistant professor of molecular biology at Duke's Nicholas School of the Environment and lead author of the study.

The study was conducted at the Pivers Island Coastal Observatory at the Duke Marine Lab in Beaufort, N.C., as part of a long-term coastal monitoring program. Researchers collected seawater samples from Beaufort Inlet weekly for a year and on a daily and hourly basis for shorter periods to track changes in the water's pH and dissolved inorganic carbon on multiple time scales.

Dana Hunt, assistant professor of microbial ecology, who co-authored the study, added that this acidification could have drastic effects on the environment and aquatic life. "We may see significant changes in biological processes such as primary production. Some organisms, such as phytoplankton, may benefit. Many others, including shelled organisms and corals, will not."

When the effects of long-term ocean acidification and short-term natural variation combine, they can create "extreme events" which may be especially harmful to coastal marine life, Johnson said. As such, the Duke team's analysis showed that a wide range of natural variables, including changes in temperature, algal production and respiration, and water movement caused by tides and storms, triggered sharp spikes in the inlet's acidity. Some changes occurred over the course of a season; others took place on a daily or hourly basis.

"Understanding to what extent pH naturally varies in coastal ecosystems worldwide will be essential for predicting where and when the effects of increasing ocean acidity will be most profound, and what organisms and ecosystems may be most affected," Hunt said. "Our research demonstrates we have to take into account a wide range of environmental variables, not just pH."

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