Effects of acidification especially strong in ocean waters off Alaska, NOAA says

The Arctic Ocean and the northern Pacific Ocean, along with Antarctic waters, are acidifying faster than the rest of the world’s marine waters, a new National Oceanic and Atmospheric Administration-led study finds.

The study, which analyzed measurements from thousands of monitoring stations across the globe, found these bodies acidified faster as carbon dioxide absorbed from the atmosphere combines with natural sources of carbon swept into them by marine currents and held fast by low temperatures.

Ocean acidification is the chemical transformation seawater undergoes as it absorbs and stores more carbon. The increasingly acidic water more easily dissolves the calcium carbonate from which many marine species make their shells — affecting not only commercially important shellfish, such as oysters and clams, but also smaller creatures, such as tiny pteropods, upon which marine food webs depend. That could upend entire ecosystems, harming other important species, including salmon.

The new study, published online in the journal Global Biogeochemical Cycles, uses data from 11,431 sampling stations to evaluate aragonite saturation levels in oceans worldwide — the degree to which aragonite, a form of calcium carbonate that sea creatures use to build shells, is held in the water.

When water is saturated, it holds the maximum amount of dissolved aragonite. When it is supersaturated, it holds excess suspended aragonite. All the world’s oceans, measured down to a depth of 50 meters, are supersaturated with aragonite, according to measurements from the Global Ocean Data Analysis Project used in the study.

Still, those measurements and other large-scale programs monitoring ocean conditions, show that aragonite saturation levels have slipped globally, a troubling sign, the study’s lead author said.

“A decline in the saturation state of carbonate minerals, especially aragonite, is a good indicator of a rise in ocean acidification,” Li-Qing Jiang, an oceanographer at NOAA’s Cooperative Institute for Climate and Satellites at the University of Maryland, said in a statement issued by the agency.

The study found that at depths shallower than 100 meters, aragonite saturation levels declined by an average rate of 0.4 percent a year from the decade spanning 1989 to 1998 to the decade after then, spanning the years 1998 to 2010.

Low levels of aragonite saturation were pronounced in the North Pacific Ocean at latitudes above 50 degrees north, according to the study. At depths of 200 meters and below, all the sections measured in that part of the Pacific showed undersaturated states for aragonite, the study said. Aragonite undersaturation, a condition normally found in the very deep parts of the world’s oceans, can be a troubling sign when it occurs in shallower waters, scientists say.

The Arctic Ocean also showed lowered aragonite saturation states, though not as low as those at corresponding depths of the North Pacific. Less data was available from the Arctic Ocean, researchers noted.

In contrast, the Atlantic Ocean was found to have aragonite-supersaturated waters down to much deeper levels, thanks to a lower level of lingering carbon from decaying organisms, according to the study.

“The deepest saturation horizon and youngest water are found in the North Atlantic. The shallowest saturation horizon and oldest deep waters are found in the North Pacific. This is because older water has had more time for CO2 accumulation from organic matter remineralization,” the study said.

In the polar regions, even the surface waters — though supersaturated with aragonite — held far less of the mineral than did waters in more temperate latitudes, the study found.

The Arctic, Antarctic and North Pacific are vulnerable to acidification in part because of their cold waters, which hold in carbon dioxide, the study said. Those regions, along with some other marine areas in the world, such as a region off the coast of Africa, are more vulnerable because the pattern of ever-moving ocean currents brings in carbon-dioxide-rich waters from elsewhere in the world and causes that older water to rise up to shallower levels closer to the surface, the study said.

That’s especially the case in the fish-rich North Pacific, site of major Alaska-based commercial seafood catches, which has the distinction of being at the very end of the Global Thermohaline Circulation, the pattern sometimes called the “ocean conveyor belt,” Jiang said in an email.

Yereth Rosen, Alaska Dispatch News, 12 November 2015. Article.

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