The world's most acidic ocean waters are found in the winter in the Bering Sea, according to studies by scientists at Columbia University's Lamont-Doherty Earth Observatory who have released a series of maps tracking changes in global marine acidity.
With its broad continental shelf, shallow waters, ocean currents that deposit nutrients from around the world, and profusion of tiny plankton that form the base of the food web for marine life, the Bering Sea is home to "wonderful fisheries" as well as large populations of marine mammals, said Taro Takahashi, a Lamont-Doherty geochemist who led a project mapping ocean acidity levels.
But those characteristics make the Bering Sea ideal for acidity, particularly in the winter when low temperatures allow water to hold maximum carbon dioxide, Takahashi said.
"You can't have it both ways," he said.
The Bering Sea's waters can be acidic enough to register pH readings of about 7.7, according to data collected by Takahashi, his colleagues and by scientists with the National Oceanic and Atmospheric Administration, the University of Alaska Fairbanks and other organizations. Some spots in the Gulf of Alaska register similarly acidic readings, according to the new maps.
The maps created by Takahashi and his colleagues are based on 40 years' worth of measurements. They show how acidity in marine waters around the globe changes with the seasons, using 2005 as a main reference. The maps were published in August in the journal Marine Chemistry.
As with other far-north waters, the Bering Sea has acidity levels that vary dramatically from season to season. The summer blooms of plankton absorb large amounts of carbon dioxide from the water, dramatically raising pH levels at least temporarily each year. Then upswellings of carbon-rich water from the ocean depths in winter help lower pH levels in that season, according to the Lamont-Doughty scientists.
Even the cold Antarctic waters, though also vulnerable to acidification, are not as acidic as the Bering Sea, Takahashi said. Unlike the Antarctic region, the Bering Sea receives large amounts of runoff from rivers, which dump carbon-rich sediments along with fresh water, he said. And the Bering Sea is partially enclosed by land, pinched off at the Bering Strait where Alaska and Russia nearly touch and bordered on the south by the Aleutian Islands, he noted. So while more acidic water can easily flow out of the Antarctic region, the Bering waters are more trapped, he said.
In the tropics, where water temperatures do not vary much throughout the year, there is a correspondingly small seasonal variation in acidity, the Lamont-Doherty scientists found in their mapping. The tropical and temperate oceans have pH levels that generally range from 8.15 to 8.05, the scientists found.
The good news, potentially, for the Bering Sea? Acidification there, though already high, appears to be increasing less quickly than that in the rest of the world's ocean waters, Takahashi said.
"The Bering Sea is very acidic and the most acidic water in the global ocean, but it's not changing fast," he said.
For much of the world, ocean acidification is a looming threat, so much so that NOAA has established a special program devoted to it.
In the past 200 years, the oceans have absorbed about a quarter of human-caused atmospheric emissions of carbon, according to the Lamont-Doherty Observatory. While the oceans have been able to soak up some of those anthropogenic greenhouse gases, thus buffering climate change, that has come at a cost to marine waters. Average pH in the world's oceans has fallen from 8.2 to 8.1 since the start of the industrial era, a 30 percent increase in acidity, and pH is expected to fall to an average of 7.8 to 7.9 by 2100, according to NOAA. The changes threaten marine species that build shells of calcium, including the tiny copepods that are eaten by salmon and other fish, according to NOAA.
The ocean changes put fishing-dependent regions in Alaska at particular risk, according to a recent NOAA study.