Growing Ocean Acidity May Erode Coastal Ecosystems

John Roach
for National Geographic News

May 22, 2008

Ocean waters along North America's west coast are becoming more acidic than expected in response to atmospheric carbon emissions, which will likely cause significant changes to economically vital marine ecosystems, a new study says.

At one spot in northern California, waters acidic enough to corrode seashells now rake the shore, researchers point out.

"The models suggested they wouldn't be corrosive at the surface until sometime during the second half of this century," Richard Feely, a chemical oceanographer with the Pacific Marine Environmental Laboratory in Seattle, Washington, said via email.

Scientists have long known that the oceans serve as a giant carbon sink, moderating the effects of global warming by absorbing about a third to a half of human-caused carbon dioxide (CO2) emissions.

But the added carbon dioxide is lowering the oceans' pH, changing their chemistry and biology, explained Feely, whose lab is run by the National Oceanic and Atmospheric Administration.

Acidic waters inhibit marine organisms from producing the calcium carbonate that makes up their exoskeletons and shells.

"Scientists have also seen a reduced ability of marine algae and free-floating plants and animals to produce protective carbonate shells," Feely said.

For example researchers have seen a decline in swimming mollusks called pteropods that are eaten by creatures ranging from shrimplike krill to whales. The mollusks are particularly vital to juvenile salmon and other commercial fish.

"The impact of ocean acidification on fisheries and coral reef ecosystems could reverberate through the U.S. and global economy," Feely said.

Oxygen-poor oceans

Olive Heffernan Nature Reports Climate Change Published online: 8 May 2008

Oxygen, one of the essential elements for most of life on Earth, has become increasingly scarce across large expanses of the tropical oceans over the past half century. A new study, based on real-world observations, supports the predictions of climate models that the ocean will become oxygen-depleted as a consequence of global warming.

Lothar Stramma of the University of Kiel, Germany, and colleagues combined historical records with recent data taken from ships and buoys to reconstruct oxygen concentrations since 1960 at selected tropical ocean sites. They focused their study on waters of intermediate depth, where oxygen supply is weak and where changes would be expected to have larger consequences than in the oxygen-rich areas. Over the past 50 years, oxygen decreased by 0.09–0.34 micromoles per kilogram per year in the layer at 300–700 metres depth. Depletion was most severe in areas with the greatest increase in temperature.

Oxygen depletion is likely to worsen in the tropical oceans as temperatures rise, creating large 'underwater deserts'. The authors warn that this could decrease the survival of large mobile marine species such as tuna and could disrupt nutrient cycles in the tropical oceans, reducing the productivity of entire marine ecosystems.