Ocean pH dropping faster than expected

Thanks to rising carbon dioxide (CO₂) levels, some Arctic waters are already experiencing pH dips that could be harmful to sea life. What’s more, this acidification seems to be happening more rapidly than models have predicted.

This sobering conclusion was reached by researchers who met on Wednesday to discuss ocean acidification at the Geological Society of America meeting in Denver. “Models are probably underestimating at least by a few years the impact of ocean acidification in the Arctic,” says Jeremy Mathis, a chemical oceanographer at the University of Alaska in Fairbanks. “We don’t know what the organisms’ responses are yet, but the conditions are already there to potentially be disruptive to the ecosystems.”

Marine organisms from plankton to crabs are dependent on carbonate ions in the ocean to build their skeletons and shells. But as CO₂ dissolves in the water it lowers the pH, which shrinks the pool of such ions available for animals to use.

One important source of carbonate ions is aragonite, a particularly soluble form of calcium carbonate. Seawater is usually saturated with aragonite. However a recent study in Biogeosciences estimated that by 2016, according to the IPCC’s mid-range emissions projections, aragonite will fall below this level in some Arctic waters for at least one month a year. By the end of the century, it predicts that the entire Arctic Ocean could be under-saturated with respect to aragonite.

“But we don’t have to wait until 2016,” says Mathis. “We’re already seeing places in the Arctic where these under-saturations are happening now.” High latitude waters in the Arctic and Antarctic are particularly sensitive to pH changes, as cold waters absorb more gas than warm waters.

Researchers at the symposium were particularly concerned about pteropods – tiny sea snails that are highly sensitive to acidification. Pteropods make up about half the diet of juvenile pink salmon living in Gulf of Alaska. And they could be affected at pH levels very close to those that the region is already experiencing. “It’s not going to take a great deal of CO₂ intrusion in high latitude seas to get to a point where the water could become corrosive to some marine calcifying organisms,” says Mathis.

The news gets worse. Several speakers on the panel, including Richard Feely, an oceanographer with the National Oceanic and Atmospheric Administration, who has been leading large-scale ocean surveys of acidification, warned that the acidity of seawater could double by the end of the century.

Around ten stations equipped with carbon sensors are currently being used to monitor subsurface and bottom water acidification around the globe. Scientists are now planning to deploy some 60 more.

The response of Arctic pteropods to ocean acidification was reported in the two following recent articles:

Comeau S., Gorsky G., Jeffree R., Teyssié J.-L. & Gattuso J.-P., 2009. Impact of ocean acidification on a key Arctic pelagic mollusc (Limacina helicina). Biogeosciences 6:1877-1882.

Comeau S., Jeffree R., Teyssié J.-L. & Gattuso J.-P., 2010. Response of the Arctic pteropod Limacina helicina to projected future environmental conditions. PLoS ONE 5: e11362.

Note:

I wonder why Dr Mathis thinks we don't know the impact of lower pH on organisms. Matt Ridley was recently discussing a metaanalysis of research on this subject that was quite clear that organisms are largely unaffected.