Ocean acidification updated – not much to celebrate

A news blog in Nature has some bad news:

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.

As for the pteropods, decreasing pH is not good for them, but nor is increasing termperatures. A recent study reports:

We conclude that pre-winter juveniles will be negatively affected by both rising temperature and pCO₂ which may result in a possible abundance decline of the overwintering population, the basis for next year's reproduction.

Also, another recent study estimating pH changes in the Meditteranean reports:

For the first time, the level of acidification is estimated for the Mediterranean Sea. Our results indicate that for the year 2001 all waters (even the deepest) have been acidified by values ranging from -0.14 to -0.05 pH unit since the beginning of the industrial era, which is clearly higher than elsewhere in the open ocean.

And down around Australia, for those who love their Sydney rock oysters, a study suggests that they may be replaced by the bigger Pacific oyster due to increasing CO2 in the oceans. Pay attention, rock oyster lovers.

The only “upside” are some studies arguing that some coastal phytoplankton that are already used to large swings in water pH may not suffer as CO2 increases. It doesn’t tell us much about the wider ocean, though.