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Ocean Acidification

Calcifying sea critters may pay the price for increasing levels of carbon dioxide in the atmosphere

Victoria Fabry and Brad Seibel study what’s come to be known as “the other CO2 problem.”

Most of us are familiar with the first problem: The copious discharge of carbon dioxide, the primary greenhouse gas, into the atmosphere is forcing the Earth’s temperature to rise, causing a wide range of disruptions and changes to the world’s climate.

The oceans play an integral role in mitigating some of that CO2 by absorbing about a third of it as what scientists call a “carbon sink.” But that benefit comes at a cost to marine critters and ecosystems, as the carbon dioxide begins to change the seawater chemistry of the oceans.

A leading expert in ocean acidification from California State University San Marcos, Fabry is the principal investigator for a team of scientists in Antarctica studying how Southern Ocean pteropods, small gastropod mollusks (sea snails and slugs), may respond to higher acidic levels of seawater predicted for the next century.

These animals may be particularly vulnerable to seawater chemistry change because, as the oceans become more acidified and the pH level decreases, their ability to calcify and form shells and skeletons may be severely affected.

“Ocean acidification is going to impact many organisms that calcify,” Fabry said from her office at the Albert P. Crary Engineering and Science Center in McMurdo Station. “It’s going to happen in our lifetimes. It’s not far away.”

The pH level, measured in units, is a calculation of the balance of a liquid’s acidity and alkalinity. The lower a liquid’s pH number, the higher its acidity. The pH level for the world’s oceans was stable for tens of thousands of years, but has dropped one-tenth of a unit since the Industrial Revolution in the 1800s.

That represents a significant decrease, Fabry said, and current models predict the pH level may drop by as much as four-tenths of a unit by 2100 relative to the pre-industrial value. That could mean big trouble for calcifying organisms, particularly in the higher latitudes of the Arctic and Antarctic.

The reason: Most pteropods and other calcifiers, like corals, use the calcium carbonate minerals of calcite or aragonite to construct their shell coverings or skeletons. Normally, surface seawater is not corrosive to calcite and aragonite because the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, so does the concentration of the carbonate ion.

Higher latitude waters are naturally less saturated, so the change in chemistry would affect these areas first. By 2040, under some CO2 emissions scenarios, surface waters of some regions may become undersaturated of aragonite, making those calcium carbonate structures constructed of aragonite vulnerable to dissolution.

At the end of the century, projections say most of the Southern Ocean and some regions of the subarctic Pacific will become undersaturated with respect to aragonite if CO2 emissions continue in a business-as-usual scenario. Data on the Arctic Ocean are pending.

“The high latitudes are the first areas that will have large expanses of surface waters that will be undersaturated with respect to aragonite. It’s not looking good,” Fabry said. “With increasing oceanic uptake of atmospheric CO2, we see CO2 increasing in the water and pH declining at time series stations at Bermuda, Hawaii and the Canary Islands. … And in high latitudes such as the Southern Ocean, what we’re going to have in the coming decades is surface seawater that is corrosive to aragonite. ”

In a previous experiment involving a sub-Arctic pteropod, Fabry grew the species at a lower carbonate ion saturation. Within 48 hours, the growing edge of the shell began to dissolve. “The shells start to get pitted, the upper layer peels off, and that exposes more calcium carbonate rods and crystals to dissolution, and they just dissolve,” she said.

The team is conducting similar experiments here over two field seasons, though weather for this second year stymied collection efforts for the first couple of weeks. The scientists are after two types of pteropods: one with a shell in its adult stage (euthecosomatous pteropods), and a second, carnivorous species (gymnosomatous pteropods) that feeds exclusively on the first.

Seibel, a co-principal investigator on the project, is interested in discovering how ocean acidification will affect other aspects of pteropod physiology, such as oxygen consumption or ammonia excretion.

“CO2 causes acidification in body fluids the same way it does in seawater, although not necessarily to the same extent,” said Seibel, with the University of Rhode Island. “Acidification of the body fluids can lead to changes in metabolism that could lead to reductions in growth and reproduction. 

“In other oceans, we’ve seen detrimental effects of CO2 on squid metabolism,” he added. 

Very preliminary results show little effect on the pteropod physiology to high levels of CO2 — 1,000 parts per million, about triple the concentration in the oceans today. However, Seibel emphasized that the experiments are short duration. The studies on squid, whose blood has a protein that binds to oxygen to transport it around the body, showed pronounced responses to acidified water.

“That protein is very sensitive to pH, so we are able to see changes in oxygen consumption with these levels of CO2 in squids,” he said.

The team’s method of specimen collection is pretty low-tech. Members put on chest waders and walk into the water from shore. They then use a long broom handle with a beaker at one end to gather the pteropods. “We just dip them up, because they’re very, very fragile,” Fabry said.

The shelled pteropods don’t live long in captivity because they feed by means of a mucous web that is suspended above their bodies, sort of like a free-floating, omnivorous spider surfing its web through the water as it feeds.

In the lab, the scientists measure how much the pteropods calcify under varying levels of ocean acidification based on predictions from the Intergovernmental Panel on Climate Change. For Seibel’s purposes, the researchers track rates of oxygen consumption and ammonia excretion, as well as measure the acidification in animal tissues. 

Fabry said it is too early to say how well the organisms may be able to adapt as worldwide surface ocean pH drops. She noted that oceans could absorb CO2 and eventually neutralize it but that process takes thousands of years. Ocean acidification, beginning in the 1800s, is occurring over the span of a few centuries.

“The rate of release of CO2 to the atmosphere is critical. We could put more CO2 in the ocean, if we did it slowly,” she explained. “The biggest unknown is how fast humans will put CO2 in the atmosphere.”

NSF-funded research in this story: Victoria Fabry, California State University San Marcos; and Brad Seibel, University of Rhode Island.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs