Carbonated Oceans

An overload of carbon dioxide is acidifying seawater, posing a subtle but profound threat to marine invertebrates.

March 2009

By Robert Monroe

Like a sinkful of hard water deposits suddenly doused with vinegar, the shells oftiny marine snails in Victoria Fabry’s test tanks don’t stand a chance.

Fabry, a biological oceanographer and visiting researcher at Scripps Institution of Oceanographyat UC San Diego, studies the effects of ocean acidification on the mollusks knownas pteropods. In one experiment, only 48 hours of exposure to slightly corrosiveseawater caused normally smooth shells to become frayed at the edges on their wayto eventual dissolution, severely diminishing their owners’ chances of survival.

The acidity of the water in Fabry’s lab had been ratcheted up to levels thatmight not be seen until the end of the century, but she and other scientistsfear that ongoing acidification of ocean water could be causing a slow-motiondestruction of ocean ecosystems now.

The loading of carbon dioxide into oceans is a consequence of fossil fuel use thathas only begun to be widely recognized as problematic in the past decade. Its subsequenteffects on seawater chemistry have the potential to spread ecological disaster to avariety of industries dependent on the seas.

To understand what the world might expect, several Scripps research teams are drawingon the institution’s expertise in long-term climate data collection and on new technologiesthat will help them understand when, where, and how ocean chemistry changes when the seasare overwhelmed by increasing infusions of carbon dioxide. They are joining a growingnumber of international scientists who are turning their attention to the issue. Theircollective hope is to understand whether the oceans are approaching a tipping point ofwidespread damage and to see what can be done to prevent it.

“We know the oceans are getting more acidic. We know lab experiments have shown thatorganisms find living more difficult as the CO2 increases,” said Scripps marine chemistAndrew Dickson, who is collaborating with Fabry to build a network of observing stationsoff the California coast. “Studies that can clarify how important this is for ecosystemsremain to be designed and done.”

Impairing Nemo

As humans burn oil and coal, carbon dioxide is released and accumulates in the atmosphere.A little less than half of it stays in the sky and about a third enters the oceans,dissolving into seawater at the ocean surface.

When ocean water absorbs CO2, the two react to form carbonic acid. The acid reactswith carbonate ions, making the ions less available in ocean waters to shell-formingorganisms. Robbed of sufficient quantities of a main ingredient for their shells,these organisms may become less hardy and less able to replenish their numbers.

The trend scientists are seeing might seem small. The average pH of water at theocean’s surface has fallen from 8.16 to 8.05 since the beginning of the IndustrialRevolution and the advent of fossil fuel use. Pure water in comparison has a pH of 7.

But marine organisms that build shells have grown accustomed to a certain chemicalbackground and they do not take such a decrease well, especially at the pace scientistsare documenting. The rate of change that marine creatures have endured in fewer thanthree centuries is 100 times faster than the rate of change over the preceding 850,000years. And once the lower pH water is there, it will be there for a long time becauseof the slow pace of ocean circulation. Dickson likens itto pouring cream in a cup of coffee and stirring it once every 1,000 years.

As a relatively well-studied example of acidification’s effects, the pteropod hasbecome what ocean acidification researchers consider their canary in the coal mine.The snails, however, are not the only organisms that are sensitive. Nearly all marinelife forms that build calcium carbonate shells are jeopardized by the rising acidityof the oceans. That long list includes corals as well as commercially important marineinvertebrates such as abalone, sea urchins, clams, and mussels.

Even fishes mightbe susceptible to problems as carbonic acid amasses in their tissues. A new studyled by Australia’s James Cook University found that acidification diminishes the abilityof larval clownfish, the colorful species popularized in the film “Finding Nemo,” to usethe olfactory cues they need to locate suitable habitats.

And though they are not themselves harvested as food, pteropods and other vulnerablezooplankton and phytoplankton have an indirect but profound value to fisheries, being akey part of the diet of pink salmon, mackerel, and cod.

Scientists are concerned less about a sudden mass die-off of shelled organisms thanabout a persistent assault on their health that won’t relent for centuries. Coralreefs, for instance, may reach the point at which they erode faster than they growby the mid-21st Century, according to some estimates.

“These organisms are likely to have difficulty in secreting their shells in afully functional way which could alter their reproductive success and theirpopulation abundances,” said Mark Ohman, a Scripps biological oceanographerwho recently added carbon dioxide measurements to the data he regularly collectsthrough the California Current Ecosystem Long-Term Ecological Research (LTER)program led by Scripps.

But at this point, only the chemical basics of acidification are well understood.New discoveries have the feel of breaking news. Richard Feely and otherresearchers at NOAA’s Pacific Marine Environmental Laboratory made headlinesin 2008 when they discovered that masses of acidic water were encroaching onthe continental shelf off the West Coast to a surprising extent. The onslaughtappears to change throughout the year and scientists still do not understandfully how much pH fluctuates seasonally in coastal waters. They also aren’tsure how far masses of lower pH water stretch geographically.

Acidification’s potential threat to the California Current, which encompassedmost of the area studied by Feely, is making the economically vital ocean regiona research target zone. Because Scripps is the home of a key repository oflong-term data about the current and greenhouse gases, the institution isbecoming a center of ocean acidification studies.

From the Archives

Two key monuments to fastidious long-term data gathering reside at Scripps.The California Cooperative Oceanic Fisheries Investigations (CalCOFI) waslaunched just after World War II after the West Coast sardine fishery collapsed.It provides a continuous record of temperature, plankton abundance, and otherkey indicators of the state of the ocean. For most of the program’s existence,these data have been gathered quarterly. In 1958, Charles David Keeling initiatedmeasurements of atmospheric carbon dioxide levels at a weather station atopHawaii’s Mauna Loa. Monthly averages are plotted to this day on the iconic graph knownas the Keeling Curve.

Both time series and their offshoots help characterize trends in ocean acidification that were presentbefore scientists knew to look for them. Ohman notes that records from CalCOFI are providing”six decades of context” in the form of proxy data ranging from temperature to nutrient concentration.This information will help scientists reconstruct the rate of change in the California Current’s acidity.Scientists also hope that the record can tell them not only about acidity trends but how climate cycleslike El Niño cause such trends to fluctuate.

The Keeling Curve provides indirect evidence that not all human carbon dioxide emissionsremain in the atmosphere and present-day researchers credit that record for promptingscientists to look for signs of acidification in the oceans. Papers about the phenomenonfirst started appearing in the 1970s and a decade later, Keeling started a similartime-series of seawater carbon dioxide content and alkalinity levels near Bermuda.Subsequent work by Dickson established the reference standards for measurements ofcarbon dioxide content and alkalinity of ocean water that have helped researchersuniformly measure trends in acidification.

“The Mauna Loa CO2 time series is the most famous example of what impact you get if youcollect very long time series,” said Uwe Send, a Scripps physical oceanographercollaborating with Ohman, Fabry, and Dickson, “and we are trying to do the same kindof thing in the ocean in important and representative or critical locations.”

Dis-solution

Last November, Send, Ohman and NOAA researcher Chris Sabine deployed a mooring withcarbon dioxide sensors 250 kilometers (155 miles) southwest of Point Conception, Calif.Contributions from those sensors and several others attached to the mooring feed intodata collected by the LTER project, a National Science Foundation-supported programbuilding from CalCOFI that aims to answer scientists’ questions about the interplaysbetween California Current organisms and their changing ocean environment.

Fabry and Dickson are leading efforts to deploy two more carbon dioxide sensors thisyear, one in Carlsbad, Calif., and the other off the Northern California city ofTrinidad. The latter launch is part of a California Ocean Protection Council-fundedproject in which Fabry will conduct complementary tests in the lab to understand howvarying pH levels affect marine organisms at different stages of development.

The moored sensors off California are a small contribution to much larger effortson the global scale. Send is co-leading the international OceanSITES program, whichis building a network of stations around the world oceans to collect long time seriesof changes in ocean climate, carbon, and ecosystems, including acidification.

It took 50 years of case-building on the part of scientists before the world began to respondaggressively to the global warming threat resulting from atmospheric CO2 increases. Becausethe chemical consequences of adding fossil fuel-derived greenhouse gases is indisputable, Fabryand Dickson hope that the wait for prudent actions will be a shorter one when it comes to the seas.

“What we’re doing now will have impacts in our lifetime,” said Fabry. “We are certainlyleaving a legacy to our children and grandchildren and they’re going to ask what did we do about it.”