Changing ocean chemistry threatens to harm marine life

By Bruce Lieberman
September 14, 2006

Census of Marine Life

[fotos] Florida Keys National Marine Sanctuary Oceans are absorbing rising levels of carbon dioxide and becoming more acidic. The change could be devastating for organisms that build their skeletons from carbonate ions. These include (from top) pteropods, foraminifera, coccolithophores and corals.

Fifty-five million years ago, Earth endured a period of rapid global warming, a shift so dramatic it altered ocean and atmospheric circulation, driving plankton in the seas and mammals on land to extinction.

The event, called the Paleocene-Eocene Thermal Maximum, may have been caused by volcanic eruptions that flooded the atmosphere with billions of tons of carbon dioxide. Or, methane gas frozen beneath the sea on continental shelves could have destabilized, diffusing into the atmosphere where it was oxidized into CO.

As the oceans absorbed much of the carbon dioxide, their pH fell and they grew increasingly acidified.

Thirty percent to 40 percent of a major class of plankton, called foraminifera, went extinct.

Today, scientists look at that turn in Earth's history with worry. Rising carbon dioxide emitted by the burning of gas, oil and other fossil fuels is being absorbed by the oceans – making them increasingly acidified once again.

Every day, about 22 million tons of CO generated from human activities – primarily from the burning of fossil fuels – are entering the world's oceans. That's 10 times the rate at which carbon dioxide would be absorbed by the oceans if humans did not burn fossil fuels.

Only 200 years after the dawn of the Industrial Revolution, scientists fear that human-generated CO is altering the chemistry and biology of the oceans – perhaps irreversibly. Many types of plankton that form a key part of the ocean food web, as well as coral reefs, could be devastated by falling pH. And it could happen within decades, scientists say.

“It's stunning. I don't know how else to describe it,” said Victoria Fabry, a researcher at California State University San Marcos who studies how ocean acidification threatens tiny and abundant plankton called pteropods. “The impact we're having is very alarming.”

Acidification of the oceans is the sleeping giant of global warming. Scientists are only beginning to understand what the changing chemistry could mean.

But some of the science is clear. As atmospheric CO is absorbed by the oceans, it forms carbonic acid, lowering the pH of seawater. The lower pH, in turn, decreases the availability of chemical building blocks called carbonate ions that many marine organisms need to make their calcium carbonate shells and skeletons.

Threatened plankton include coccolithophores, foraminifera and pteropods, which lie at the bottom of the ocean food chain. Both warm-water corals, such as those at the Great Barrier Reef off Australia, and deep cold-water corals, which provide critical habitats for numerous species of fish, also are in danger.

“There's nothing really controversial about ocean acidification,” Fabry said. “The chemistry is very, very well known. The only unknown is how organisms will respond and how those changes will ripple through ecosystems.”

Scientists had long thought that the world's oceans amounted to a near limitless reservoir that could absorb any CO that humans put into the atmosphere. But the oceans are highly stratified, with layers varied by temperature and salinity, and it takes time for CO to mix completely in the oceans – as many as 1,500 years, said Christopher Sabine, a researcher at the Pacific Marine Environmental Laboratory, a division of the National Oceanic and Atmospheric Administration (NOAA), in Seattle.

In fact, 50 percent of the man-made carbon dioxide that's been absorbed by the oceans is still in their upper 1,300 feet, he said.

That is where the oceans have grown more acidified.

Falling pH

The pH scale measures acidity and alkalinity. On the scale, 7.0 is neutral, lower numbers are more acidic, and higher numbers are more alkaline. Battery acid has a pH of about 0, while household bleach has a pH of about 12.

The scale is akin to the Richter scale in that increments in one direction or another change logarithmically – not linearly. Seemingly small differences in pH, then, are large and can have big consequences for ocean chemistry and sea life.

The oceans are actually alkaline, with a pH today of 8.05.

During the last ice age, peak concentrations of CO in the atmosphere measured 180 parts per million, and the corresponding pH of the upper oceans was 8.32, Fabry said.

Just before the Industrial Revolution, the concentration of carbon dioxide in the atmosphere had risen to 280 parts per million and the pH of the upper oceans had fallen to 8.16, toward the acidic side of the pH scale.

Today, the CO concentration is 380 parts per million and the pH of the upper oceans has dropped further to 8.05.

If CO2 concentrations reach 560 parts per million by the end of the century as some scenarios predict, the pH of the upper oceans will fall more, to 7.91. At 840 parts per million, it will drop to 7.76.

“To put this in historical perspective, this ocean surface pH decrease (to
7.76) would be lower than it has been for more than 20 million years,” said Steve Murkowski, director of scientific programs and chief science adviser at NOAA, during a Congressional hearing in April.

“These are systems that have been in very delicate balance,” Sabine said of the relationship between atmospheric CO and pH in the oceans. “Whenever you mess with that balance, things can go wrong very quickly and in very unexpected ways.”

Dissolving shells

Fabry first noticed that something wasn't right 20 years ago, while on a research cruise in the Gulf of Alaska. While examining a glass jar filled with seawater and swimming pteropods, she observed that the translucent shells of the graceful, snaillike animals were dissolving.

“I thought, 'Wow, that's strange,'” Fabry said. “I couldn't quite believe it.”

CHARLIE NEUMAN / Union-Tribune Victoria Fabry, of California State University San Marcos, sorted through samples of water collected at various locations throughout the Pacific Ocean. She first recorded changes in ocean chemistry due to carbon dioxide 20 years ago. Consumed with other work, Fabry filed the observation away in the back of her mind. Nearly two decades later, scientists in the journal Nature reported that rising CO in seawater inhibited the ability of coccolithophores to form their calcium carbonate shells.

Fabry dug out her pteropod samples and confirmed that their shells had in fact dissolved under elevated CO conditions. A paper on her findings was published in Nature in 2005.

Fabry regards pteropods as “canaries in the coal mine” that signal what might happen throughout the oceans as they acidify. They are a key plankton species in high-latitude oceans, and they are extremely abundant. They're a key food resource for salmon, mackerel, herring and cod.

Impacts to sea life will likely be more severe in higher-latitude seas, because colder water can absorb more carbon dioxide and because seawater there mixes downward into the ocean interior, carrying CO into deeper water, Fabry said. “The problem we have in the high latitudes is that we have low species diversity already, so if you pull (pteropods) out, you know, it's not good,” she said.

Fabry, Sabine and their colleagues referred to these possible shifts in a landmark report in June that summarized the specter of ocean acidification and the need for more study. The report is called “Impacts of Ocean Acidification on Coral Reefs and other Marine Calcifiers.”

Mussels, clams, scallops and more

Scientists are quick to note that much isn't known about precisely how marine life will react to acidifying seas.

Data exist on less than 2 percent of the plankton that build calcium carbonate shells “so that really means we can't make any sweeping statements” about what might happen to them, Fabry said.

But some studies show alarming results. Even small reductions in skeleton-building elements can reduce by 50 percent corals' ability to form their skeletons.

Studies by Chris Langdon at the University of Miami have shown that corals do not adapt to acidification, at least in the short term.

The ocean uptake of CO follows a basic principle of chemistry. If you increase the concentration of a gas over a body of water, the two seek equilibrium, and the water will passively absorb the gas from the air.

Since 1800, about 525 billion tons of carbon dioxide, or one quarter of all the carbon dioxide produced by human activities, has been absorbed by the oceans.

About half remains in the atmosphere, and the other 25 percent has been absorbed by trees and other plants on land.

Even at this astounding rate, the oceans have taken up only about 15 percent of their total capacity.

“The oceans will continue to take up CO for thousands of years,” Sabine said. “We do not have to worry about the oceans running out of capacity.”

But in near-term time scales, acidification in the upper oceans could make the seas a very different place.

“The oceans are performing this great service for mankind, absorbing carbon dioxide out of the atmosphere, and they will continue to do that,” Sabine said. “But the consequence of that is that we're changing the chemistry of the oceans.”

For people in the San Diego region, a trip to tidal pools at the coast may give them an idea of what they might lose. Numerous bottom dwellers such as mussels, clams, scallops, sea urchins and starfish develop during a planktonic period in which their calcium carbonate shells are critical to their survival, Fabry said.

“If they have increased mortality due to elevated carbon dioxide, that will change all (bottom-dwelling) coastal communities,” she said. “When you just start thinking about this – it's really overwhelming.”

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