03 April 2009
Acidification Threatens Future of Ocean Protein Sources
Undersea environment could change for marine life critical to food web
This is the second article in a series about the effects of rising atmospheric CO2 concentrations on the world’s oceans.
Washington — Over the past 200 years, industrial and agricultural activities on Earth have pushed carbon dioxide (CO2) concentrations in the atmosphere higher than they have been for 800,000 years, causing the planet to warm. And because atmospheric CO2 diffuses into the sea, the oceans absorbed 525 billion tons of carbon dioxide in the same period.
This natural process takes CO2 out of the atmosphere and minimizes some of the effects of global warming on land, but CO2 is what scientists call an acid gas that reacts with seawater to form carbonic acid. An excess of the mild acid, delivered by the rising CO2 concentrations, is beginning to alter the ocean’s fragile chemistry and interfere with its animals and organisms. (See “Rising Carbon Dioxide Concentrations Make Oceans More Acidic.”)
“Think about a carbonated beverage,” Joanie Kleypas, a research scientist at the National Center for Atmospheric Research’s Institute for the Study of Society and Environment, based in Colorado, told America.gov. “CO2 is added to a liquid under pressure. That drives CO2 into the liquid and creates a carbonated beverage. So that’s our atmosphere. We’re adding CO2 into the atmosphere like the headspace on a soda bottle, and it’s driving more of that CO2 into the ocean.
“In my field I work on corals and organisms that live on the ocean bottom,” she said. “A lot of those organisms secrete shells that are made out of calcium carbonate. If you drop a piece of coral or shell into a carbonated system and let it sit there for a while, the system is acidic and eats away at the shell. You can see it etch away. That’s what we’re so afraid of in the oceans.”
SEA LIFE AND CO2
Acidification could have the most visible effects on undersea organisms that form calcium carbonate shells and other hard parts. The shells are formed from chemical forms called ions — calcium ions and carbonate ions — in seawater.
The organisms range from planktonic algae called coccolithophores and planktonic snails called pteropods to echinoderms like sea stars and sand dollars to corals and red coraline algae. Plankton are any drifting organisms (animals, plants, single-celled archaea or bacteria) that inhabit the upper portion of oceans, seas and other bodies of water.
Pteropods, for example, an abundant source of food for other marine life, form a shell that is made out of a form of calcium carbonate called aragonite. Aragonite dissolves more readily than other kinds of shell material and could be one of the first to dissolve in an acidic ocean, Victoria Fabry, a biological oceanographer in the Department of Biological Sciences at California State University–San Marcos, told America.gov.
“We think pteropods are very sensitive to ocean acidification, and where you have an animal that’s so abundant, they’re likely to play an important role in the food web,” she said. “In the sub-Arctic Pacific, in fact, shelled pteropods are one important food source for juvenile pink salmon and pollock, cod and other important commercial fishes.”
“Phytoplankton, which is the real base of the food chain, and other microorganisms, including bacteria that can be so important in recycling nutrients,” Fabry said, “all of those populations and species could also change in response to ocean acidification.”
Increasing acidification will also affect larger commercially important fish and shellfish larvae.
“Protein resources from the sea contribute a significant amount of protein for about a third of the world’s population,” Richard Feely, a senior scientist with the Pacific Marine Environmental Laboratory (PMEL) in Seattle, part of the National Oceanic and Atmospheric Administration (NOAA), told America.gov. “Ocean acidification is a serious concern for the food resources of the future and the sustainability of these resources.”
PARTS PER MILLION
One way to determine the extent of ocean acidification is to analyze data from direct measurements of seawater and from ice cores.
Another way, Scott Doney, a senior scientist in the Department of Marine Chemistry and Geochemistry at the Woods Hole Oceanographic Institute in Massachusetts, told America.gov, is to “combine the data with computer models of ocean behavior to deduce how much acidification there has been from the pre-industrial period to the present and how much may occur into the future.”
One of the main things the models are showing involves thresholds, or tipping points.
“Right now the surface ocean is supersaturated [with calcium and carbonate ions], so it’s relatively straightforward for organisms to take calcium and carbonate out of seawater and make calcium carbonate, and it doesn’t dissolve back into solution,” Doney said.
“But as we’re changing ocean chemistry, there will be a threshold where the water will go from supersaturated to undersaturated and the shells will start to dissolve unless they’re protected somehow. You reach a point where suddenly things change fairly dramatically.”
“When you add CO2 to the seawater,” Christopher Sabine, a supervisory oceanographer at PMEL, told America.gov, “it destroys carbonate ions. That makes it more difficult for organisms to produce their calcium carbonate shells. If you decrease carbonate ions to the point where there is very little carbonate ion, and if there are shells in the water, it will actually dissolve the shells to release more carbonate back into the water.”
TIPPING POINTS
Tipping points are different for each marine organism but all result from rising atmospheric CO2 concentrations. Today, according to calculations by scientists at NOAA’s Earth System Research Laboratory, the average global concentration of atmospheric CO2 is 383.9 parts per million by volume of air.
Before 1750, known as the beginning of the Industrial Age, atmospheric CO2 concentrations were about 280 parts per million.
“Most predictions so far show that if CO2 levels get much beyond 500 or 550 parts per million,” Feely said, “then corals, for example, will no longer be able to grow fast enough to sustain themselves above sea level rise or coastal erosion processes.”
He added, “If we were very rigorous we could keep CO2 levels below about 450 parts per million, which is what we are recommending to the community. If we follow most of the plans that exist right now for reducing CO2 emissions, the target is about 550 parts per million.”
“To some extent we are committed to a certain amount of change,” Fabry said, “but we need to reduce CO2 emissions as soon as possible so we don’t push some of these organisms over the edge. I’m quite sure that if we do nothing it will be much worse. Now is better than never.”
More information about ocean acidification is available at the Pacific Marine Environmental Laboratory Web site.
