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Marine Organisms Threatened By Increasingly Acidic Ocean

September 29, 2005

Every day, the average person on the planet burns enough fossil fuel to emit 24 pounds of carbon dioxide to the atmosphere, out of which about nine pounds is then taken up by the ocean.  As this CO2 combines with seawater, it forms an acid in a process known as ocean acidification.

A new study by an international team of oceanographers published in the September 29, 2005 issue of Nature reports that ocean acidification could result in corrosive chemical conditions much sooner than previously thought. Within 50 to 100 years, there could be severe consequences for marine calcifying organisms, which build their external skeletal material out of calcium carbonate, the basic building block of limestone. Most threatened are cold-water calcifying organisms, including sea urchins, cold-water corals, coralline algae, and plankton known as pteropods—winged snails that swim through surface waters. These organisms provide essential food and habitat to others, so their demise could affect entire ocean ecosystems.

In the Nature study, a group of 27 marine chemists and biologists from Europe, Japan, Australia and the United States combined recently compiled global ocean carbon data with computer models to study potential future changes in the ocean CO2 system.  The addition of carbon dioxide to the ocean lowers the pH of seawater, although seawater remains slightly basic with a pH greater than 7. The models project that the ocean’s coldest surface waters, such as in the Weddell Sea of Antarctica, will become corrosive to pteropods much sooner than thought. Shells of these marine organisms may simply dissolve as soon as atmospheric CO2 reaches the levels that are expected to occur in about 50 years under the IS92a business-as-usual CO2 emissions scenario.

“We have recognized for several decades that the build-up of carbon dioxide in the atmosphere from fossil-fuel combustion will lead to ocean acidification,” said Scott Doney, a senior scientist in the Marine Chemistry and Geochemistry Department at Woods Hole Oceanographic Institution and one of the study authors. “Previous studies have noted that this change in ocean chemistry will hurt warm water species such as corals that build shells out of calcium carbonate but on relatively long time-scales of hundreds of years. We bring a new focus on the impacts to cold water ecosystems, which appear to be even more sensitive to ocean acidification and on shorter time-scales of the next few decades.”

Doney says the increased sensitivity is driven by two factors: organisms build shells out of a more soluble form of calcium carbonate called aragonite, and the baseline (pre-industrial) water composition at high latitudes is already less conducive to building shells. “The key ecological role of many of  these organisms, which include planktonic mollusks called pteropods and cold-water corals, are just starting to be understood. And in large parts of the Southern Ocean, North Atlantic and North Pacific, they may disappear before the end of this century.”

As atmospheric CO2 continues to rise, the projection is that by the end of this century the entire Southern Ocean and part of the North Pacific would become so corrosive that these organisms may not be able to grow their shells.  That has not happened for  millions of years, and the authors say the current rate of ocean acidification is unprecedented.

“Basic chemistry tells us that within decades there may be serious trouble brewing in the polar oceans,” said James Orr, lead author and ocean modeler from the French Laboratoire des Sciences du Climat et de l’Environnement. “Unlike climate predictions, the uncertainties here are small.”

As a complement to model projections, one of the study coauthors, Victoria Fabry from the Department of Biological Sciences at California State University San Marcos, set up two-day shipboard experiments and demonstrated how shells of live pteropods begin to dissolve when the corrosive conditions that are projected to occur by 2100 are met.  “Those results,” Fabry says, “suggest that for subpolar and polar pteropods to survive, they will need either to adapt to the expected changes in seawater chemistry or move to warmer, lower-latitude surface waters,”

If populations of polar pteropods decline significantly, the researchers say that decline could provoke a chain reaction of events through complex ocean ecosystems.  Pteropods are eaten by organisms ranging in size from zooplankton to whales and provide part of the diet of many fish, including commercially important species such as North Pacific salmon.

The material that makes up pteropod shells is aragonite, a common mineral form of calcium carbonate which is also secreted by other marine organisms to form external skeletal material.  Such organisms include varieties of stony corals that grow throughout the cold, dark recesses of the ocean. Unlike their better-known tropical cousins which grow in warm surface waters, these cold-water corals grow very slowly and can live to be hundreds of years old. Previous studies have already shown that ocean acidification will make tropical corals less able to build skeletal material, even before waters become corrosive. However, the cold-water corals will be the first to be bathed in waters that are actually corrosive to aragonite.

In recent years, human occupied and remotely controlled submersibles have begun to provide scientists with photographs of the beautiful skeletal structures of cold-water corals. These calcium carbonate skeletons are essential not only for their survival, but also for providing the habitats for diverse ecosystems, including deep-sea fish, eels, crabs, and sea urchins.

Cold-water corals are already threatened by open-ocean trawling for bottom fish.  Ocean acidification will add further pressure on cold-water corals, especially those made of aragonite. These corals are most abundant in the North Atlantic, where they form massive deep reefs. Unfortunately, North Atlantic polar and subpolar waters that now offer hospitable refuge down to depths of 3 kilometers, or  about  two miles, will become mostly corrosive by the end of the century due to the invasion of fossil fuel CO2.

Other marine organisms among the first to show signs of corrosion from ocean acidification are those that construct external skeletons out of another variety of calcium carbonate rich in magnesium.  These organisms include sea urchins and coralline algae, which are common on the Arctic and Antarctic sea floor.

This new study has demonstrated that cold polar surface waters will start to become corrosive to these calcifying organisms once the atmospheric CO2 level reaches about 600 parts per million. Although  that  number is 60% more than the current level, Doney and colleagues say it could be attained by the middle of this century and note there is now urgency for new research to respond to a much tougher question: To what extent will ocean acidification alter marine ecosystems and biodiversity?