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News Release : Marine Organisms Threatened By Increasingly Acidic Ocean Corals and Plankton May Have Difficulty Making Shells |
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September 29, 2005
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Enlarge Image Lophelia pertusa, the dominant stony cold-water coral. (Photo by Andre Freiwald)
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| September 29, 2005Source: Media Relations
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?
Last updated: November 19, 2009 |
