Ocean acidification causing coral 'osteoporosis' on iconic reefs

Date:
August 27, 2020
Source:
Woods Hole Oceanographic Institution
Summary:
Scientists have long suspected that ocean acidification is
affecting corals' ability to build their skeletons, but it has
been challenging to isolate its effect from that of simultaneous
warming ocean temperatures, which also influence coral growth. New
research reveals the distinct impact that ocean acidification is
having on coral growth on some of the world's iconic reefs.



FULL STORY ========================================================================== Scientists have long suspected that ocean acidification is affecting
corals' ability to build their skeletons, but it has been challenging to isolate its effect from that of simultaneous warming ocean temperatures,
which also influence coral growth. New research from the Woods Hole Oceanographic Institution (WHOI) reveals the distinct impact that ocean acidification is having on coral growth on some of the world's iconic
reefs.


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In a paper published Aug. 27, 2020, in the journal Geophysical Research Letters, researchers show a significant reduction in the density of coral skeleton along much of the Great Barrier Reef -- the world's largest
coral reef system -- and also on two reefs in the South China Sea,
which they attribute largely to the increasing acidity of the waters surrounding these reefs since 1950.

"This is the first unambiguous detection and attribution of ocean acidification's impact on coral growth," says lead author and WHOI
scientist Weifu Guo. "Our study presents strong evidence that 20th
century ocean acidification, exacerbated by reef biogeochemical processes,
had measurable effects on the growth of a keystone reef-building coral
species across the Great Barrier Reef and in the South China Sea. These
effects will likely accelerate as ocean acidification progresses over
the next several decades." Roughly a third of global carbon dioxide
emissions are absorbed by the ocean, causing an average 0.1 unit decline
in seawater pH since the pre-industrial era. This phenomenon, known as
ocean acidification, has led to a 20 percent decrease in the concentration
of carbonate ions in seawater. Animals that rely on calcium carbonate to
create their skeletons, such as corals, are at risk as ocean pH continues
to decline. Ocean acidification targets the density of the skeleton,
silently whittling away at the coral's strength, much like osteoporosis
weakens bones in humans.

"The corals aren't able to tell us what they're feeling, but we can see
it in their skeletons," said Anne Cohen, a WHOI scientist and co-author
of the study.

"The problem is that corals really need the strength they get
from their density, because that's what keeps reefs from breaking
apart. The compounding effects of temperature, local stressors, and
now ocean acidification will be devastating for many reefs." In their investigation, Guo and his co-authors examined published data collected
from the skeletons of Porites corals -- a long-living, dome-shaped species found across the Indo-Pacific -- combined with new three-dimensional CT
scan images of Porites from reefs in the central Pacific Ocean. Using
these skeletal archives, which date back to 1871, 1901, and 1978,
respectively, the researchers established the corals' annual growth and density. They plugged this information, as well as historical temperature
and seawater chemistry data from each reef, into a model to predict the
corals' response to constant and changing environmental conditions.



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The authors found that ocean acidification caused a significant decline
in Porites skeletal density in the Great Barrier Reef (13 percent)
and the South China Sea (7 percent), starting around 1950. Conversely,
they found no impact of ocean acidification on the same types of corals
in the Phoenix Islands and central Pacific, where the protected reefs
are not as impacted by pollution, overfishing, runoff from land.

While carbon dioxide emissions are the largest driver of ocean
acidification on a global scale, the authors point out that sewage
and runoff from land can exacerbate the effect, causing even further
reductions of seawater pH on nearby reefs. The authors attribute the
declining skeletal density of corals on the Great Barrier Reef and
South China Sea to the combined effects of ocean acidification and
runoff. Conversely, reefs in marine protected areas of the central
Pacific have so far been shielded from these impacts.

"This method really opens a new way to determine the impact of ocean acidification on reefs around the world," said Guo. "Then we can focus
on the reef systems where we can potentially mitigate the local impacts
and protect the reef." Co-authors of the paper include Rohit Bokade (Northeastern University), Nathaniel Mollica (MIT-WHOI joint program),
and Muriel Leung (University of Pennsylvania), as well as Russell
Brainard of King Abdullah University of Science and Technology and
formerly at the Coral Reef Ecosystem Division of the Pacific Islands
Fisheries Science Center.

Funding for this research was provided by the National Science Foundation,
the Tiffany & Co. Foundation, the Robertson Foundation, the Atlantic
Donor Advised Fund, and WHOI's Investment in Science Fund.



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Key Takeaways An innovative numerical model developed by researchers at
the Woods Hole Oceanographic Institution demonstrates the distinct impact
of ocean acidification -- separate from ocean warming -- on coral growth.

The model shows that ocean acidification has caused a 13 percent decline
in the skeletal density of Porites corals in the Great Barrier Reef,
and a 7 percent decline in the South China Sea since 1950.

Pollution and land runoff can exacerbate the effects of ocean
acidification, causing corals in local reefs to weaken more quickly than
those located farther away from human settlements.

A global-scale investigation of coral CT scans could help to target
protections for vulnerable reefs.


========================================================================== Story Source: Materials provided by
Woods_Hole_Oceanographic_Institution. Note: Content may be edited for
style and length.


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Link to news story: https://www.sciencedaily.com/releases/2020/08/200827122110.htm

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