July 16, 2010

Red Sea Coral Species Under Threat

Scientists say that a species of coral in the Red Sea could stop growing by 2070 if current warming trends continue.

A team of researchers said they used 3D technology to find that the rate of growth of Diploastrea heliopara declined by 30 percent since 1998.

They found that rising sea surface temperatures were already "driving dramatic changes" in the growth rate of the important reef-building organism.

Co-author Anne Cohen, a research specialist at Woods Hole Oceanographic Institution (WHOI), said the team was able to measure the decline in growth by examining core samples from coral skeletons.

"The coral is an animal, and the colony made up of millions of tiny, little animals - and they together build this huge thing that is seven meters in diameter," she told BBC News.

"As they are growing, they are building this calcium carbonate skeleton that the animal is basically leaving behind. If you cut through a colony, only the very top layer is actually living - the rest of it is all dead."

"What is really cool is that everything that the colony has experienced in its life, which can be very long - these colonies can live four or five hundred years - is recorded in the skeleton," Cohen said.

"It is recorded in annual growth bands, so we know exactly the year in which certain things happened."

The researchers collected biopsies from six colonies, which were then put under a computerized tomography (CT) scanner to be examined.

"The scan reveals variations in density in calcium carbonate because the growth rings are caused by changes in density; what the CT scanner is effectively doing is revealing the annual growth bands for us, which you cannot see with the naked eye," Cohen said.

WHOI used the technique because many of the corals had very complicated skeleton growth patterns, which were too complex to examine using 2D images.

The researchers were able to see how much the coral had grown in any given year by building up a profile of the growth bands.

"We can also work out the density of the band, which tells us how much calcium carbonate the coral has put down," Cohen told BBC.

"We say in our paper that the upper growth rate has decreased by 30%, and the amount of calcium carbonate produced has decreased by 20% since 1998."

The team compared coral growth with sea surface temperature (SST) record by using precise chronology provided by the CT scans.

There was a critical temperature of 86.9 degrees Fahrenheit, above that the growth rate "basically plummeted."

"So we have identified a threshold temperature for growth," said Cohen.

The team calculated that the coral would "cease calcifying together by 2070" by using future climate change scenarios.

However, the researchers said this timescale was likely to be conservative.

"One reason why we say this is because we think the corals will bleach long before this," Cohen told BBC.

"They are going to lose their symbiotic algae (zooxanthellae) long before they stop calcifying, and many of these corals will die at that point."

However, she pointed out that the paper only considered the impact of rising SST on one species from the 250 or so in the Red Sea.

"It is a very important species - a dominant reef-building species - but it is only one species," she said.

"I expect that there are corals that are doing much worse than this species, but there might be others that are doing better in terms of thermal tolerance."

"Before we can talk about what the coral reefs are doing to do in the future, we really need more information about more species," she observed.

The researchers plan to carry out similar studies on more species.  Cohen said they collected samples from another dominant reef-building species, which they were planning to examine in the next few months.

The findings were reported in the journal Science.


Image 1: Diploastrea heliopora in its natural habitat in the Red Sea. The coral's growth has slowed considerably in the last 10 years. (Woods Hole Oceanographic Institution)

Image 2: Neal E. Cantin and Anne L. Cohen examine a Red Sea coral specimen just outside the hollow tube of a CT scanner. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)

Image 3: CT scan reveals two high-density (light-shaded) bands in the skeleton of the D. heliopora coral, which coincide with high sea-surface temperature anomalies in 1998 and 2001. (Woods Hole Oceanographic Institution/Science Magazine)


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