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Global warming killing off coral in Red Sea

26/07/2010 05:28:18

CT scans reveal coral has been under ‘chronic stress' for ten years

July 2010: A major Red Sea coral species is being killed off by global warning, according to the latest study.

As summer sea surface temperatures have remained about 1.5 degrees Celsius above ambient over the past ten years, the growth of the coral - Diploastrea heliopora - has declined by 30 per cent. The scientists, who used CT scans to assess the skeletal health of the coral, suggest that the coral could cease growing altogether by 2070.

‘The warming in the Red Sea and the resultant decline in the health of this coral is a clear regional impact of global warming,' said Neal E. Cantin, a Woods Hole Oceanographic Institution postdoctoral investigator and co-lead researcher on the project. In the 1980s, he said, ‘the average summer [water] temperatures were below 30 degrees Celsius. In 2008 they were approaching 31 degrees'.

Looks healthy – but skeleton tells a different story
Cantin and the other lead investigator Anne L. Cohen said that although the coral looked healthy, CT scans of its skeletal structure revealed ‘the secrets that the skeletons are hiding'. ‘The CT scans reveal that these corals have actually been under chronic stress for the past ten years,and that the rates of growth were the lowest in 2008,' the final year of the study.

Historically, scientists have used X-rays to examine coral skeletons, which display annual growth bands much like tree rings, Cantin explained. But that method usually entails cutting into the skeleton, he said. CT allows non-invasive 3D observation of the skeletons and bands. ‘The biggest advantage over X-ray is that we can scan intact cores without cutting the core into thin slices,' said Cantin. ‘Since corals do not grow in a straight line, when the core is cut, inevitably the growth axis will be lost from a thin cut. Maintaining the vertical growth axis is crucial for us to visualise the annual density banding patterns.

The use of CT scans is a huge leap forward
‘With CT scanning we are able to work with a complete 3D reconstruction of the entire core, from which we can make "virtual" cuts on the computer that have the exact thickness, orientation and location that we need for a particular coral to get the most precise measurements. X-ray requires that we cut the core ‘blind' beforehand, before we know what the orientation of growth is. Whole cores can be sacrificed this way. This is a huge leap forward."

The researchers scanned six skeletal cores of D. heliopora and were able to pinpoint two high-density growth bands, indicating high thermal stress in 1998 and 2001. This correlates with an abrupt drop in skeletal growth after 1998, which has continued steadily since then. The corals are building skeleton, or calcifying, at a progressively slower rate because they are losing symbiotic algae that live in the coral tissue. By performing photosynthesis, the algae provide the fuel for the corals to make new skeleton.  But, says Cohen, ‘when the corals are thermally stressed, they lose algae and many will eventually starve and die. When corals lose enough algae, they actually turn white, and that's what bleaching is. We think these corals are on their way to bleaching'.

It was the CT technique that enabled early detection of the problem. ‘The corals look healthy, but looking inside at the skeleton gives you an idea of things to come,' she said. ‘It's like osteoporosis. You look at a person and, on the outside, everything seems fine, but inside there are signs of trouble.'

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