El Nino threatens Great Barrier Reef

The warm current may kill a rugged coral variety that would have otherwise lived for centuries

By Aprajita Singh
Published: Tuesday 08 July 2014

Photo courtesy UNESCO

Home to several endangered species of marine flora and fauna, including up to 1,500 species of fish, the Great Barrier Reef is a wonder to behold, and deservedly, one of the seven natural wonders of the world, as designated by the United Nations Educational Scientific and Cultural Organization (UNESCO). The reef is home to the world’s most extensive coral reef ecosystem, housing about 200 species of coral.

Of these, Porites, a genus of hard corals, is an important habitat for reef fauna and has been known to spread across several metres and live for centuries. A recent study by scientists from the University of Queensland, Australia, however, predicts that the corals may die untimely due to extreme surface temperatures brought by El Nino this year.

The team collected samples from 50 dead colonies and approximated their times of death using a Uranium-Thorium dating method to ascertain periods of high mortality in the past. They found that there were two main periods of mortality: 1989-2001 and 2006-2008.

Peaks in coral deaths corresponded to an increase in sea-surface temperatures and there was a general increase in mortality post-1980s, indicating that the inshore Great Barrier Reef region may be facing severe decline. The timings of the deaths corresponded to major flooding in the Burdekin river, implying that terrestrial runoff may also have been a contributing factor, the scientists say in the study to be published in the August 1 issue of Geochimica et Cosmochimica Acta.
According to Professor Jian-xin Zhao, who led the study, the devastating 1997-1998 period of coral bleaching followed a strong El Nino, which was compounded by poor water quality and global warming that pushed the corals beyond their capacity. The results are particularly worrying because Porites have long been considered robust, and would not have been expected to be impacted by such an extent.

A similar trend was observed in a study conducted by Zhao in the South China Sea, which suggests that the phenomenon is global.

El Nino causes anomalously cold or hot ocean temperatures around the globe. In Australia, it is associated with warm waters and severe droughts. The current predictions by the European Centre for Medium-range Weather Forecasts for a strong El Nino event this year are currently at 90 per cent, which may spell disaster for the Great Barrier Reef.

According to Zhao, the focus should now be on factors that can be controlled, such as pollution of ocean waters and overfishing, to minimise damage to the reefs.

Reconstructing the history of coral colonies, as the team has done, is a useful tool for reef managers, who can ascertain periods of previous disturbances and what caused them, enabling them to prepare for any such events in the future.


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