Coral growth across the Great Barrier Reef has suffered a "severe and sudden" slowdown since 1990 that is unprecedented in the last four centuries, according to scientists.

The researchers analyzed the growth rates of 328 coral colonies on 69 individual reefs that make up the 1,250 mile-long Great Barrier Reef, off north-east Australia. They found that the rate at which the corals were laying down calcium in their skeletons dropped by 14.2 percent between 1990 and 2005.

Corals around the world are severely threatened by coastal pollution, warming seas and over-exploitation, but the most probable explanation for the drop in the growth rate of the corals' calcium carbonate skeletons is acidification of the water due to rising carbon dioxide levels in the atmosphere. More acid water makes it more difficult for the coral polyps to grab the minerals they need to build their skeletons from the seawater.

"Our data shows that growth and calcification of massive Porites in the GBR (Great Barrier Reef) are already declining and are doing so at a rate unprecedented in coral records reaching back 400 years," wrote Dr Glenn De'ath from the Australian Institute of Marine Science in Townsville, Queensland, and his colleagues in the journal Science.

Porites corals can be centuries old and grow into 6m tall mounds. Rather like a tree ring, each year's growth is visible as a band, so by drilling into the corals the scientists could examine the extent of growth in specific years. The team measured annual growth and skeletal density and calculated the amount of calcification annually. They found that the calcification rate rose 5.4 percent between 1900 and 1970, but this dropped by 14.2 percent between 1990 and 2005. The drop was mainly due to a growth slowdown from 1.43cm a year to 1.24cm.

"This study has provided the first really rigorous snapshot of how calcification might be changing," marine biologist Ove Hoegh-Guldberg of the University of Queensland in Australia told Science. "The results are extremely worrying."

(China Daily January 4, 2009)