Corals unable to recover from bleaching if sea temperatures rise: study

Corals on Australia's Great Barrier Reef risk not being able to recover from bleaching events if sea temperatures permanently rise due to human-induced climate change.

Coral reefs are one of the world's most important and productive ecosystems, however human-induced climate change and changing weather patterns have been increasing the frequency of mass coral bleaching events.

Published in the journal Science on Friday, a joint-study by Australian researchers and the U.S.-based National Oceanic and Atmospheric Administration (NOAA) found corals take a 'practice run' to a heat shock as waters warm, reducing the severity of bleaching and mortality.

"Corals that are exposed to this pattern are then less stressed and more tolerant when bleaching does occur," lead author and research fellow at James Cook University's Coral Centre of Excellence, Dr Tracy Ainsworth said in a statement.

However these pre-stress conditions disappear when seawater temperatures rise by as little as 0.5 degrees Celsius, a similar temperature to that predicted under various Climate Change scenarios, directly exposing the sensitive ecosystem to stress.

"When corals lose the practice run, there is no break or 'relaxing' for the corals as summer stress develops," Dr Scott Heron from NOAA's Coral Reef Watch program said.

"In future summers, bleaching events will occur more often and, without the practice run, become even more severe - with a greater risk for coral mortality and a fast decline in coral cover across reefs."

The research is timely, given the great barrier reef is in the midst of its worst bleaching event on record from the El Nino reinforced body of warm water travelling around the equatorial pacific, causing mass bleaching in Fiji, New Caledonia and Kiribati, among others.

The researchers said the early evidence shows roughly three quarters of the reef had exhibited the same mechanism, but some not previously exposed to bleaching stress missed out on the "practice run", suggesting damage could be even greater.

The full extent of the bleaching won't be known for some months, however scientists on Friday warned it is now reaching into subtropical waters in northern New South Wales state, with the hardest hit corals also being the most dominant.

"Should mortality ensue for these two dominant corals on the reef, Australia's subtropical reefs could undergo major changes in their biotic composition," University of Queensland Professor of marine studies, John Pandolfi said.

"On the positive side, many of the hardest hit corals on the Great Barrier Reef, such as Acropora, are showing only minimal stress to the sub-tropical bleaching event." Endit