More bad news for one of Earth’s largest marine eco-systems. As unprecedented mass bleaching spreads across Australia’s coral reefs, more than half of the corals in many parts of the Great Barrier Reef are dead or dying.
Corals consist of tiny animals, known as polyps, which have microscopic plants from the genus Symbiodinium, living inside their cells. The polyps are translucent, but the tiny plants (referred to as symbionts) inside them are brightly colored, and give healthy coral reefs their robust hue.
In warmer temperature water, the symbionts begin to throw off large amounts of oxygen through the photosynthesis the normally sustains the coral polyps. When the waters reach a tipping point, there is so much oxygen produces that the host polyps survival is threatened, so they spit out the symbionts in a desperate attempt to defend themselves, thus turning white. This is what a bleaching event is.
Aerial surveys, which can readily spot the brightly bleached corals, are showing 93 percent bleaching.
Bleaching events in 2014 and 2015 shocked marine biologists by their scope and back to back occurrence, and now recently a third was confirmed.
University of Queensland Global Change Institute Director Professor Ove Hoegh-Guldberg, who has studied the impact of climate change on coral for more than three decades, said:
“From the tip of Cape York to the Whitsundays, the Great Barrier Reef in the east to the Kimberley’s in the west and Sydney Harbour in the south, Australia’s corals are bleaching like never before.
This is the worst coral bleaching episode in Australia’s history, with reports of coral dying in places that we thought would be protected from rising temperatures.”
All but a tiny section of the reef has been affected, a Great Barrier Reef survey coordinated by the ARC Centre for Excellence for Coral Reef Studies has revealed. According to coral expert Dr Tyrone Ridgway, the frequency of bleaching events over the past 20 years is reducing the recovery time between them and weakening the corals’ ability to bounce back.
“Previously, scientists thought the reefs off Western Australian could withstand bleaching and that southern waters around Sydney would be too cool for bleaching — this year has shown that is not the case,” Dr Ridgway said.
“It will already take several decades for coral reefs to recover from this bleaching event. If we have another harsh summer or a serious cyclone, it will set the clock back even further.”
Roger Bradbury, an ecologist from Australia National University has said that were coral reefs to disappear, the oceans would be similar to what they were in the Precambian era before fish had evolved; a slimy soup of plankton and algae. Globally, it has been estimated that around five hundred million people rely on coral reefs for income, protection, or food, with monetary impact valued at estimates of up to 375 billion dollars per year.
“Similarly to issues such as defence, we need bipartisan support to meet our environmental responsibilities to reduce greenhouse gas emissions and in turn reduce the impacts of future bleaching events on our precious coral reefs,” Professor Hoegh-Guldberg said.
“We still have time to turn this problem around but implementing deep cuts in CO2 emissions must start today.”
Photo: a dead coral sitting next to one that is bleached yet still alive. Ove Hoegh-Guldberg, Global Change Institute, University of Queensland