Coral Bleaching takes hold of the Great Barrier Reef

The Great Barrier Reef “is one of the most spectacular, complex, but fragile ecosystems in the world”, Sir David Attenborough remarked in the landmark documentary ‘The Great Barrier Reef with David Attenborough’.  When filming began in 2014, no one could predict just how poignant the timing of the first broadcast – December 2015 in the UK and April 2016 in Australia, would be. 2016 has seen the worst bleaching event every recorded on the Great Barrier Reef. 

Bleached coral - like the staghorn in this photo - can be recognised by its white colour. Credit  Matt Kieffer/Flickr  (CC BY-SA 2.0)

Bleached coral - like the staghorn in this photo - can be recognised by its white colour. Credit Matt Kieffer/Flickr (CC BY-SA 2.0)

As of 20th April the National Coral Bleaching Taskforce, convened by Professor Terry Hughes, reported that only 7% of individual reefs that make up the Great Barrier Reef are unbleached.  Coral mortality rates around 50% are reported in the northern section of The Reef were waters are warmest, and is expected to increase.  In mid-April, scientists from University of Technology Sydney and Macquarie University reported bleaching inside Sydney harbour – some 1,300 km from the southern tip of the Great Barrier Reef.  The proximate cause of the bleaching - a strong El Niño and warming sea surface temperatures which has stressed the corals.  The ultimate culprit – us.

Coral polyps form a symbiotic relationship with zooxanthellae – microscopic unicellular algae that reside inside the coral.  In return for supplying carbohydrates and other organic products of photosynthesis, the corals provide zooxanthellae with a safe environment and compounds (the coral’s metabolic waste) for photosynthesis.  As waters warm, zooxanthellae metabolic rates increase.  Combined with sunlight, increasing photosynthesis means more oxygen production, even to levels toxic to the corals, at which point the corals eject the zooxanthellae.  Since zooxanthellae also provide coral with colours, their ejection also means the coral turns white – just like they have been bleached.  As long as warming isn’t prolonged, corals can recover if they regain zooxanthellae, albeit with reduced growth and reproductive output.  If corals stay bleached for a long time, the corals run the risk of starvation, disease, and death.

This article was written for (and can be read in full in) The Marine Professional - a publication of the Institute of Marine Engineering, Science & Technology (IMarEST)