These last few weeks have appeared particularly bleak for the state of corals in the Pacific Ocean. With headlines hailing the Great Barrier Reef coral bleaching as the “worst bleaching in history”, and a prediction that this event would leave a lasting effect on coral life in the region, it was the inevitable news that those who study and love the ocean feared, and a news story that shocked the wider world. The long-term survival of one of the natural wonders of the Earth, and a peak tourist destination and dive haven, appeared to be in question.

A study carried out by Reef & Rainforest Research Centre provides further information on the northern section of the Great Barrier Reef, which is currently undergoing a major coral bleaching event – a stress response to poor water quality and higher-than-average water temperatures mainly as a result of the present El Niño climate phenomenon. This specific region of the reef is experiencing severe, large scale bleaching for the first time. But the study reports that this isn’t necessarily the end of the coral reefs that form the vast underwater metropoles in the Southern Hemisphere; whilst a significant portion of the coral may die as a result of the bleaching, the corals that survive will be more tolerant of high temperatures in the future.

Coral bleaching explained

Those vibrant underwater coral landscapes of a thousand colours come from the presence of symbiotic algae (zooxanthellae) that live in their tissues and produce 90 percent of the coral’s energy requirement through photosynthesis. Zooxanthellae and the corals share a mutualistic relationship: the coral provides the algae with a protected environment and the compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove wastes.

Bleaching occurs when corals expel the majority of their zooxanthellae, and turn to white. Different varieties of coral have different temperature tolerance levels, but all coral is vulnerable to bleaching.

“There are four levels of bleaching,” says Col McKenzie, the Executive Director of the Association of Marine Park Tourism Operators. “Type one is when only the ‘ends’ and ‘surface coral’ is involved, type two is when the entire coral pales, type three is when the coral bleaches white, and type four is the death of the coral.”

“In types one and two bleaching, the coral should recover with minimal mortality but will then be slow to reproduce.” He then adds that most of the current bleaching is either type one or two, with around five percent to 10 percent of type three, and around one percent of type four.

Currently the waters of the northern section of the Great Barrier Reef are stressful conditions for the resident coral, and what determines their fate is all down to how long the event lasts, and if they have suffered from bleaching before. If the stressful conditions persist for an extended period, the corals will eventually starve to death.

If temperature and water quality conditions return to normal, corals can be recolonised by zooxanthellae that have survived the high temperatures, and then return to their normal colour and survive. Although these corals will not reproduce for one or two years, they will likely be more tolerant to higher temperatures, and later El Niño events.

Past GBR bleaching events 

These past two decades have seen several incidents of widespread mass coral bleaching on multiple coral reefs around the world. There have been seven major bleaching events alone on the Great Barrier Reef, with three being particularly widespread – 1981-82, 1997-98 and 2001-2.

Previously, the 1997-98 mass bleaching event saw 74 percent of inshore and 21 percent of offshore reefs having moderate to high bleaching. However, most reefs recovered fully, with an estimated two to five percent of inshore reefs experiencing high coral mortality – that’s an area one-third of the size of London. The central GBR was most affected, with the Palm Island site suffering up to 70 percent coral mortality.

In the summer of 2001-2, there was another mass bleaching event, with evidence of bleaching at 54 percent of the reefs that were surveyed. This time, 41 percent of offshore reefs and 72 percent of inshore reefs showed moderate to high bleaching and an estimated five percent of corals died. Once again the worst affected reefs were situated in the central GBR region.

Again in 2006, sea temperatures in the southern GBR around the Keppel Islands reached 1-2°C above the seasonal average, and bleaching was observed in up to 98 percent of corals with approximately 39 percent coral mortality on the reef flats and 32 percent mortality on deeper reef slopes.

What’s in the future for the reef?

“As long as the water temperature comes down and we get the water quality right, the reef will recover,” Col states, “although there might be some biodiversity lost as the more susceptible types could die out.” 

Previous mass bleaching events have shown that significant coral recovery can occur even after severe bleaching. Importantly, prior exposure of corals to high temperatures is a key determinant in whether they will bleach, and how much they can recover from the bleaching. The GBR is expected to experience more frequent and more serious temperature stresses in the future.

The Australian Institute of Marine Science and the University of Queensland are researching the ability of individual corals to increase their temperature threshold by “shuffling” their zooxanthellae to preferentially host the zooxanthellae that are more tolerant of high temperatures.

It’s difficult to determine the exact state and absolute future of the reefs, but Col McKenzie is adamant to highlight the positives. “There is some good news in that the corals that survive can become more resilient, but a lot more work needs to be done in this field. While large bleaching events do occur as long as the event does not last for weeks on end, the coral can recover very quickly.”

The severity of coral bleaching continues to divide expert opinions. Truly only time will have the final say.