How the 2016 bleaching altered shape of Great Barrier Reef

Different colour morphs of Acropora millepora, each exhibiting a bleaching response during mass coral bleaching event

The Conversation

In 2016 the Great Barrier Reef suffered unprecedented mass coral bleaching – part of a global bleaching event that dwarfed its predecessors in 1998 and 2002. This was followed by another mass bleaching the following year.

This was the first case of back-to-back mass bleaching events on the reef. The result was a 30% loss of corals in 2016, a further 20% loss in 2017, and big changes in community structure.

New research published in Nature today now reveals the damage that these losses caused to the wider ecosystem functioning of the Great Barrier Reef.

Fast-growing staghorn and tabular corals suffered a rapid, catastrophic die-off, changing the three-dimensional character of many individual reefs.

In areas subject to the most sustained high temperatures, some corals died without even bleaching – the first time that such rapid coral death has been documented on such a wide scale.

The research team, led by Terry Hughes of James Cook University, carried out extensive surveys during the two bleaching events, at a range of scales.

First, aerial surveys from planes generated thousands of videos of the reef. The data from these videos were then verified by teams of divers in the water using traditional survey methods.

Finally, teams of divers took samples of corals and investigated their physiology in the laboratory. This included counting the density of the microalgae that live within the coral cells and provide most of the energy for the corals.

The latest paper follows on from earlier research which documented the 81% of reefs that bleached in the northern sector of the Great Barrier Reef, 33% in the central section, and 1% in the southern sector, and compared this event with previous bleaching events.

Another previous paper documented the reduction in time between bleaching events since the 1980s, down to the current interval of one every six years.

Different colour morphs of Acropora millepora, each exhibiting a bleaching response during mass coral bleaching event
Different colour morphs of Acropora millepora, each exhibiting a bleaching response during mass coral bleaching event

Although reef scientists have been predicting the increased frequencyand severity of bleaching events for two decades, this paper has some surprising and alarming results. Bleaching events occur when the temperature rises above the average summer maximum for a sufficient period.

We measure this accumulated heat stress in “degree heating weeks” (DHW) – the number of degrees above the average summer maximum, multiplied by the number of weeks. Generally, the higher the DHW, the higher the expected coral death.

The US National Oceanic and Atmospheric Administration has suggested that bleaching generally starts at 4 DHW, and death at around 8 DHW.

Modelling of the expected results of future bleaching events has been based on these estimates, often with the expectation the thresholds will become higher over time as corals adapt to changing conditions.

In the 2016 event, however, bleaching began at 2 DHW and corals began dying at 3 DHW. Then, as the sustained high temperatures continued, coral death accelerated rapidly, reaching more than 50% mortality at only 4-5 DHW.

Many corals also died very rapidly, without appearing to bleach beforehand. This suggests that these corals essentially shut down due to the heat. This is the first record of such rapid death occurring at this scale.

This study shows clearly that the structure of coral communities in the northern sector of the reef has changed dramatically, with a predominant loss of branching corals.

The post-bleaching reef has a higher proportion of massive growth forms which, with no gaps between branches, provide fewer places for fish and invertebrates to hide. This loss of hiding places is one of the reasons for the reduction of fish populations following severe bleaching events.

The International Union for Conservation of Nature (IUCN), which produces the Red List of threatened species, recently extended this concept to ecosystems that are threatened with collapse.

This is difficult to implement, but this new research provides the initial and post-event data, leaves us with no doubt about the driver of the change, and suggests threshold levels of DHWs. These cover the requirements for such a listing.

Predictions of recovery times following these bleaching events are difficult as many corals that survived are weakened, so mortality continues. Replacement of lost corals through recruitment relies on healthy coral larvae arriving and finding suitable settlement substrate.

Corals that have experienced these warm events are often slow to recover enough to reproduce normally so larvae may need to travel from distant healthy reefs.

Although this paper brings us devastating news of coral death at relatively low levels of heat stress, it is important to recognise that we still have plenty of good coral cover remaining on the Great Barrier Reef, particularly in the southern and central sectors. We can save this reef, but the time to act is now.

This is not just for the sake of our precious Great Barrier Reef, but for the people who live close to reefs around the world that are at risk from climate change. Millions rely on reefs for protection of their nations from oceanic swells, for food and for other ecosystem services.

This research leaves no doubt that we must reduce global emissions dramatically and swiftly if we are save these vital ecosystems.

We also need to invest in looking after reefs at a local level to increase their chances of surviving the challenges of climate change. This means adequately funding improvements to water quality and protecting as many areas as possible.

Source: The Conversation. Reproduced with permission.


2 responses to “How the 2016 bleaching altered shape of Great Barrier Reef”

  1. Joe Avatar

    That the GBR, a UNESCO World Heritage Site, is dying is a National Disgrace and An International Disaster. But it is also coral reefs all around the world that are feeling the effects of warming sea temps. I can count myself fortunate indeed to have visited and enjoyed The GBR when I visited over 10 years ago. Anyone wanting to visit The GBR to see the remaining splendour had better make their plans quickly.

  2. Syd Walker Avatar

    Policy of both “sides of politics” in northern Queensland has long been to placate the polluters, especially the farming lobby. Sediments, excess nutrients and pesticides pour into the Coral Sea every year. Then there’s the mining and fishing industries..

    The jobs that came through World Heritage declarations were welcomed as freebies; there has never been a real sense up here that there’s a cost (well worth paying, in my opinion) associated with maintaining world class environmental heritage for future generations. That cost is we need to stop treating “sustainability” like a bit of annoying legalese to get around, and start closing down industries and municipalities if they can’t or won’t practise genuine sustainability. Run-off of anything to the sea should not exceed natural levels. Surely that’s not too much to ask?

    These mass bleachings, however, do appear to be more or less completely related to sea temperature (and acidity?). They’ve damaged such vast areas of reef, including areas that had previously remained relatively unaffected. It’s a terrible tragedy, requiring an urgency of response that’s simply lacking at all levels of government.

    People in the north of Queensland need to look ourselves in the mirror. We are the bogans on whose watch the Great Barrier Reef has fallen into a grossly diseased state. In majority votes, we elected the bogans who presided over this collapse and who have done zip to address the problem. Sorry, future generations.. we screwed up the reef while pretending we were trying not to..

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