Coral Bleaching on the Great Barrier Reef

Fantastic news from the long-term monitoring program of the coral Reef condition on the Great Barrier Reef is that the GBR is recovering after decades of disruption.

The Aims Great Barrier Reef condition report states that the hard coral increased in the last two years in 2021.

Dr Mike Emslie is the Reef monitoring program’s Long-term Monitoring Program Team leader.    Please click here to read the whole report.

Assessing the long-term health of the Great Barrier Reef

Determining the status of the GBR requires robust long-term datasets collected using standard methods.  Long-term data are critical to avoid the “shifting baseline” syndrome, as the results each year are always considered in the context of long-term trends.

The last couple of years has revealed that recovery is underway across much of the GBR.  This a good sign that the GBR still has the capacity and necessary  functions to recover.

Central and Southern Great Barrier Reef

The Central and Southern GBR had periods of recovery within the last decade, curtailed by disturbances, arresting recovery and causing further coral declines.  The sustained recovery of the GBR back to historically high coral cover requires the next few years to be disturbance-free to allow corals to continue to grow and increase their inhabitants.

While the hard coral cover has increased across all three regions over recent years, the Northern and Southern GBR is still below the highest recorded coral cover in the 1980s, and preliminary analyses have documented shifts in the dominant corals on some reefs.

2021 has been a low disturbance year, while 2014 to 2020 was an intense period of widespread disturbances.  There were numerous severe tropical cyclones and three mass coral bleaching events in five years.

The fourth wave of crown-of-thorns starfish outbreaks began around 2010 between Lizard Island and Cairns and, by 2020, had progressed south to reefs offshore from Townsville.

The prognosis for the future distress regime under climate change is one of the increasingly frequent and longer-lasting marine heat waves and tropical cyclones.  Mitigation of these climatic threats requires immediate global action on climate change.

Crown-of-thorns starfish as coral predators are a significant cyclic disturbance on the GBR, and when left unchecked, outbreaks can destroy coral populations.  However, it is one of the few threats to the GBR that can be managed locally.

The Crown-of-thorns Control Program has been active on the GBR during the current outbreak.  It seeks to reduce starfish numbers at essential reefs to reduce the coral loss and diminish the broodstock that propagates the episode ‘wave’ southward through most of the GBR.


In 2021, the number of active outbreaks declined from previous years.  Attacks were recorded in the Swain sector of the Southern GBR.  The LTMP aims to assist in analyses of the effectiveness of the Crown-of-thorns Control Program in the next few years.

The predicted consequences of climate change, which include more frequent and intense mass coral bleaching events, are now a contemporary reality.

At the same time, chronic stressors such as increasing ocean temperatures and changing ocean chemistry can all negatively affect recovery rates.  More frequent acute distributions mean that the intervals for recovery are becoming shorter.

What is Coral Bleaching

Corals eject the symbiotic algae dwelling in their tissues.  This causes them to turn entirely white when under pressure by environmental changes like temperature, light, or nutrients.

Can coral withstand a bleaching incident?

Coral has recovered n to recover if bleaching is not severe.

If the stress and algae loss are both protracted, the coral perishes

The result of warmer water temperatures can cause coral bleaching.

Corals will eject the algae (zooxanthellae) dwelling in their tissues if the water is too warm, turning the coral white.   The term for this is coral bleaching.  Whitening of the coral is not already dead.

Corals may endure a bleaching episode, although they are more vulnerable to stress and death.

In 2016 and 2017, Australia’s Great Barrier Reef was under stress due to high temperatures.  Most of the Great Barrier Reef was affected.  Marine Scientists were concerned the Great Barrier Reef would not recover.

In 2021 the Great Barrier Reef marine scientists that have been monitoring the Great Barrier Reef for over thirty years said they recorded the highest level of coral cover since the program began.  This is fantastic news for the Great Barrier Reef.

What is coral bleaching?

Healthy Coral

Algae and coral both rely on one another to live.

Corals and zooxanthellae share a symbiotic relationship.

The coral’s main food supply and source of colour are these algae.

Coral under strain

When the coral is stressed, the algae leave the coral.

Algae depart the coral tissue when the symbiotic relationship is challenged by high ocean temperature or pollution.

The Coral turns white when it is left exposed and vulnerable.

Algae is the coral’s primary food source.  Without algae, the coral turns white or pale and is prone to illness.

Why does Coral Bleaching happen?

An increase in the heat of the Ocean can cause Coral bleaching caused by rising ocean temperatures.

Pollution and runoff

Storm-generated rain can quickly dilute ocean water, and runoff can transport chemicals that may cause stress on the coral close to the coast.  High temperatures and the sun in shallow water may cause coral stress.

Low Tides

Shallow water coral may bleach when exposed to the air during shallow tides.

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