What Coral Bleaching on the Great Barrier Reef means

Dr Anjani Ganase, coral reef ecologist, looks at the bleak picture through the lens of the degradation of coral reefs worldwide, and what we must do to reverse the trend towards an unsustainable existence.

 

El Niño climate conditions aggravated by man-made global heating caused mass bleaching on coral reefs in the northern hemisphere during the summer of 2023, and brought devastation and mortality to reefs throughout the Caribbean. In Tobago, above average temperatures persisted for three months (September to November 2023). Over the past month, these conditions have delivered a deadly blow to the Great Barrier Reef in Australia. Anomalously high ocean temperatures have been washing over the reefs for at least ten weeks, similar to the heat waves experienced in Caribbean.

 

Initial reports from the Australian Institute of Marine Science who conducted aerial surveys state that broadscale mass bleaching has occurred along the shallow reefs in the impacted regions particularly the northern and southern sections of the Great Barrier Reef (GBR). The full extent of the GBR spans 2,300 km, covering an area that is equivalent to the size of Japan. The reefs span tropical and sub-tropical reef systems (in the south) and are critical to the surrounding ocean biodiversity, not to mention the Australian economy. While aerial surveys can easily detect the bleaching in the shallow reefs, in-water surveys are essential to determine bleaching on deeper sections of the reef.

 

Coral bleaching on the Great Barrier Reef, 2017. Credit: The Ocean Agency / Ocean Image Bank

Coral reefs of the GBR can extend beyond 30 m depth for many outer reef sites. A total of 215 in-water surveys conducted during the last week in February confirm severe bleaching on southern reefs with mild to moderate bleaching in the northern and central Great Barrier Reef. At the moment, there are no disease outbreaks but there are localised incidents of crown-of-thorns starfish (COTS) outbreaks to the south and offshore areas of the central GBR. This starfish is natural predator of corals, but population explosions essentially results in mass grinding of corals skeletons to dust. Such added impacts limit recovery and contribute to severe degradations.

 

The last bleaching event on the GBR occurred two years ago in 2022, following 2020, 2017 and 2016. Before this, the previous mass bleaching events were in 2002 and 1998. In 2016, our research team from the University of Queensland surveyed large sections of the reefs that were basically killed by coral bleaching. Fields of branching corals extending for kilometres on end such as on Opal Reef and The Great Detached Reef that were home to vibrant marine life became barren dead fields of coral skeletons – no corals, no fish. During the 2016/ 2017 bleaching, the reefs of the Far Northern GBR were the most severely devastated. This was a clear sign of water simply becoming too hot, as these pristine reefs were remote from land-based pollution and other aggravators.

 

These mass bleaching events in the largest coral reef system most visible and most frequented by man clearly indicate the accelerating warming trend of climate change over the last 30 years. The shorter periods of relief between heating events means that there is too little time for reef recovery. Full recovery of coral reefs can take over a decade. This means that many of the reefs are now dominant in fast growing weedy or pioneer species. For comparison, consider the forests.  After a fire, the recovery of the forests begins with the growth of grasses; natural succession leads to larger plants and then trees take over. For coral reefs, the fast weedy species also make way to the long-lived, reef building coral species. But these processes take many seasons, decades even.

 

A pair of clownfish in their bleached sea anemone home on the Great Barrier Reef. Credit: The Ocean Agency / Ocean Image Bank

While this bleaching event occurred during an El Niño (warmer, dry regional conditions), previous bleaching events on the GBR have become more common under La Niña (cooler, wetter regional) conditions. Similar scenarios were also experienced in the Caribbean during the cooler La Niña climate period from 2019 to 2022 where Tobago experienced mild to moderate bleaching of certain coral species. Bleaching under La Niña conditions is a sign that climate change continues to push global and regional temperatures to higher ranges.

 

Unfortunately, there is no quick fix to climate change and its ocean impacts. It requires strong action to move away from fossil fuels; and foresight to develop the management and restoration plans to bring ecosystems to recovery. When will we realise that taking care of earth’s diverse natural networks takes care of us?

 

The window for viable recovery is closing quickly. In our lifetimes, before our eyes, reef ecosystems are becoming permanently altered. The Great Barrier Reef is one of the best protected and managed reef systems in the world; and the strategies for management are the same that must be applied in the Caribbean and Tobago. In the aftermath of coral bleaching, management to curb chronic stressors are implemented, including protection of fish, especially herbivorous fish that clean that reef of algae and encourage coral recruitment. Other strategies include the education and awareness of reef tour operators  and users to take part in the recovery process.

 

Of course, the other most important act, as was stated by the Great Barrier Reef Marine Park Authority, is to get to net zero carbon emissions as soon as possible. That requires the agreements of nations, the actions of communities and the awareness of human populations around the world. Can we do it? We must.

 

 

Coral bleaching on Heron Island, Great Barrier Reef, 2015. Credit: The Ocean Agency / Ocean Image Bank