The Great Barrier to understanding the Reef

Aug 13, 2022
Great Barrier Reef climate change climate action ocean
(Image: Unsplash)

The Great Barrier Reef is a vast and complex ecosystem. Sometimes headline statements about its health can appear confusing and even contradictory.

Earlier this year, the Marine Park Authority responsible for its welfare announced a fourth coral bleaching event, despite a cool La Nina year. Then in early August, the Australian Institute of Marine Science announced that coral recovery was leading to 36-year highs across two-thirds of the Great Barrier Reef. One could be forgiven for being confused.

Yet behind the headlines, there was a great deal of consistency between the two statements. I’ve written previously about the coral bleaching event earlier this year that lead to widespread bleaching but not widespread death. On the whole, the water temperature was not lethal to corals, unlike the marine heatwaves of 2016 and 2017. The effects of this year’s heatwave are not known yet but could lead to reduced growth and reproduction. That means fewer and weaker baby corals.

The announcement by AIMS in early August concerned its annual survey of the Reef. The surveys, which began in the 1980s, measure one thing: coral cover. It’s an important metric, but it has its limitations. It simply measures the amount of living coral on the surface of the seafloor. It does not tell us anything about the diversity or composition of the coral communities.

In theory, one square metre could be 100% covered by one species of coral, or it could be 100% covered by 10 species. The latter would provide a more complex habitat for reef fish and invertebrates than the former and would probably be more beautiful to the human eye.

Yet a simple measure of coral cover doesn’t distinguish between the two. Back in the 1980s, this was not an issue. But with rapidly increasing climate impacts, a simple coral cover measure becomes a major issue in communicating the state of the Reef to the public.

The survey found that most of the recovery is of one group of species: Acropora. This group includes the famous table corals and staghorn corals. These species are the ‘pioneer’ species – the ones that grow rapidly after disturbance. They are also the most vulnerable to damage from cyclones and further coral bleaching, and are the preferred diet of the coral-eating Crown-of-Thorns starfish.

The AIMS report finds that where Acropora species are the dominant growth, the rapid increase in coral cover can be quickly overturned. So, while the topline message sounds positive, the underlying message is of vulnerability and temporary reprieve. The prognosis for the Reef remains alarmingly negative. It’s one of increasingly frequent, severe, and long-lasting marine heatwaves of lethal effect.

One coral expert said to me recently: if the Reef can bleach in a La Nina year, I am terrified of the next El Nino.

It is also worth noting that while the AIMS surveys select coral reefs that are representative of the larger ecosystem, they cover only 87 out of approximately 3,000 coral reefs that make up the largest living organism on Earth. The report notes that there are reefs not surveyed in the last year that would have accumulated heat stress above thresholds at which coral mortality would be expected.

While we are all deeply relieved about the imminent passage of the Albanese climate bill through Parliament, no one is under any illusion that much more ambition is needed to keep parts of the Great Barrier Reef alive. The question remains: will the World Heritage Committee keep up the pressure on the Australian Government to align with the critical threshold for coral reef survival, 1.5C of global warming?

This question will be answered whenever the World Heritage Committee meets. At the moment, it is paralysed due to Russia being the Chair and a large number of countries refusing to attend as a result.

Regardless, it is likely that in the next 2-3 months UNESCO will release a report on the Great Barrier Reef written by two experts who visited Australia in March, one from UNESCO, the other from IUCN. The report’s findings may provide Australia with another incentive to enhance its new role as a leader on climate action by working in partnership with our neighbouring countries who also have climate-vulnerable World Heritage listed tropical coral reefs.

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