Oceans: Great Barrier Reef: Coral bleaching in rapid succession

Australians love fish and seafood. At the top of the popularity scale of maritime delicacies are oysters, which are not on the menu in luxury restaurants down under, but are often also on the menu at ordinary fish and chip stands. But before Easter, the popular rock oysters, found from the southern end of the Great Barrier Reef (GBR) in Queensland to southern New South Wales, with its capital Sydney, were in short supply because authorities closed a number of oyster farms. The reason for this was water temperatures of 29 degrees Celsius, which meant there was a risk of strong growth of the temperature-dependent, disease-causing Vibrio bacteria.

The oyster problem is not a small matter, but a symptom of a much larger drama that is currently unfolding in the GBR and beyond in the world’s oceans. »The alarm bells should be ringing because this is not just about corals dying on the Great Barrier Reef and marine ecosystems starting to suffer. It’s about our oceans starting to fail,” warns reef scientist and member of the Australian Climate Council Dean Miller to Australian media. Global sea surface temperatures have already broken historical records for 365 days in a row. This raises concerns that the damage to tropical coral reefs could become irreversible. Corals are very sensitive molluscs that can only live in water temperatures between 23 and 29 degrees Celsius.

The 344,400 square kilometer GBR is currently experiencing its fifth mass bleaching event in eight years. Particularly affected is the southern part of the reef, which has been virtually untouched by previous bleaching events and actually consists of 2,900 individual structures. The current heat wave on the reef and the threat of death of many corals is alarming many GBR researchers. The waters of the Australian Coral Sea are so heated due to the climate phenomenon El Niño, according to scientists led by Australian geoscientist Zhi Huang in their publication in ScienceDirect Study on maritime heat waves on the GBR.

Normally an El Niño comes and goes and with it higher temperatures and a simultaneous rainfall deficit. The current El Niño is also in its final phase, but this is still not good news for the reefs, which were able to recover after previous bleaching phases. “MHWs (maritime heat waves) have increased in frequency, intensity and spatial extent in this region,” says the study. The authors continue: »El Niño, especially in the phase with a positive Indian Ocean Dipole, is the main driver of intense MHWs. But the strong MHWs in the recent past (e.g. 2017 and 2022) occurred without these climate events. They are signs of the effects of long-term climate change and local factors. A positive Indian Ocean Dipole means that the western part of the Indian Ocean is warming.

Like Zhi Huang, the reef experts from the Australian Institute of Marine Science (AIMS) and the authority for the management and conservation of the GBR protected area “Marine Park” GBRMPA are also recording the situation of the coral banks. “Aerial photography from the Reef Authority in collaboration with (…) AIMS, covering two-thirds of the Marine Park, confirms that coral bleaching has spread widely across the Great Barrier Reef,” says AIMS coral expert Neil Cantin, adding: ” “We now need to combine aerial photography of the spatial extent with underwater surveys to assess the extent of coral bleaching in deeper reef habitats in different regions of the Marine Park.”

There is no quick end to the maritime heat wave in sight. Longer, more intense heat could result in the death of corals, many scientists warn. Roger Beeden, chief scientist at the Reef Authority, is optimistic about corals’ ability to recover from previous disasters. But he also warns: “Climate change is the greatest threat to the Great Barrier Reef and coral reefs worldwide.”