Coral species considered to be resistant to bleaching also found to be affected and dying
Satellite degree heating weeks data clearly outline the record intensity and impact of the 2024 mass coral bleaching event on the Great Barrier Reef. Photo: Lizard Island in mid March, posted by Jez Roff on X (formerly Twitter)
New evidence from the Australian Marine Conservation Society revealed that the coral bleaching observed earlier this year in the southern part of the Great Barrier Reef is far worse than expected.
A new footage indicated that the coral bleaching has extended to deeper parts of the reef, with corals at 18 metres of depth affected and even starting to die as record marine heatwaves scourge the ocean.
Coral bleaching is caused when corals get stressed by extreme conditions such as temperature, light and nutrients. As a stress response, they release symbiotic algae called as zooxanthellae, habituating in their living tissues, causing them to turn white. Bleaching of coral does not imply it is dead, but long term bleaching or stress can lead to their mortality.
According to Copernicus Climate Change Service, the global sea surface temperatures averaged around 21.07 degrees Celsius for March, the highest monthly value on record. In February this year, the temperature was 21.06°C.
Scientists said that such extended marine heatwaves have never been observed at the reef before. The US National Oceanic and Atmospheric Administration observed that some parts of the southern reef experienced extreme temperatures, escalating as high as 14.5 degree heating weeks. This is significantly higher than the previous record of 11.8 heating weeks in 2020. ‘Degree heating weeks’ is a widely used indicator that helps to estimate thermal stress of corals and predict coral bleaching. It is estimated using long-term temperature data and remote sensed sea surface temperature data.
Terence Hughes, a professor of marine biology at James Cook University in Queensland, Australia, posted on X April 12 that the ongoing mass coral bleaching and mortality event on the Great Barrier Reef is “very severe”.
He noted, “It is the strongest event so far (highest heat exposure on record for the southern region).”
In a press statement issued by the Australian Marine Conservation Socity, reef coral expert Selina Ward, the former academic director of the University of Queensland’s Heron Island Research Station, said coral bleaching was extensive at all 16 sites she has just visited across the southern section.
Ward noted that the “severe” bleaching event was worse than any she witnessed since 1992, when she started working on the Reef.
Many of the species that are seen to be bleached this year, she added, were considered to be resistant towards bleaching.
AMCS Great Barrier Reef Campaign Manager Lissa Schindler said that the new footage reveals that apart from the extensive coral bleaching event witnessed in the southern reefs, images showed that central and northern parts were also experiencing extensive and severe bleaching in some areas.
The ongoing event is the fifth mass coral bleaching in eight years.
Schindler stated the current event to be worse than the previous bleaching observed in 2020 and 2022.
He said he suspects this episode can be as bad as the one in 2016, considered the worst bleaching in history. Coral dying is already being witnessed and may increase across multiple coral species, he added.
Besides, aerial footage showed that 75 per cent of the Great Barrier Reef has been impacted due to the current bleaching event. Much of it falls under high to extreme bleaching category.
Hughes, in his X posts, mentioned that almost half of the world’s largest coral reef system was recorded to be hotter and longer during this summer than ever with respect to record high DHW.
Mass mortality has already been observed since February and will continue in the coming months, he said.
Jez Roff, a PhD student at the Centre for Marine Studies, University of Queensland wrote on X April 9 that preliminary analysis showed 46 per cent of the reefs experienced highest DHW, registering a new record of 17.7 DHW from the southern part of the reef.
Ward said that the current bleaching event indicated that humans are running out of time and urged the need to reduce greenhouse gas emissions immediately.
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