“Untouchable” coral reefs in the deep Indian Ocean waters faced bleaching after 2019 warming, study finds

Tools and fire were probably just extremely fortunate means to an end for ancient humans. But as hammerstones evolved to AIs and bullet trains, we were somehow left cleaning up our mess from even the "pristine" expanses of space and the deep ocean. But as our influence gradually leeches over more and more of the planet, the list of things left unharmed also continues to shorten every day.

In a groundbreaking study, researchers have uncovered an alarming revelation — the deepest evidence of coral bleaching at depths previously deemed "untouchable" by global warming! The team found that up to 80% of certain reef areas 90 metres below the Indian Ocean were affected, likely due to an unprecedented 30% rise in the region's sea temperatures.

Trouble deepens for marine ecology

In recent years, 75% of the world’s tropical coral reefs have experienced bleaching due to increased heat stress. Scientists estimate that 70–90% of the reefs will be lost with a 1.5 °C global warming while almost all of them would be bleached out if the temperatures increase beyond 2 °C. But most of these impacts have always been thought to be restricted to shallower regions.

The recent study, published in the reputed scientific journal Nature, presents one of the first evidence of coral bleaching in deeper coral reefs. The team attributed this spike in temperatures to the positive state of the Indian Ocean Dipole (IOD) in 2019.

The IOD refers to a temperature gradient between the eastern and western parts of the Indian Ocean. A positive IOD — often associated with the El Niño phase over the Pacific Ocean — means warmer than normal water in the western parts and it helps enhance the monsoon rainfall across India. Even this year, the same phenomenon has helped offset the impact of the current El Niño phase and prevent a full-blown drought in India.

However, the growing intensity and frequency of positive IOD events may have spelt disaster for our deep corals!

"There are no two ways about it; this is a huge surprise," remarks Dr Phil Hosegood, the project lead. "Deeper corals had always been thought of as being resilient to ocean warming because the waters they inhabit are cooler than at the surface and were believed to remain relatively stable."

Being a reservoir of rich marine biodiversity, they support an estimated 25% of known marine species! Bleaching represents a break in the symbiotic relationship between the corals and the vibrant algae, limiting their ability to support numerous marine species. Therefore, such events can spell doom for the entire marine ecosystem of the region.

Unprecedented ocean warming to blame

The first evidence of coral damage was observed during a research cruise in November 2019 when remotely operated underwater vehicles equipped with cameras transmitted live images of the bleached corals. Ironically enough, no observable harm was evident in the region's shallow water reefs.

Subsequent analysis revealed that a deepening thermocline, driven by a positive IOD (regional equivalent of El Niño), caused the bleaching in deeper reefs. In simpler words, while the temperature at the ocean's surface stalled, mercury levels beneath the surface sneakily jumped seven massive notches from 22-29°C — too much for most organisms to adapt to in such a short span of time.

Scientists have always expected such deep water corals — spanning depths between 30-150 metres under the surface — to pick up the ecological slack as shallower reefs die out. But this new finding shows they might be just as vulnerable to bleaching. The fact that they remain so painfully understudied, mainly due to the complexity of deep ocean exploration, only adds to the worrisome uncertainty.

Fortunately, however, the team also noticed that many large swaths of the reef managed to recover when they went back for expeditions in 2020 and 2022. However, this does mean we're out of hot water yet, the authors warned. With the oceans warmer than they’ve ever been, another ongoing positive IOD phase and a “super strong” El Niño on the cards, conditions could worsen for the Indian Ocean reefs once again.

"While there is no way we can stop the thermocline from deepening, what we can do is expand our understanding of the impacts that these changes will have throughout these environments of which we have so little knowledge. In the face of fast-paced global change, that has never been more urgent," explains Dr Hosegood.

The findings of this research have been published in Nature Communications and can be accessed here.