Warmer oceans have bleached corals at depths previously thought impossible

Coral bleaching has been discovered at record depths, over 90 metres below the surface of the Indian Ocean.

These depths were previously thought to be resilient to the effects of ocean warming but scientists have now found severe damage to these vital ecosystems. Up to 80 per cent of the reef was damaged in certain areas.

“This is an unequivocal revelation,” says Dr Phil Hosegood, Associate Professor in Physical Oceanography at the University of Plymouth and the project’s lead.

“Deeper corals were conventionally seen as impervious to ocean warming, primarily because their underwater environments are cooler than the surface and considered to be more stable.”

“Nonetheless, this has been proven incorrect, and consequently, reefs at similar depths across the globe may be under threat from comparable climatic shifts.”

Why are corals bleaching at this record depth?

Researchers from the University of Plymouth say the bleaching is due to a 30 per cent increase in sea temperatures in the Indian Ocean.

In 2019, scientists used cameras attached to underwater robots to assess the health of coral below the surface of the ocean. Live images broadcast back to a research vessel on the surface revealed the first signs of bleaching at these depths.

At this time, reefs in shallow waters didn’t show the same damage highlighting a stark contrast in their vulnerability.

A wide array of data was collected during the voyage in addition to data from satellites that monitor ocean conditions and temperatures. Analysis revealed that, while surface ocean temperatures remained relatively stable during this period, temperatures beneath the surface rose significantly from 22°C to 29°C.

The thermocline, a layer of the ocean where temperatures rapidly change with depth, is getting deeper, scientists say.

“Our findings unequivocally demonstrate that this bleaching resulted from a deepening of the thermocline,” says Clara Diaz, the study’s lead author.

Diaz explains that this is due to climate patterns, like the Indian Ocean’s equivalent of El Niño, which are being intensified by climate change.

“Consequently, bleaching in the deeper ocean, both in this region and elsewhere, is likely to become more frequent.”

Co-author of the study, Dr Nicola Foster adds that the findings “highlight the vulnerability” of these coral ecosystems and provide “fresh evidence of the far-reaching impacts of climate change on every corner of our ocean.”

Could similar unnoticed bleaching events be happening elsewhere?

Though parts of the reef appeared to have recovered when scientists visited again in 2020 and 2022, the researchers say more detailed monitoring is needed to keep track of corals in the deep ocean.

As damage to reefs in shallow waters increases in frequency and severity, it was hoped that corals at depths of 30 to 150 metres could help to compensate for this loss.

But this research shows that this may not be the case as the largely uncharted deep-sea corals across the planet may be experiencing similar, unnoticed bleaching events.

“The oceanography of the region is influenced by naturally occurring cycles that are being exacerbated by climate change,” Dr Hosegood concludes.

“Presently the region is grappling with comparable, if not more severe, consequences due to the combined impact of El Niño and the Indian Ocean Dipole.”

Though we can’t prevent the thermocline from deepening, he says, we can expand our knowledge of the consequences of this change within these environments which remain largely unexplored.

“Given the rapid pace of global change, this has never been more imperative.”

Video editor • Aisling Ní Chúláin