Deepest known evidence of coral reef bleaching found in Indian Ocean

Deep corals, which were always thought to be more resilient to rising ocean temperatures, sustained heavy damages, researchers find

Scientists have discovered the deepest known evidence of coral reef bleaching in the Indian Ocean, more than 90m below the surface.

This unprecedented evidence of bleaching at depths previously thought to be resilient to ocean warming reveals the severe levels of damage to coral reefs that play a crucial role in the ocean ecosystem.

The study, published in Nature Communications, reveals that coral reefs situated more than 90m below the surface of the Indian Ocean have experienced severe damage, affecting up to 80 per cent of the reefs in certain areas.

Researchers from the University of Plymouth who carried out the study say this damage was due to a 30 per cent rise in sea temperatures in the Indian Ocean.

“This is a huge surprise,” Dr Phil Hosegood, an associate professor in physical oceanography at the University of Plymouth and the lead researcher of the study said.

“Deeper corals had always been thought of as being resilient to ocean warming, but that is clearly not the case.”

“Reefs at similar depths all over the world are likely at risk from similar climatic changes."

The researchers have been studying the Central Indian Ocean for well over a decade and employed a combination of underwater robots, and satellite-generated oceanographic data to study damages to the coral reef.

During a research cruise in November 2019, scientists say they observed the first signs of coral bleaching using remotely operated underwater vehicles equipped with cameras.

While shallow water reefs exhibited no harm, deeper reefs were found bleaching, highlighting the stark contrast in their vulnerability.

The bleaching was caused by a deepening of the thermocline, a layer where temperature changes rapidly with depth, experts say. This deepening was linked to regional climate patterns, akin to an El Nino, which are being intensified by the worsening climate crisis.

“Due to climate change, these cycles of variability are becoming amplified. Bleaching in the deeper ocean here and elsewhere will likely become more regular,” Clara Diaz, the lead author of the study, said.

While parts of the affected reef recovered between 2020 and 2022, scientists emphasise the urgency of increasing monitoring efforts in the deep ocean.

With damage to shallow water corals on the rise, mesophotic corals were expected to compensate by delivering essential ecosystem benefits. However, the findings challenge that assumption and reveal the vulnerability of deep-sea corals.

In the recent decades tropical Indian Ocean has experienced a rapid increase in ocean warming with an average rise in Sea Surface Temperature (SST) of about 1C over the period of 1951-2015, according to the Indian government data.

This warming has been happening at a rate of 0.15C per decade.