Suprising study finds coral bleaching reaches far below the ocean surface

Deep under the Indian Ocean sits a coral reef ecosystem almost in the dark. Despite its sheltered nature and cooler water, there’s evidence it has been subject to bleaching more than 90 metres below the surface.

This is the deepest evidence of coral reef bleaching ever found.

“There are no two ways about it, this is a huge surprise,” says Dr Phil Hosegood, who is part of a team of researchers at the University of Plymouth in the UK.

“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.

“However, that is clearly not the case and – as a result – there are likely to be reefs at similar depths all over the world that are at threat from similar climatic changes.”

These deep reefs known as Mesophotic Coral Ecosystem (MCE), live between 30 and 140 metres under tropical and subtropical water. Both light-dependant coral and algae and deep-water corals that don’t require much light, are present at these depths.

You can think of it as the ‘twilight zone’, as there’s just a trickle of sunlight that makes it all the way down.

But it’s not the light that causes the damage, it’s the temperature of the water.

The coral damage has been attributed to a 30 percent rise in sea temperature – specifically due to the Indian Ocean dipole – and the researchers believe that it harmed up to 80 percent of the reefs in certain parts of the seabed.

“What we have recorded categorically demonstrates that this bleaching was caused by a deepening of the thermocline. This is down to the regional equivalent of an El Nino, and due to climate change these cycles of variability are becoming amplified,” said Clara Diaz, the lead author on the study.

“Bleaching in the deeper ocean here and elsewhere will likely become more regular.”

The team discovered the damage in November 2019, while on a research cruise. The researchers used remote operated underwater vehicles with cameras that could monitor the health of the reef.

The images were transmitted live onto the research vessel, and the team knew straight away something was wrong. Interestingly, while deeper reefs were harmed, shallow water reefs didn’t have the same bleaching issues at the time.

They found that temperatures on the ocean surface had barely changed during the period, but temperatures beneath the surface had climbed from 22°C to 29°C.

The team then went back to the reef in 2020 and 2022 and found that large parts of the coral had recovered.

“Our results demonstrate the vulnerability of mesophotic coral ecosystems to thermal stress and provide new evidence of the impact that climate change is having on every part of our ocean,” said one of the researchers, Dr Nicola Foster.

“Increased bleaching of mesophotic corals will ultimately lead to coral mortality and a reduction in the structural complexity of these reefs. This will likely result in a loss of biodiversity and a reduction in the critical ecosystem services that these reefs provide to our planet.”

The research has been published in Nature Communications.