Coral Bleaching Found at New Ocean Depths

Coral bleaching has reached a new low, literally.

Scientists from the University of Plymouth have found coral reef bleaching 90 meters below the surface of the Indian Ocean – a new record depth for coral damage.

The bleaching, which has been attributed to a 30% rise in the local sea temperature, serves as a stark warning of the harm caused by rising ocean temperatures, say the scientists.

Their paper was published in Nature Communications.

Coral bleaching at new depths

Coral bleaching occurs when the surrounding water is too warm, forcing the corals to expel the algae living in their tissues. Without their colorful symbionts, the coral turns completely white. The animal (yes, a coral is an animal) is still alive, but is now more vulnerable to disease and starvation.

Deep-sea corals have been considered more resilient to bleaching, but the findings of the new study have challenged that notion.

“There are no two ways about it, this is a huge surprise,” Dr. Phil Hosegood, an associate professor in physical oceanography at the University of Plymouth and co-author of the paper, said.

“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.”

Hosegood and his colleagues have been studying the Central Indian Ocean for over a decade, supported by the Garfield Weston Foundation and the Bertarelli Foundation.

They’ve used a combination of in situ monitoring, underwater robots and satellite-generated oceanographic data to understand more about the region’s unique oceanography and the life it supports.

They first found evidence of coral damage during a research cruise in November 2019. Images from their underwater cameras gave the team their first glimpse of bleached corals deep in the ocean. Strangely, at the same time as the deeper reefs were bleaching, the team observed that the shallower reefs exhibited no signs of harm. 

Over the subsequent months, the researchers assessed a range of other data collected during the cruise and information from satellites monitoring the ocean conditions and temperatures.

Their data highlighted that, while temperatures on the ocean surface had barely changed during the period, temperatures beneath the surface had climbed from 22 °C to 29 °C due to the thermocline deepening across the equatorial Indian Ocean.

“What we have recorded categorically demonstrates that this bleaching was caused by a deepening of the thermocline,” Clara Diaz, a PhD candidate at the University of Plymouth and lead author of the study, said.

“This is down to the regional equivalent of an El Nino, and due to climate change these cycles of variability are becoming amplified. Moving forward, bleaching in the deeper ocean here and elsewhere will likely become more regular.”

Reference: Diaz C, Foster NL, Attrill MJ, Bolton A, Ganderton P, Howell KL, Robinson E, Hosegood P. Mesophotic coral bleaching associated with changes in thermocline depth. Nat. Commun. 2023;14(1):6528. doi:10.1038/s41467-023-42279-2  

This article is a rework of a press release issued by the University of Plymouth. Material has been edited for length and content.