Deepest Known Evidence of Coral Bleaching Discovered in Indian Ocean

Coral bleaching occurs when stressors such as changes in temperature, nutrients or light cause corals to expel the zooxanthellae algae living in their tissue, turning them white. The most common cause of coral bleaching is warming ocean temperatures due to climate change.

Now, a new study by scientists from England’s University of Plymouth has found the deepest known evidence of the bleaching of a coral reef, nearly 300 feet below the surface in the Indian Ocean.

“As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30–150 m), cooler water and were thought to offer a refuge to shallow-water reefs,” the scientists wrote in the study. “Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative.”

The bleaching event was attributed to the Indian Ocean dipole — an irregular oscillation of ocean surface temperatures where the western and eastern parts of the ocean become alternately warmer, then colder. This led to a 30 percent elevation in ocean temperatures that damaged 80 percent of the coral reefs in parts of the seabed at depths that were thought to be resilient to warming, a press release from the University of Plymouth said.

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

The study, “Mesophotic coral bleaching associated with changes in thermocline depth,” was published in the journal Nature Communications.

Studies in the Central Indian Ocean have been conducted by researchers from the University of Plymouth for more than a decade, the press release said. During their research cruises, a combination of satellite-generated oceanographic data, in situ monitoring and underwater robots have been used to learn more about the distinct oceanography of the region, as well as its biodiversity.

Evidence of coral damage was first discovered during a November 2019 research cruise. During the cruise, remotely operated underwater vehicles equipped with monitoring cameras were used to observe coral health. Images from the cameras were transmitted live to the research vessel, giving the team its first look at the bleached corals.

Unexpectedly, during the same time frame, shallow water reefs showed no sign of bleaching.

Over the following months, the team looked at an array of additional data collected during the cruise, as well as satellite information on ocean temperatures and conditions.

The information showed that, while ocean surface temperatures had hardly changed during that period, temperatures below the surface had risen from 71.6 to 84.2 degrees Fahrenheit due to the deepening of the thermocline — the transition layer between the upper mixed layer and the water below — across the Indian Ocean at the equator.

“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 Niño, 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,” said lead author of the study Clara Diaz, who is a University of Plymouth Ph.D. student on mesophotic coral ecosystems, in the press release.

University of Plymouth researchers returned to the area in 2022 and 2022 and discovered that large portions of the reef had recovered from the bleaching event.

The researchers emphasized that, despite its complexities and challenges, monitoring the deep ocean seafloor is of dire importance.

“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. 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,” said Dr. Nicola Foster, co-author of the study and a professor of marine biology at the University of Plymouth, in the press release.

The research team had expected that mesophotic corals — those found between about 98 to 492 feet below the surface — would continue to deliver ecosystem benefits even as shallow water corals suffered more frequent and severe damage due to rising sea surface temperatures caused by the climate crisis.

The study demonstrated the possibility that might not be true, however. Deep water corals around the world remain mostly understudied, and similar bleaching events could be happening.

“The oceanography of a region is impacted by naturally occurring cycles that are becoming amplified by climate change. Currently, the region is suffering similar, if not worse, impacts due to the combined influence of El Niño and the Indian Ocean Dipole,” Hosegood said. “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.”