Unseen Depths of Coral Bleaching in the Indian Ocean Reveal Climate Change’s Wider Impact

Evidence of significant damage to corals was noted during a November 2019 research expedition led by scientists from the University of Plymouth. They employed remotely operated underwater vehicles fitted with cameras to assess the state of coral health beneath the ocean’s surface. Source: University of Plymouth

Unprecedented instances of deep coral bleaching have been identified in the Indian Ocean, attributed to the escalation in ocean temperatures. This revelation questions prior assumptions about the ability of deep corals to withstand warming conditions, while emphasizing the expansive influence of climate change on marine environments.

Extensive Coral Bleaching Discovered in the Indian Ocean

Researchers have uncovered the deepest known instances of coral bleaching, situated over 90 meters (approximately 300 feet) beneath the Indian Ocean’s surface.

The degradation, caused by a 30% spike in sea temperatures due to the phenomena known as the Indian Ocean dipole, impacted as much as 80% of the coral reefs in certain seabed regions. These depths were formerly considered to be impervious to the effects of oceanic warming.

The scientific community warns that this serves as a grim indicator of the dangers that rising sea temperatures present to marine life. It also highlights the concealed toll that climate change is exacting on ecosystems worldwide.

These observations were detailed in a study released on October 16 in the journal Nature Communications, conducted by researchers affiliated with the University of Plymouth.

Live feed from underwater cameras sent to the research vessel allowed the Plymouth University team to witness firsthand the bleaching of corals. Interestingly, no evidence of damage was observed in shallow water reefs during the same period. Source: University of Plymouth

Dr. Phil Hosegood, Associate Professor of Physical Oceanography at the University of Plymouth and project lead, stated: “The assumption that deeper corals were resistant to oceanic warming due to the relatively stable, cooler water conditions they inhabit has been discredited. Consequently, it is likely that coral reefs at analogous depths globally are also under threat from similar climatic variations.”

Prolonged Study on Coral Reefs by the University

For over a decade, the University’s research team has been investigating the Central Indian Ocean, supported by both the Garfield Weston Foundation and the Bertarelli Foundation.

During their expeditions, they utilize a blend of direct monitoring, automated underwater vehicles, and satellite-derived oceanographic information to acquire a deeper understanding of the region’s unique oceanic conditions and its supported life forms.

Initial signs of coral deterioration were spotted during the 2019 research voyage, where the team utilized remotely operated underwater vehicles with camera capabilities to assess the condition of corals beneath the ocean surface.

Subsequent analyses of diverse data sets collected during the expedition and satellite observations confirmed that subsurface ocean temperatures had risen from 22°C to 29°C (72°F to 84°F), a result of the thermocline deepening across the equatorial Indian Ocean.

Root Causes and Ramifications

Clara Diaz, the study’s primary author, commented: “The data unequivocally show that the bleaching was induced by a deepening of the thermocline, attributable to regional climatic variations similar to an El Nino. These cycles are becoming more intense due to climate change, leading to the likelihood that deep ocean bleaching will become increasingly frequent.”

Dr. Nicola Foster, a co-author and Lecturer in Marine Biology, added: “Our findings expose the susceptibility of mesophotic coral ecosystems to thermal stress, offering new evidence of climate change’s pervasive influence on all oceanic layers. This will likely result in coral death and a decrease in reef complexity, leading to reduced biodiversity and diminished ecosystem services.”

Recovery Prospects and Future Concerns

Returning to the same locales during planned research cruises in 2020 and 2022, the researchers found some coral recovery. However, they insist that intensified monitoring of deep ocean floors is imperative, despite the complexities involved.

As the frequency and severity of damage to shallow water corals increase, it had been anticipated that mesophotic corals—located between 30-150m (approximately 100-500 feet) beneath the surface—would compensate by providing essential ecosystem services. Yet, this new research suggests that may not be the case, highlighting the need for more focused studies on deep-water corals, which have generally been under-examined.

Dr. Hosegood elaborated: “Natural cyclical phenomena, now exacerbated by climate change, have significant impacts on a region’s oceanography. Our limited understanding of these deep marine environments must be broadened to grasp the far-reaching consequences of these climatic changes. Given the rapid global changes we are experiencing, this is more urgent than ever.”

Reference: “Mesophotic coral bleaching associated with changes in thermocline depth” by Clara Diaz, Nicola L. Foster, Martin J. Attrill, Adam Bolton, Peter Ganderton, Kerry L. Howell, Edward Robinson, and Phil Hosegood, published on October 16, 2023, in Nature Communications.
DOI: 10.1038/s41467-023-42279-2

Frequently Asked Questions (FAQs) about Deep Coral Bleaching in the Indian Ocean

More about Deep Coral Bleaching in the Indian Ocean

• University of Plymouth Coral Research
• Nature Communications Journal
• Indian Ocean Dipole Explained
• About Coral Bleaching
• Climate Change and Marine Ecosystems
• Garfield Weston Foundation
• Bertarelli Foundation
• Thermocline and its Impact
• Global Climate Change Effects on Oceans