Never-before-seen deep coral bleaching has been discovered in the Indian Ocean, linked to rising sea temperatures. This challenges previous beliefs about the resilience of deep-sea corals and highlights the widespread impacts of climate change on marine ecosystems.
Evidence of coral damage was observed during a research cruise in November 2019, during which University of Plymouth scientists used remotely operated underwater vehicles equipped with cameras to monitor coral health beneath the ocean's surface. Image source: University of Plymouth
Scientists have discovered the deepest known evidence of coral reef bleaching more than 90 meters (300 feet) below the surface of the Indian Ocean. This damage - attributed to a 30% increase in sea temperatures caused by the Indian Ocean Dipole - has damaged up to 80% of coral reefs in some parts of the seafloor at depths previously thought to be resistant to ocean warming.
However, scientists say it is a stark warning of the harm rising ocean temperatures are causing to our oceans, and of the hidden damage climate change is causing to nature as a whole.
Researchers at the University of Plymouth highlighted the findings in a study published Oct. 16 in Nature Communications.
Images from underwater cameras were streamed live to the research vessel, giving the University of Plymouth team a first look at bleached coral. In contrast, while deep-sea reefs were bleaching, they observed no signs of damage to shallow-water reefs. Source: University of Plymouth
Dr Phil Hosegood, associate professor of physical oceanography at the University of Plymouth and project leader, said: "There is no doubt that this is a huge surprise. It has long been thought that deep-sea corals have a strong adaptability to ocean warming because they inhabit waters that are cooler than sea surface temperatures and were thought to remain relatively stable. However, this is clearly not the case, so coral reefs at similar depths around the world may be threatened by similar climate changes.
With support from the Garfield Weston Foundation and the Bertarelli Foundation, researchers at the university have been studying the central Indian Ocean for more than a decade. During the research cruise, they used a combination of in-situ monitoring, underwater robots, and satellite-generated oceanographic data to learn more about the region's unique oceanography and the life it supports.
The first evidence of coral damage was observed during a research cruise in November 2019, during which scientists used remote-controlled underwater robots equipped with cameras to monitor coral health below the surface.
This video shows damage to seafloor coral 90 meters below the surface in the central Indian Ocean. The video was taken by scientists from the University of Plymouth during a research cruise in November 2019, during which they used remotely operated underwater vehicles equipped with cameras to monitor coral health below the surface. Image source: University of Plymouth
Images from underwater cameras were streamed live to the research vessel, giving the team their first look at bleached coral. Instead, while deep-water reefs were bleaching, they observed no signs of damage to shallow-water reefs.
In the months that followed, the researchers evaluated a range of other data collected during the expedition, as well as information from satellites monitoring ocean conditions and temperatures.
The study highlights that while ocean surface temperatures changed little during this period, subsurface temperatures rose from 22 to 29°C (72°F to 84°C) as the thermocline deepened across the equatorial Indian Ocean.
"Our records clearly show that this bleaching event was caused by a deepening of the thermocline," said Clara Diaz, lead author of the study. "This is the regional equivalent of El Niño, and these cyclical changes are being amplified due to climate change. In the future, deep-sea bleaching events here and elsewhere may become more regular."
Dr Nicola Foster, lecturer in marine biology and co-author of the study, added: "Our results show that Mesozoic coral ecosystems are vulnerable to thermal stress and provide new evidence of the impact of climate change on all parts of the ocean. Increased bleaching of mesozoic corals will ultimately lead to coral death and a reduction in the structural complexity of these reefs. This is likely to lead to a loss of biodiversity and a reduction in the important ecosystem services these reefs provide to our planet."
During cruises planned for 2020 and 2022, the university's researchers returned to the same areas and found that much of the reef had recovered. Still, they say it's crucial to improve monitoring of the deep seafloor, even though it's a challenging and complex undertaking.
With the frequency and severity of damage to shallow-water corals increasing, it had been expected that mesopelagic corals - corals found between 30-150 meters (100-500 feet) below the surface - would fill the gap and bring benefits to the ecosystem.
However, this study highlights that this may not be the case - as deep-water corals around the world remain largely understudied, similar devastating bleaching events may go unnoticed.
Dr Horsgood added: "The oceanography of a region is affected by natural cycles, which are amplified by climate change. Currently, the region is suffering similar or even more severe impacts from the combined influence of El Niño and the Indian Ocean Dipole. Although we have not There are ways to stop the deepening of the thermocline, but what we can do is expand our understanding of the impacts these changes will have across environments of which we understand so little, which is more urgent than ever in the face of rapidly evolving global change."