The evidence of coral damage was observed during a research cruise in November 2019, during which scientists from the University of Plymouth were using remotely operated underwater vehicles equipped with cameras to monitor the coral health below the ocean surface. Credit: University of Plymouth
Key points:
- On a research cruise in 2019, researchers found evidence of the deepest coral reef bleaching known, more than 90 meters below the surface.
- Remotely operated underwater vehicles equipped with cameras enable the team to monitor coral health below the ocean surface over a period of years.
- The reef seems to have recovered in large part after two years, but the team is still concerned.
Scientists have discovered the deepest known evidence of coral reef bleaching, more than 90 meters below the surface of the Indian Ocean. The team says it serves as a stark warning of the harm caused in our ocean by rising ocean temperatures, and also of the hidden damage being caused throughout the natural world as a result of climate change.
Researchers from the University of Plymouth (UK) discovered the evidence during a research cruise in November 2019. They have been studying the Central Indian Ocean for well over a decade, with their work supported by the Garfield Weston Foundation and the Bertarelli Foundation.
On their research cruises, the team uses 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. On that 2019 trip, images from the underwater cameras gave the research team its first glimpse of the corals that had been bleached. Conversely, at the same time as the deeper reefs were bleaching, they observed shallow water reefs exhibiting no sign of harm.
Over the subsequent months, the researchers assessed a range of other data collected during the research cruise and information from satellites monitoring the ocean conditions and temperatures. The 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. 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,” said Clara Diaz, the lead author on the study.
However, there is some good news. The team returned to the same areas during planned cruises in 2020 and 2022 only to find that large parts of the reef had recovered.
Even so, the researchers say it is critically important to increase monitoring of the seafloor in the deep ocean, even if it is a hugely challenging and complicated undertaking.