Temporary hope that deep coral would offset coral bleaching dashed

Axios is an unheralded, but excellent source, of factually accurate information. I tend to focus on their environmental discussions, but you might take a look.

Deeper corals aren’t refuges after all

Researcher Norbert Englebert surveys a coral reef.
Researcher Norbert Englebert surveys a coral reef. Photo: Pim Bongaerts/California Academy of Sciences

Deep-water coral reefs may not offer adequate protection to corals being degraded by heatwaves at the surface of the ocean, according to new research.

Why it matters: Those reefs, at between 100–500 feet underwater, are frequently viewed as conservationists’ best hope in saving vulnerable corals.

  • The widespread degradation of coral reefs — which nurture creatures that provide critical food supplies for millions of people — is one of the clearest impacts from climate change affecting humanity.

The details: The study, published Tuesday in Nature Communications, examines the extent to which coral bleaching penetrated to deeper waters during the Great Barrier Reef’s devastating mass bleaching event in 2016.

How they did it: Using temperature measurements at various depths at nine sites on the Great Barrier Reef and in the western Coral Sea, plus surveys conducted by divers down to 130 feet, the researchers concluded that deeper depths offered some protection from intense bleaching. However, this protection came with caveats:

  • Initially, cooler waters moving up from deeper waters kept the deep-water corals from bleaching. But once these currents shut down as the austral summer ended, the water temperatures skyrocketed in these zones, too.
  • The researchers characterized the impacts on the deep reefs as “severe” — 40% of the coral colonies studied were bleached, and 6% died.
  • The toll closer to the surface: 69% of coral colonies bleached at a depth of just 16 feet.
  • Researchers also found the species of corals change with water depth. That hints at the difficulty of a leading conservation strategy involving relocating vulnerable corals from the surface to deeper waters.

The context: Several studies published in the past few years raise doubt that deep-water corals can serve as refuges for coral species deemed vulnerable at the surface.

  • For example, a study published in the journal Science in July, found that deep-water reefs are “distinct, impacted, and in as much need of protection as shallow coral reefs.”
  • Another recent paper reported reefs on the island nation of Palau have seen an uptick in bleaching events, though at different intervals and with varying severity compared to surface reefs.

Yes, but: One limitation of the new study, according to study author Pedro Rodrigues Frade from the University of Algarve, is that it does not indicate what ultimately happened to the deep coral reefs after the bleaching event.

  • The research team hopes to return to their monitoring sites to see if these corals bounced back or suffered longer-term damage, like many of the surface reefs did.

Thomas Frölicher, a climate researcher at the University of Bern who was not affiliated with the study, tells Axios the new research also shows that scientists need to better understand local ocean currents, which can control the fate of deeper reefs.

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