In shallow waters, the Caribbean coral Montastraea cavernosa exhibits a boulder-like morphology, shown at 5m. Credit: John Reed.

Shallow coral reefs up to 40 metres deep are the tip of the iceberg that comprises the ocean’s extensive coral ecosystem. Now, a United Nations report co-authored by the University of Sydney’s UNESCO Chair in Marine Science provides a glimmer of hope for those managing the impact of bleaching on the world’s coral reefs, including the Great Barrier Reef.

Coral bleaching has affected virtually the entire Great Barrier Reef and many other coral reef systems globally, a result of the continuing rise in global temperatures and exacerbated by the summer’s major El Niño event. The 35 authors of the United Nations Environmental Programme report launched today – including the University’s Professor Elaine Baker in the School of Geosciences – say the deeper, mesophotic coral ecosystems (MCEs) may act as a lifeboat for shallow coral reefs.

MCEs are intermediate depth reefs starting at about 40 metres depth and continuing to around 150 metres. The report – Mesophotic Coral Ecosystems A lifeboat for coral reefs? – looks at the role MCEs could play in the preservation of shallower reefs.

The report asks if MCEs can provide a refuge for the species under threat in shallower reef ecosystems and whether they can provide the stock to re-populate shallow reefs if they continue to decline.

“More research needs to be done to firmly establish the role of MCEs in preserving our reefs,” said Professor Baker. “They aren’t a silver bullet but they may be able to resist the most immediate impacts of climate change and help replenish destroyed surface reef and fish populations.

“It may be that the cooler, deeper water in MCEs could be more hospitable to many species than the warmer surface water,” she said. “They also are less prone to waves and turbulence, therefore potentially offering a more stable environment in which to replenish coral.

The review brought together information on the geology, biology, distribution and socio-economic aspects of mesophotic reefs in order to examine their potential resilience. It found some deep mesophotic coral ecosystems may be immune from the most extreme ocean warming, but other ecosystems are just as vulnerable as their shallow counterparts and cannot be relied on to act as life boats.