Known as mesophotic coral ecosystems (MCEs), these areas are found between 30 and 150 metres below the surface. The UN's mesophotic coral ecosystem report collates reports from around the world. Credit:UNEP Globally, coral systems are reeling from some of the worst bleaching events ever recorded. Bleaching has affected all but 7 per cent of the Great Barrier Reef, caused by a rise in global temperatures and last summer's El Nino event. But collated global research suggests that coral ecosystems can survive the extremes of some events. The 35 authors of the UN Environmental Program report include the University of Sydney's Professor Elaine Baker and James Cook University's Tom Bridge. Together they ask if these deeper coral systems can "provide this ecosystem service and act as 'lifeboats' for nearby, connected shallower reefs that have been damaged".

The answer is: it depends. Coral in shallow waters - such as this Montastraea cavernosa at 5 metres - can be more susceptible to bleaching. Credit:John Reed/UNEP Professor Terry Hughes from James Cook University, some of whose research was compiled to make the UN report, said that while deeper ecosystems could act as "spatial refuges" for shallower coral, it was important to remember that MCEs were also affected by bleaching events. "It's less intense, but not trivial," he told Fairfax Media. He also said that most species found in shallow waters were not found in the mesophotic areas. He said that even if they could provide a bank from which corals could repopulate shallow reefs, that could mean "a huge shift in species make-up". Mesophotic coral, such as this in Pohnpei, Micronesia, might act as "lifeboats" for nearby damaged reefs - but that could come with changes to species populations. Credit:Sonia Rowley/UNEP

Dr Bridge said: "The deeper you go, the more dissimilar are the species." He said that mesophotic corals occupied about the same area as shallow coral ecosystems in the Great Barrier Reef. Professor Baker said: "More research needs to be done to firmly establish the role of MCEs in preserving our reefs; they aren't a silver bullet but they might be able to resist the most immediate impacts of climate change. "They could provide a refuge for some species and potentially help replenish destroyed surface reef and fish populations." In a commentary published in Nature Climate Change in 2013, Professor Hughes and his colleague Tom Bridge wrote: "The unexpectedly large areal extent of mesophotic reefs around the world is only now beginning to emerge, because historically reefs were mapped only if they presented a hazard to navigation." Professor Hughes and Dr Bridge point out that the UN report has no new data, but has collated reports from around the world, including data from the Great Barrier Reef from before the most recent bleaching event.

He and his colleagues at James Cook University expect to shortly announce coral mortality estimates from the latest bleaching events.