There were two big pieces of news for the future of coral reefs this week, and both were bad. A study of the world’s largest coral reef system found that the recovery time after damaging events has increased six-fold. On top of this, the deep-water reefs many people (IFLScience included) hoped would prove shallow reefs' salvation have been found to be ill-suited to the task. Although marine scientists still hold out hope we can save the most precious reefs on the planet, it looks increasingly likely that the vast majority of coral reefs are doomed within decades.
The Great Barrier Reef is easily the largest coral reef system in the world, and should also be one of the most immune to human activities since so much of it is remote from population centers. However, a report in Science Advances reveals the reef is recovering so slowly from disasters like cyclones, crown-of-thorn starfish outbreaks, and heat-induced bleaching effects that it is increasingly unready to face the next blow.
What is worse, the 84 percent fall in recovery rates took place between 1992 and 2010, before the drastic bleaching events of 2016-17. It's likely to be worse now, but measurements are not yet available. Some of the 81 individual reefs studied are now even experiencing coral cover decline between disturbances.
Ocean acidification is probably a contributing factor, but co-author Professor Peter Mumby, also of the University of Queensland, noted: “Our results indicate that coral recovery is sensitive to water quality, and is suppressed for several years following powerful cyclones. Some reefs could improve their recovery ability if the quality of the water entering the reef is actively improved.”
Optimists have wondered whether reefs' salvation might lie in deeper waters. We've only recently come to realize how widespread deep-water coral reefs are, thanks to improved diving equipment. These mesophotic reefs are in cooler waters, the thinking went, so global warming may do them less damage. They're also more protected from cyclones and often far enough offshore to be safe from other human activities.
If endangered species could survive more than 30 meters (100 feet) below the surface, they might eventually repopulate the shallower reefs that are so important for tourism, fishing, and the planet's biodiversity.
Sadly, Dr Luiz Rocha of the California Academy of Sciences has crushed these hopes in Science. “Deep reefs feel it all.” Rocha said. “Besides storm impacts, we saw the familiar signs of heavy fishing, sedimentation, coral bleaching, and invasive species in deep reefs.”
Rocha admits that especially remote reefs may be spared this damage, but for those not too far from shore, he said: “Reef troubles don't stop at 100 feet deep.”
Moreover, Rocha found there is little overlap between the species that inhabit mesophotic and shallow reefs, so even if the deeper ones survive, they are unlikely to prove a useful source for repopulating shallower locations.
Rocha and co-authors hope this news will not cause people to ignore the plight of mesophotic reefs. The reefs they studied in the Pacific and Caribbean were rich in species that have not been scientifically described. It would be as tragic to lose these as the fish and corals we know about.
Both teams are keen to counsel against despair, arguing it is not too late to save these sites of much biological richness and beauty. However, to do so will require a level of will that has so far been lacking.