Two years after a long-lasting undersea heat wave scalded large sections of the Great Barrier Reef, scientists now have found that because so many corals died, much of the reef likely has been altered “forever.”
“What we just experienced is one hell of a natural selection event,” said Terry Hughes, director of the Australian Research Council Center of Excellence for Coral Reef Studies at James Cook University in Queensland.
In a notably blunt study in the journal Nature — laden with words like “unprecedented,” “radical” and “catastrophic” — Hughes and 15 colleagues report that in 2016 alone, about 30 percent of the Great Barrier Reef’s corals were lost, with the most severe damage in the isolated northern sector. In 2017, another ocean heat wave claimed another roughly 20 percent of corals, Hughes said.
Many corals died faster than expected and at a lower level of sustained heat than had been predicted to be deadly.
The researchers add that since losses in certain species were much greater than in others, the entire ecological identity of much of the reef system likely has changed. In particular, elaborate branching corals that provide key fish habitat are being replaced by bulky, less intricate “dome-shaped” corals, Hughes said.
And because it takes about 10 years for even the fastest growing corals to recover, the study warns that there is probably no reversing the sweeping change to the most damaged sectors of the world’s largest barrier reef. Not before yet another bleaching event occurs.
That certainly doesn’t mean the end of the reef as a whole. The south, in particular, escaped much of the bleaching in 2016 and 2017. But it does mean that much of the reef will probably shift into a new ecological state with a less diverse, but more resilient, set of corals.
“The 2016 marine heatwave has triggered the initial phase of that transition on the northern, most-pristine region of the Great Barrier Reef, changing it forever as the intensity of global warming continues to escalate,” reads the study, written by scientists from numerous Australian institutions as well as the U.S. National Oceanic and Atmospheric Administration.
In 2016 and 2017, the Great Barrier Reef was pummeled by two successive summers in which ocean temperatures far exceeded normal for key portions of the reef — and stayed that way for a considerable time.
Extremely warm ocean temperatures stress corals and cause a phenomenon called “bleaching,” in which tiny algae called zooxanthellae abandon the corals they live with, causing the corals to lose their color.
The consequence is not just outward — zooxanthellae are partners with coral in an ancient symbiotic relationship, conducting photosynthesis necessary for the corals to survive. If the algae are gone for too long, the corals die.
Hughes and his colleagues have been directly studying this extreme die-off at the reef since its beginning during the 2016 El Nino event, when they took observations through aircraft surveys and dives.
The researchers then reported back dire dispatches about the destruction, documenting coral reefs’ surprisingly immediate vulnerability to warming ocean waters.