Australia's Great Barrier Reef was hit hard by warm ocean water in 2016 and 2017, causing back-to-back disasters filled with widespread coral die-offs. During marine heatwaves, corals are forced to expel the single-celled symbiotes that photosynthesize inside them. This leaves the corals a pallid white color—a process called "bleaching."

If this bleaching lasts too long, the corals die. That's what happened along much of the Great Barrier Reef (the southern end has largely avoided bleaching). And, if the reef doesn't recover quickly, it could suffer from a long-term degradation.

The question—asked with fingers firmly crossed—was whether the reefs would find a way to bounce back quickly, repopulating with a new generation of corals. Unfortunately, a new study published this week shows that the first year's data is not encouraging.

Post-bleach

A team led by James Cook University's Terry Hughes used panels that are set out to collect and count baby corals, referred to as "recruits." Because this has been done since 1996, the first batch of recruits after the bleaching double-whammy can be compared to previous years.

Part of this method is not just tallying up the total but monitoring the balance of different species. There are two categories of corals that differ in their sexual protocol: "brooders" and "spawners." Brooders fertilize eggs internally, releasing larvae that will only travel for about a day before settling in a spot to make a run at life. Spawners release their sperm and eggs to the mercy of the current, with fertilized eggs traveling up to a week before setting up shop.

In the Great Barrier Reef, almost all corals are spawners like the branching Acropora. Brooders play a minority role.

Overall, there was a dramatic drop in recruits in 2018, with numbers dropping an average of 89 percent (the southernmost portion of the reef, which was not bleached much, actually had a bumper crop). Numbers varied a bit from place to place, but the striking thing is the systematic difference between spawners and brooders. Spawning recruits were down 93 percent on average, while brooding recruits declined an only slightly less jaw dropping 63 percent.

The differing decline between spawners and brooders meant that 2018's recruits reversed the usual mix. The historically less-common brooders actually accounted for the majority of recruits last year. The reason for the decline is straightforward: these numbers correlate with the portion of adult coral that died during the bleaching.

No refuge

While you might hope that the refuge of surviving corals at the south end of the reef could help repopulate the rest, this unfortunately isn't the case. For most of the reef, the distance is too far, and the water temperatures are different enough that the south has different corals.

These back-to-back bleaching events had a much larger impact on the coral population than other types of disasters. Two tropical cyclones hit the reefs of Lizard Island in 2014 and 2015, for example, but the number of young recruits stayed strong. Bleaching, however, reduced the number of recruits by over 95 percent.

The researchers don't have anything optimistic to say about this pattern. When it comes to the resilience of these ecosystems, past events aren't always a good guide to the future. Given a decade, the corals of the Great Barrier Reef could perhaps recoup their losses, but the odds of a decade without a major bleaching event are now long. These corals have seen four bleaching events in the last 20 years. Under a scenario of high greenhouse gas emissions, projections show that bleaching could be an annual event by 2050.

"According to the most recent assessment by the Intergovernmental Panel on Climate Change," the researchers write, "70-90 percent of the world's coral reefs could disappear by as soon as 2030 owing to global warming, unless decisive action is taken to reduce greenhouse gas emissions."

Nature, 2016. DOI: 10.1038/s41586-019-1081-y (About DOIs).