The Australian Great Barrier Reef was hit hard by warm seawater in 2016 and 2017. Disasters in the background were filled with widespread coral extinctions. During the heat of ocean waves, corals must drive out the single-celled symbiots that are photosynthesized in them. The corals thus get a pale white color ̵
If this bleaching takes too long, the corals will die. So it happened along the Great Barrier Reef (the southern end has largely avoided fading). And if the reef does not recover quickly, it could suffer from a long-term worsening.
The question – with fingers firmly crossed – was whether the reefs would find a way to quickly get back on their feet and fill themselves with a new generation of coral. Unfortunately, a new study released this week shows that the data for the first year is not encouraging.
A team led by Terry Hughes of James Cook University used plates designed to collect and count baby corals as "recruits". Since this has happened since 1996, the first recruitment of the recruits after the double whammy bleaching can be compared with previous years.
Part of this method is not just to find the grand total, but to monitor the balance of different species. There are two categories of corals that differ in their sexual protocol: "Brooders" and "Spawners". Breeders fertilize eggs internally and release larvae that only fly for about a day before settling in a place to start a life. Spawners release their sperm and their eggs from the mercy of the current, with fertilized eggs traveling up to a week before the store was founded.
In the Great Barrier Reef, almost all corals are spawning birds such as the branching Acropora . Brooder play a minority role.
Overall, there was a dramatic drop in recruits in 2018, with the average falling by 89 percent (the southernmost portion of the reef, which was not actually bleached, had a bumper harvest). The numbers varied a little from place to place, but the striking feature is the systematic difference between lay and breeders. The number of spawning recruits averaged 93 percent, while the number of recruits declined 63 percent.
The differential decline between spawners and brooders caused the recruits of 2018 to reverse the usual mix. The historically rare breeding birds were responsible for the majority of the recruits last year. The reason for the decline is straightforward: these numbers correlate with the proportion of adult corals that died during bleaching.
While you're hoping that the survival of the surviving coral at the southern end of the reef can help fill up the rest, that's unfortunately not the case. For most of the reef, the distance is too large and the water temperatures are so different that the south has different corals.
These bleaching events had a much greater impact on the coral population than other types of disasters. For example, in 2014 and 2015, two tropical cyclones had the reefs of Lizard Island, but the number of young recruits remained strong. By fading, however, the number of recruits could be reduced by over 95 percent.
The researchers have nothing to say optimistic about this pattern. When it comes to the resilience of these ecosystems, past events are not always a good guide to the future. In the wake of a decade, the corals of the Great Barrier Reef could perhaps make up for their losses, but the chance of a decade without a major bleaching event is now great. These corals have experienced four bleaching events over the last 20 years. Under a scenario of high GHG emissions, forecasts indicate that bleaching could be an annual event by 2050.
"According to the latest assessment by the Intergovernmental Panel on Climate Change," the researchers write, "70-90 per cent of the world's coral reefs could disappear as early as 2030 due to global warming unless decisive measures to reduce greenhouse gas emissions are taken.
Nature 2016. DOI: 10.1038 / s41586-019-1081-y (via DOIs).