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The Great Barrier Reef Australia was hit hard by the warm waters of the ocean in 2016 and 2017, causing successive disasters that resulted in widespread mortality of corals. During marine heat waves, corals are forced to expel unicellular symbionts that photosynthesize within them. This leaves the corals a pale white color – a process called "whitening".
If this bleaching lasts too long, the corals die. This is what has happened along most of the Great Barrier Reef (the south end has largely avoided bleaching). And if the reef does not recover quickly, it could suffer from long-term degradation.
The question – posed with fingers crossed firmly – was whether the reefs would find a way to bounce back quickly by repopulating with a new generation of corals. Unfortunately, a new study released this week shows that first year data are not encouraging.
Post-laundering
A team led by Terry Hughes of James Cook University used working groups to collect and count the coral babies, called "recruits". As has been done since 1996, the first group of recruits after double whitening can be compared to previous years.
Part of this method is not simply totaling the total but monitoring the balance of different species. There are two categories of corals that differ in their sexual protocol: "brooders" and "broodmares". Brooders fertilize eggs internally, releasing larvae that will only travel for about a day before settling in a wildlife-friendly location. Spawners release sperm and eggs at the mercy of the current. The fertilized eggs travel up to a week before they settle.
In the Great Barrier Reef, almost all corals are spawners, like branches Acropora. Breeders play a minority role.
Overall, the number of recruits fell sharply in 2018, with an average decrease of 89 percent (The southernmost part of the reef, which has not been much bleached, has had an exceptional harvest). The numbers vary a little from one place to another, but what is striking is the systematic difference between breeders and breeders. Spawning recruits dropped 93% on average, while brood recruits dropped slightly by 63%.
The different decline between spawners and breeders means that recruits in 2018 reversed the usual mix. The historically less common breeders actually accounted for the majority of recruits last year. The reason for this decline is simple: these numbers correlate with the portion of dead adult coral during bleaching.
No refuge
Although you may hope that the shelter of the surviving coral at the southern end of the reef can help repopulate the rest, this is unfortunately not the case. For most of the reef, the distance is too great and the water temperatures are sufficiently different for the south to have different corals.
These consecutive bleaching episodes had a much greater impact on the coral population than other types of disasters. For example, two tropical cyclones hit the reefs of Lizard Island in 2014 and 2015, but the number of young recruits remained high. However, laundering has reduced the number of recruits by more than 95%.
Researchers are not optimistic about this pattern. With regard to the resilience of these ecosystems, past events are not always a good guide for the future. In the space of a decade, Coral Great Barrier Reef could possibly recover their losses, but the chances of a decade without major whitening episode are now long. These corals have experienced four bleaching episodes in the last 20 years. In a scenario of high greenhouse gas emissions, projections show that money laundering could be an annual event by 2050.
"According to the latest assessment of the Intergovernmental Panel on Climate Change," the researchers wrote, "70 to 90 percent of the world's coral reefs could disappear by 2030 due to global warming." planet, unless decisive action is taken to reduce the greenhouse gas emissions. "
Nature, 2016. DOI: 10.1038 / s41586-019-1081-y (About DOIs).
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