It is too early to tell if the bleaching of the Great Barrier Reef was a disaster

In a heat wave, humans sweat and try to avoid heat exhaustion. But in a coral reef, a wave of warm ocean temperatures can trigger a response very different from ours. Corals harbor unicellular photosynthetic symbionts called zooxanthellae that provide food in exchange for shelter. During a heat wave, the corals are forced to expel the zooxanthellae (lest the coral be poisoned by their stressed excretions), losing the corals' food source as well as most of their color – from the coral reefs. where the name "coral discoloration".

If the hot water stays too long, the corals starve and leave lifeless carbonate skeletons behind them. Young corals can repopulate the area over time, although algae often claim abandoned structures during this time.

Recent years have affected the Great Barrier Reef in Australia, with a mbadive bleaching episode in 2016 and consistently high temperatures. respite-part of a trend in a climate of warming. A team led by the University of Tasmania Rick Stuart-Smith hit the water almost a year after the bleaching to have a first look at the recovery process.

The Complex Changes

What they saw varied considerably from one place to another. Of the 186 sites visited, all of which had been studied before bleaching, 44 still lacked more than 10% of their corals. The variability is in part due to the fact that the northern end of the Great Barrier Reef experienced the hottest water, but local factors were obviously important.

Where the living coral surface has decreased, the algae coverage increased. The sites that have lost the most corals have also unsurprisingly shown a decline in fish that eat coral.

Beyond these common patterns, there were different changes at the north and south ends of the Great Barrier Reef. In the north, fish grazing on algae were declining. This can potentially hinder coral recovery as they compete for the same resources. The number of fish species present in the south, oddly enough, has increased to more closely resemble the northern ecosystem. The number of sea urchins has also dropped in the north while increasing in the south.

The researchers suspect that these models have more to do with the direct effect of hot water on these species of fish rather than being a consequence of coral shedding. They found that the missing species in the north tended to be close to the upper limit of their comfortable temperature ranges. It is possible that some creatures from the north have headed south for cooler water, but researchers have seen no sign of it

Other influences

could be other things that would mask the impact of coral bleaching. The researchers note that they found a higher species of fish rely on 40 percent of the sites, the opposite of what normally happens. It is possible that fish populations in these areas will experience other influences on their size, in addition to the bleaching effects.

Overall, the researchers found a significant reshuffling of local ecosystems, varying from one place to another. Some regions have experienced warmer water. Some hosted species were more vulnerable to high temperatures or coral shedding. And some areas still came back from the hurricane damage. A complex mix of factors leads to uneven impacts that challenge the explanation at first glance – and complicate the prognosis of recovery. "The extent to which the 2016 mbad laundering event turns out to be ecologically catastrophic remains uncertain," the researchers write, "just like the sum of the cumulative effects of multiple whitening events."

This is not to say that we have no option but to raise our hands and say, "It's life." The team emphasizes that this knowledge is immediately useful. For example, local fisheries managers could determine whether reducing the capture of some species could help re-establish reefs in the region – or at least prevent us from adding even more human difficulties to reef problems.

Nature 2018. DOI: 10.1038 / s41586-018-0359-9 (About DOIs).