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 very different response from ours. Corals host single-celled, photosynthetic symbiotes, called zooxanthellae, that provide food in exchange for shelter. During a heat wave, the corals are forced to kick the zooxanthellae out (when stressed, the zooxanthellae secrete toxins) and thus lose their source of food as well as the majority of their color—hence the name “coral bleaching”.

If the water stays warm too long, the corals starve and leave behind lifeless carbonate skeletons. Young corals may repopulate the area in time, although algae will often claim the abandoned structures in the meanwhile.

The last few years have hit Australia’s Great Barrier Reef pretty hard, with a massive bleaching event in 2016 and persistently elevated temperatures giving no respite—part of a trend in a warming climate. A team led by the University of Tasmania’s Rick Stuart-Smith hit the water almost a year after the bleaching to get a first look at the recovery process.

Complex changes

What they saw varied drastically from place to place. Of the 186 sites they visited—all of which had been surveyed before bleaching—44 were still missing more than 10 percent of their corals. (The worst site was down by half.) The variability is partly due to the fact that the northern end of the Great Barrier Reef experienced the warmest water, but local factors were obviously important.

Where live coral area decreased, algae cover increased. The sites that lost the most coral also unsurprisingly showed a decline in fish that munch on coral.

Beyond these common patterns, there were different changes at the north and south ends of the Great Barrier Reef. In the north, fish that graze on algae were down. This can potentially hamper coral recovery, since they'll be competing for the same resources. The number of fish species present in the south, oddly enough, increased to become more like the northern ecosystem. The number of sea urchins also dropped in the north while rising in the south.

The researchers suspect these patterns had more to do with the direct effect of the warm water on these fish species rather than being a consequence of coral loss. They found that the species missing in the north did tend to be near the upper limit of their comfortable temperature ranges. It’s possible that some northern critters headed south for cooler water, but researchers saw no sign of that.

Other influences?

There could be other things going on, masking the impact of the coral bleaching. The researchers note that they found a higher fish species count at 40 percent of the sites—the opposite of what normally happens. It could be that the fish populations in those locations were experiencing other influences on their size, in addition to the bleaching effects.

Overall, what the researchers found was a significant “reshuffling” of local ecosystems, varying from place to place. Some areas experienced warmer water. Some hosted species that were more vulnerable to high temperatures or coral loss. And some areas were still coming back from cyclone damage. A complex mix of factors leads to patchy impacts that defy explanation at first glance—and complicate the prognosis for recovery. “The extent to which the 2016 mass bleaching event proves ecologically catastrophic remains uncertain,” the researchers write, “as does the sum of accumulated effects from multiple bleaching events.”

That doesn’t mean we’re left with no option but to throw up our hands and say, “C’est la vie.” The team points out that this knowledge is immediately useful. For example, local fishery managers could consider whether reducing the catch of certain species might help with reef recovery in the area—or at least prevent us from adding even more human-caused hardship to the reef’s troubles.

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