In the hottest days of Australia's summer, just after a full moon, the Great Barrier Reef experiences an underwater snowstorm.
Along the massive stretch of reefs corals release millions of sticky eggs and sperm that float up to the water's surface and combine. They will eventually form larvae. Some will float down and settle on the nearby ocean floor, while others might be swept along by a current.
New research published Wednesday in Nature finds that warmer waters are making it harder for corals to reproduce en masse. In fact, following a major coral bleaching event in 2017 the amount of reproductive material collected in the water after a mass spawning event was down by 89 percent in the Great Barrier Reef in 2018. It will take five to 10 years for coral to fully recover, the researchers estimate.
After mass bleaching events, coral reefs struggle to recover, the research shows. That decline in new young is also hitting some corals harder than others, meaning the composition of reefs could be drastically altered by climate change.
Corals that reproduce by releasing a mass of reproductive material into the water are called spawners. Most species of coral in the Great Barrier Reef reproduce in this way, but some, a group labeled brooders, reproduce by releasing larvae that settle nearby.
To assess which species of adult coral were the most impacted, researchers conducted underwater surveys by draping tape over reef beds and measuring the topography. To sample larval recruitment, they set out panels across the Great Barrier Reef in the days following a mass spawning event.
“We put out a thousand settlement panels,” says Terry Hughes, a coral reef scientist from James Cook University and the lead researcher. “Between the north and southern tip, that’s a distance of 1,800 miles.”
In previous surveys, they gathered panels that each contained 50 to 100 gametes. This year, he says, “the most common numbers were between zero and one.”
Less time to recover
Warm temperatures and pollution trigger coral to expel the algae that lives in their tissues. This algae provides food to each coral polyp. When water doesn't cool or pollution doesn't dissipate, the algae doesn't return to the coral, leaving them to starve to death. Weather patterns like El Niño can make already warm waters unbearable for coral.
Coral bleaching was first recorded in the early 1980s, but the Great Barrier Reef has experienced four mass bleaching events that have devastated huge swaths of the reef. The first was in 1998 and the second in 2002. From 2002 to 2016, corals were able to recover what had been lost to bleaching.
“We were lucky to have a 14-year gap between the second and third,” says Hughes. “And unlucky to have no gap after 2016.”
Back-to-back bleaching events in 2016 and 2017 devastated the Great Barrier Reef, and research published by Hughes last year in the journal Science suggests the gap between extreme warming is getting shorter.
“It's like getting hit by a serious disease every couple of years, or at such short intervals that you don't have time to recover in between,” study author and marine biologist Julia Baum told National Geographic at the time.
A rocky future
The estimated 10 years corals need to recover only holds true if no other mass bleaching events occur in that time. Hughes says it's unlikely for that to occur in a warming world.
“I’m reasonably positive that we’ll still have reefs [in the future],” says Hughes. “But already we’re seeing the shifts that are occurring with one degree of warming. If we go to two or more, then the reefs will become more and more degraded and more unrecognizable.”
Certain species of spawning corals, like Acropora corals, were more impacted. Recruitment for that species declined by 93 percent. Acroporas are shaped like tables; they're responsible for much of the reef's three dimensionality, and researchers say they support thousands of other species.
“We’ve always anticipated that climate change would shift the mix of coral,” says Hughes. “What’s surprised us is how quickly that is now happening. It’s not happening in the future. It’s something that we’re now measuring.”