A team of research divers surveying the Great Barrier Reef and the reefs of the Coral Sea have produced alarming findings that raise questions about how Australia’s iconic coral ecosystem and others throughout the world will fare as the planet warms.
In a new study published in the journal Nature, the scientists found that populations of herbivorous parrotfishes – a group of keystone species that protects coral reefs from becoming smothered in algae – declined markedly on the northern portions of the reef following a severe marine heatwave in 2016.
“When parrotfish eat the algae off dead coral, they allow new corals to recruit and grow,” said Rick Stuart-Smith, a research fellow with the University of Tasmania. “If you lose parrotfish and other herbivores, you’re potentially at risk of algae taking over the reef.”
As marine heatwaves grow in duration and severity, they threaten coral reefs around the world, largely by causing coral bleaching. Bleaching – in which symbiotic zooxanthellae algae are expelled by the heat-stressed coral polyps – isn’t necessarily fatal for coral communities, since the evicted algae may quickly recolonize the reef when the water temperature drops. During prolonged heatwaves, however, bleaching kills the coral. The 2016 marine heatwave, which affected reefs throughout the tropics, resulted in a 30 percent loss of coral cover on the Great Barrier Reef. It was one of the worst bleaching events documented.
Before and after that phenomenon, Stuart-Smith, several colleagues and about 20 citizen-scientist volunteers, trained and recruited through the Reef Life Survey organization, closely studied numerous locations between the southern and northern ends of the Great Barrier Reef and in the western Coral Sea. They carefully recorded fish and invertebrate presence at 186 sites throughout the world’s largest coral reef system and came away with detailed data showing how the heatwave and bleaching reshuffled fish communities, especially by reducing biodiversity in the more northerly study areas.
But what was especially important about the research is that it allowed the scientists to distinguish between the direct ecological impacts of bleaching and the direct impacts of warmer water.
“When scientists have studied bleached reefs, they often just assume the bleaching has had an effect on the fish or their habitat,” said Stuart-Smith. “But we had a nice natural experiment set up for us with reefs right next to each other, some that experienced a lot of bleaching-related coral loss and some that didn’t lose any coral. This allowed us to tease apart the impacts of coral loss from the impacts of increased water temperature.”
Even on reefs that had not suffered bleaching, the scientists saw marked declines in biodiversity – meaning that even if a coral reef survives a future heatwave, the creatures that live on it might not. They also observed that overall species richness – as a global rule of thumb, it’s greater on coral reefs closer to the equator and less on more poleward reefs – declined on the northern study sites and increased on sites farther south. The effect was a leveling out of the biodiversity gradient usually seen between warmer and cooler coral ecosystems.
“The structure of fish communities on southern reefs became more similar to those in the north, a broad-scale homogenization that resulted in a slight decline in the overall number of fish species recorded across all surveys (from 532 to 494),” the study authors wrote.
Among the fishes that fared poorly were parrotfishes, especially in the northern study sites. This came as a surprise to Stuart-Smith.
“In theory, they should thrive after a bleaching event, because you get so much algae growing on the dead coral, so they have lots of food,” he said. “But for some reason or other – we don’t know why – the parrotfish seemed to not like the warmer water more than the other herbivores and more than the plankton feeders, which seemed to do fine.”
Algae compete with coral larvae for substrate on which to grow, and that makes parrotfish a key species in fostering coral growth. The services parrotfishes provide are believed to be as essential for the maintenance of healthy coral reefs as they are in the recovery of reefs damaged by bleaching. The coral reefs of Palau seem to provide a case in point. There, major heatwaves triggered severe bleaching in 1998.
“But they have large populations of parrotfish there, and their coral reefs have seen a lot of recovery,” said Raphael Ritson-Williams, a postdoctoral fellow at the California Academy of Sciences who has closely studied coral reefs for 15 years. He said that most coral reef researchers “think parrotfish are bringing back the coral.”
Nancy Knowlton, the Smithsonian Institution’s Sant Chair for Marine Science, said that coral communities that have been shifted into an algae-dominated state by the impacts of warming and bleaching may then resist conversion back to coral dominance, even if herbivore fishes become plentiful again. She recalls diving in Panama’s San Blas Islands after a bleaching event that had killed a large bed of staghorn coral, which then had become overgrown with mature seaweed.
“And off in the distance there was this huge school of parrotfish,” she said. The grazers were busy attacking a small patch of algae. They weren’t interested in the mature seaweed, Knowlton said. She concluded that the parrotfish found adult seaweed unpalatable but that they will “nip it in the bud” when it’s young, making their constant presence on any reef crucial.
“Without them, you eventually wind up with an algal community dominated by things they don’t like to eat,” she said.
With relatively minor reductions of algae-grazing fish populations, bleached reefs may take longer to recover. That, Knowlton said, could eventually be a death sentence for coral reef communities in a warmer future in which bleaching is a routine occurrence.
“If you’re increasing the frequency of disturbance and decreasing the rate of recovery, you eventually get to the point where the reef is permanently in a pre-recovered state,” she explained.
When reefs become dominated by algae and seaweeds, coral polyp communities decline or disappear, which means coral building processes slow or stop. At this point, Knowlton said, it can take just a few decades or less for erosion to reduce a coral reef to sand.
Ritson-Williams said the new research from the University of Tasmania advances coral reef studies forward in a very important way. “We’ve generally done a very poor job of establishing baselines in coral reef ecosystems,” he said. “One of the things I liked about this study is they were able to go to a lot of different reefs and analyze them and set up baselines that they were able to look back to later, and that hasn’t happened in very many other studies.”
Stuart-Smith and his coauthors noted that that their reef surveys “could provide valuable signs of the potential trajectory of ecosystem change for a warmer future with increasingly prevalent extreme events.”
But to Ritson-Williams, it seems like the future, and what it holds for coral reefs, is already here.
“Before the bleaching events we had in 2014 and 2015, I used to tell people climate change is a really bad thing and it’s coming,” he said. “After I saw these latest major bleaching events, it was obvious – climate change is here. We don’t have to wait to see the effects. They’re happening right now.”