Small patches of tropical reefs that persist despite punishing marine heat waves could hold clues to the protection of the world’s corals, according to a new analysis.
Scientists conducted aerial surveys of Hawaiian reefs before and after a major heat wave in 2019 and found that corals at some sites fared better than their neighbors. These so-called refuges lost up to 40% fewer corals despite similar temperatures. A number of variables may explain their success, including distance from human settlements, the researchers reported May 2 at Proceedings of the National Academy of Sciences.
“We are trying to figure out where these [refugia] are and why they are there,” says Greg Asner, co-author of the study and director of the Center for Global Discovery and Conservation Science at Arizona State University in Hilo, Hawaii. “From a conservation perspective, these are tiny points of light, like little arches of biodiversity that are already in the ocean and need protection.”
Climate change has made heat waves more frequent in the sea. Prolonged exposure to these heat conditions poses a serious threat to corals. When the surrounding water becomes too warm, stressed corals undergo bleaching: Beneficial microscopic algae that inhabit corals are expelled, leaving them a ghostly white color. If the heatwave lasts long enough, the corals will eventually die.
To better understand why some corals are more resilient than others, Asner and his collaborators tracked coral mortality across 21,773 hectares (84 square miles) of reef surrounding six Hawaiian islands. The researchers flew over the reefs in January 2019, six months before a sea heat wave hit the archipelago. The team then repeated the survey in January 2020 to find out how the corals had resisted the disturbance.
The researchers used a technique called imaging spectroscopy to analyze the molecular makeup of the corals, which allowed them to distinguish living corals from dead corals. The proteins, chlorophyll and carbon compounds measured in living corals and their algal inhabitants are distinctive, Asner says. Once the coral dies, it becomes covered in macroalgae, which have a molecular composition closer to land plants.
Most previous studies have focused on bleached corals, Asner says. However, not all of these diseased corals will perish. “What we really need is to know where corals are dying, not just getting sick, so we can start formulating conservation and management planning around those that die and those that survive. “, he says.
After the heat wave, the reefs mapped by Asner and his team lost an average of 26.1% of their living corals, covering about 6.3% of the seabed. The waters around the islands of Lanai, Hawaii and Kahoolawe, particularly affected by the sea heat wave, have lost the greatest proportion of living corals.
However, the researchers also found that reefs with more abundant coral cover before the heat wave weathered the event better than reefs with sparser coral cover. These areas could serve as a long-term refuge during heat waves, the team wrote. There were several dozen refuges ranging in size from a few acres to several hundred acres, Asner said.
“The good scenarios were mostly in undeveloped areas with very little pollution and sedimentation or runoff,” he says. These corals were likely healthier when the heat wave hit than those on reefs close to residential and agricultural areas.
“Wastewater treatment in Hawaii is really bad,” says Asner. “We have a lot of human effluent – poo and pee – that is going into the ocean in some areas.” Stemming the flow of human waste, pesticides and other pollutants will be crucial to fortifying corals that don’t live in sheltered refuges, he says.
In addition, some refuges were located near natural underwater springs fed by cool, cool groundwater. “In areas where this [freshwater] escaped, the corals behaved better during the heat wave because it’s like a little thermal blanket,” says Asner. However, he adds, more research is needed on the importance of these “little protectors” and their ability to withstand increasingly intense marine heat waves in the future.
Even inside the refuge, not all the corals survived. “There were clearly winners and losers,” says Asner. Some species, such as cauliflower coral, are more vulnerable to rising temperatures than others. And some corals have genetic traits that make them more resilient than other members of their species. Tracking corals in a refuge that survive heat waves can provide conservationists with valuable information for coral reproduction and reef restoration efforts, the researchers concluded.
[Related: Coral reefs are dying, but it’s not too late to save them]
Another important step is to identify reef refuges beyond the Hawaiian Islands. How corals in a given area respond to rising temperatures will depend on their local ocean environment, the species present, and the types of pollution and other stressors the reef faces. Asner and his team hope to start measuring coral mortality using satellites next summer.
“We’re going to get worldwide coverage in this decade, maybe the first half of this decade, but right now I don’t want people to think that Hawaii is the whole planet,” he says.
The new findings echo some patterns scientists have noted in previous aerial surveys of the Great Barrier Reef, says Terry Hughes, a marine biologist at the Australian Research Council’s Center of Excellence for Coral Reef Studies at James Cook University in Townsville.
“We also showed that exposure to heat…explains why some places lose more coral than others,” Hughes said in an email. “More corals are dying where the water is the warmest for the longest time, with each bleaching episode.”
His team also found that some coral species died at higher rates than others, although these disparities decreased under more extreme conditions. “The win-lose spectrum is greatest when bleaching is relatively mild,” Hughes said. “Even so-called winners have high losses if the temperatures are high enough.”
Hughes and his colleagues also coral bleaching monitoring on the Great Barrier Reef after repeated heat waves. Parts of the northern reef escaped bleaching in 1998 and 2002. “Then the north fried in the third mass bleaching event in 2016, and has since bleached again in 2017, 2020 and 2022,” Hughes said. “To identify a refuge, you need to examine its responses to heat stress during multiple bleaching events, to see if that location consistently escapes with little or no damage.”
He warns that it remains to be seen how the newly discovered refuge in Hawaii will handle future disruptions.
“It’s premature to consider a place with comparatively lower losses in a year as a haven,” Hughes said. “The world is full of ancient coral reef refugia – places that were relatively lucky during one bleaching episode, only to be hammered away during a later episode.”