Small patches of tropical reefs that endure despite punishing marine heat waves could hold clues to protecting the world’s corals, a new analysis suggests.
Scientists took aerial surveys of Hawaiian reefs before and after a major heat wave in 2019, and found that corals at certain sites fared better than their neighbors. These so-called refugia lost up to 40 percent fewer corals despite facing similar temperatures. A number of variables may explain their success, including distance from human settlements, the researchers reported on May 2 in Proceedings of the National Academy of Sciences.
“We’re trying to understand where those [refugia] are and why they’re there,” says Greg Asner, a coauthor of the study and director of the Arizona State University Center for Global Discovery and Conservation Science in Hilo, Hawaii. “From a conservation point of view, these are tiny points of light, like little arks of biodiversity that are already in the ocean that need protection.”
Climate change has made heat waves more common in the sea. Prolonged exposure to these toasty conditions poses a serious threat to corals. When the surrounding water becomes too warm, the stressed-out corals undergo bleaching: The beneficial microscopic algae that dwell within the corals are expelled, leaving them a ghostly white color. If the heat wave continues long enough, the corals will ultimately die.
To better understand why some corals are more resilient than others, Asner and his collaborators tracked coral mortality over 21,773 hectares (84 square miles) of reefs surrounding six Hawaiian Islands. The researchers flew over the reefs in January 2019, six months before a marine heat wave descended on the archipelago. The team then repeated the survey in January 2020 to find out how the corals had weathered the disturbance.
The researchers used a technique called imaging spectroscopy to analyze the molecular composition of the corals, which allowed them to distinguish living corals from deceased ones. The proteins, chlorophyll, and carbon compounds measured in living corals and their algal denizens are distinctive, Asner says. Once the coral dies, it becomes covered by macroalgae, which have a molecular composition more akin to terrestrial plants.
Most previous studies have focused on bleached corals, Asner says. However, not all of these ailing corals will perish. “The thing we really need is to know where the corals are dying, not just getting sick, so that we can start to formulate conservation and management planning around those who die and those who survive,” he says.
After the heat wave, the reefs Asner and his team mapped lost an average of 26.1 percent of their live corals, covering about 6.3 percent of the seafloor. The waters surrounding the islands of Lanai, Hawaii, and Kahoolawe, which were hit particularly hard by the marine heat wave, lost the greatest proportion of live corals.
However, the researchers also found that reefs with more abundant coral cover before the heat wave withstood the event better than reefs with sparser coral cover. These zones might be acting as long-term refugia during heat waves, the team wrote. There were several dozen havens that ranged in size from a few acres to several hundred acres, Asner says.
“The good scenarios were mostly in the undeveloped areas with very little pollution and sedimentation or runoff,” he says. These corals were likely healthier when the heat wave struck than those in reefs near residential and agricultural areas.
“Wastewater treatment in Hawaii is really bad,” Asner says. “We have a lot of human effluent—poop and pee—going into the ocean in certain areas.” Stemming the flow of human waste, pesticides, and other pollutants will be crucial to fortify corals that don’t live in sheltered refugia, he says.
Additionally, some refugia were located near natural underwater springs fed by cool, fresh groundwater. “In areas where that [freshwater] leaked out, corals did better during the heat wave because it’s like a little thermal blanket,” Asner says. However, he adds, more research is needed on how important these “little protectors” are, and how well they will stand up to increasingly intense marine heat waves in the future.
Even within the refugia, not all corals survived. “There were clearly winners and losers,” Asner says. Some species, such as cauliflower coral, are more vulnerable to rising temperatures than others. And some corals have genetic traits that make them hardier than other members of their species. Tracking which corals within a refugium survive heat waves can give conservationists valuable information for coral breeding and reef restoration efforts, the researchers concluded.
Another important next step is to identify refugia in reefs beyond the Hawaiian Islands. How corals in a given area react to rising temperatures will depend on their local oceanic environment, which species are present, and what kinds of pollution and other stressors the reef is facing. Asner and his team hope to begin measuring coral mortality using satellites next summer.
“We’re going to get the global coverage done in this decade, or even in the first half of this decade, but right now I don’t want people to think that Hawaii represents the entire planet,” he says.
The new results echo some patterns that scientists have noted in previous aerial surveys of the Great Barrier Reef, says Terry Hughes, a marine biologist at the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University in Townsville.
“We also showed that heat exposure…explains why some places lose more corals than others,” Hughes said in an email. “More corals die where the water is hottest the longest, in each bleaching event.”
His team additionally found that some coral species died at higher rates than others, although these disparities shrank under more extreme conditions. “The winner-loser spectrum is most prominent when bleaching is comparatively mild,” Hughes said. “Even the so-called winners have high losses if the temperatures are high enough.”
Hughes and his colleagues have also tracked coral bleaching 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 look at its responses to heat stress in multiple bleaching events, to see if that location consistently escapes with little or no damage.”
He cautions that it remains to be seen how the newly-discovered refugia in Hawaii will handle future disruptions.
“It’s premature to regard a location with comparatively smaller losses in one year as a refuge,” Hughes said. “The world is full of former coral reef refuges—places that were comparatively lucky in one bleaching event, only to be hammered in a subsequent one.”