A blistering marine heat wave caused extensive coral bleaching off the coast of South Florida. And similarly damaging conditions have been projected to hit much of the Caribbean, according to the National Oceanic and Atmospheric Administration.
Much of the immediate response to a sudden and intense marine heat wave is often focused on rescuing corals from areas that are too hot — moving them on land or into deeper, cooler sites to wait out the heat — but experts say they are also thinking about how to best safeguard what’s left in the inhospitable water.
“An existing coral is worth a lot more than a new coral that you put in the water, so if you can keep them, that’s the best thing to do,” said David Mead, executive director of strategy and development at the Australian Institute of Marine Science, who has researched bleaching interventions.
Technical strategies have limitations, Mead and other experts emphasize, as the growing impacts of climate change underline that it’s critical to address planet-warming emissions. These ideas, expert say, are a small part of any overall solution to saving imperiled reefs.
“If a coral reef is under pressure from temperature, there’s really only a few things that you can do,” Mead said.
Exposure to the sun’s rays and too much heat for too long can trigger bleaching — when corals expel the beneficial plantlike organisms called zooxanthellae that live in them, causing them to lose their vibrant color and turn a deathly white. Using shade to lessen the amount of light hitting heat-stressed corals can help.
Some research suggests that high levels of direct shading using a specially designed cloth could reduce the stress placed on corals, said Neal Cantin, a research scientist at the Australian Institute of Marine Science. Tests found that 30 or 50 percent shading could be effective, said Cantin, who spoke during a recent webinar on strategies to address the ongoing bleaching event hosted by the Coral Restoration Consortium, a global group of experts.
Direct shade is most important during the middle of the day between 10 a.m. and 3 p.m., particularly when the water is calm, he said.
But when the temperatures are too hot for too long, the benefits of shading corals from the sun can start to decrease. At that point, the heat stress alone will likely cause bleaching, Cantin said.
“You’ll want to be shading your corals early before they start bleaching and before the heat stress really gets extreme,” he said. For places like Florida, it might be too late to deploy shade.
Shade structures also typically can only cover limited areas, making them useful for nurseries or specific sites, but not large swaths of reefs. Additionally, installing and maintaining the underwater structures can be time consuming. The shade cloths, for instance, should be cleaned multiple times a week, Cantin said.
Covering the surface of the ocean with fog, also known as fogging, is another method of shading that’s being explored, Mead said. The type of mist researchers are trying to produce is similar to natural sea fog, he said.
“You can create a fog at the right sort of density so that it reduces light,” he said.
While fogging isn’t suitable for use at a very large scale, it could cover more area than standard shade structures, he said. He noted that shade cloth could be limited to around tens to hundreds of square meters, whereas fogging could cover around 10,000 square meters.
In Australia, Mead and other experts are testing an industrial fog machine that — with some minor tweaks — could be used on the ocean.
“It’s still very close to being off-the-shelf technology,” he said. But, he noted, researchers are still evaluating the approach and aren’t able to say that it is guaranteed to work.
The machine would need to be mounted on a boat or another type of vessel so it can move around the water, Mead said. Fogging also can’t be done when it’s too windy as the fog needs to be able to stay spread out over the target area.
Probiotics and supplemental feeding
Work is underway to develop probiotics that can be given to corals to help them better endure hotter temperatures, said Tali Vardi, executive director of the Coral Restoration Consortium, the global group of experts.
“It’s kind of like when you’re sick and you go to get a smoothie with immunity boosters and bee pollen and whatever else,” Vardi said.
In the meantime, scientists are also trying to support heat-stressed corals with more food through a process known as supplemental feeding.
“When you have bleaching, the corals are effectively starving,” said Elizabeth McLeod, Global Ocean Director at the Nature Conservancy. “If you can supplementally feed the corals, then sometimes they’re better able to cope with that heat-stress event.”
Researchers are also looking into methods to actively cool the water around corals.
One way involves trying to move cooler water from a nearby location and mixing it with the warm water to lower the overall temperature.
While Mead said he has “yet to see any maths where this method works,” it could make some difference if used in small areas where the water isn’t moving much. A strong current would just carry the cooler water away, he noted.
A previous version of this article incorrectly referred to the Coral Restoration Consortium as the Coral Restoration Foundation. The article has been corrected.