Earth is in the midst of its fourth global coral bleaching event. Scientists expect that by 2050, the world’s oceans will have warmed so much that most reefs suffer mass bleaching every year. A surprisingly simple invention could help protect some of them.
In a study published today in the journal Frontiers in Marine Science, researchers present “underwater umbrellas” that shade corals to reduce their Sun exposure, which exacerbates bleaching caused by warming waters. They tested these crafty canopies in the Caribbean, where corals are especially vulnerable to solar radiation and heat stress, and found that shaded corals indeed suffered less bleaching. Some even recovered some of their color.
“The particular structures that we deployed are good for smaller areas, like individual high-priority corals, coral nurseries, or small outplant areas,” lead author Karen Neely, a research scientist at Nova Southeastern University, told Gizmodo in an email. “But new engineering ideas could make it suitable for larger reef areas.”
Corals in crisis
When corals are stressed by changes in environmental conditions such as temperature, light, or nutrient availability, they expel the symbiotic algae that live in their tissues. These algae, called zooxanthellae, give corals their signature bright colors and also serve as their primary food source. Without zooxanthellae, corals turn white—hence the term “bleaching”—and begin to starve.
Cooling the waters that corals live in will require global action. Humanity will need to cut its greenhouse gas emissions to net-zero and remove carbon from the atmosphere to reverse the current warming trend. Until we can achieve those feats—and we are nowhere near close to achieving them—coral reefs will continue to suffer from mass bleaching events.
The decimation of these vital ecosystems threatens numerous other marine species, natural protections for coastal communities, and the multi-billion dollar fishing industry that feeds and employs millions of people.
As Earth’s oceans continue to warm, scientists are developing strategies to mitigate the damage to coral reefs. Shading has emerged as one promising solution, and researchers have conducted experiments in the Pacific Ocean using a variety of designs, according to Neely. Her team’s design was inspired by a colleague in Dominica who created a few of the umbrella-like structures during the marine heatwave of 2023.
Shading can be life-saving
The shades, made of pipes with nylon cloths attached, get fixed to the seabed so that they float about a foot (30 centimeters) above a coral. In the summer of 2024, Neely and her colleagues set up 20 shades over 10 different colonies of two species of brain coral, Pseudodiploria clivosa and Colpophyllia natans, at Newfound Harbor in the Florida Keys. They also picked 20 matching corals that would remain unshaded to serve as controls.
The researchers found that the shades reduced corals’ light exposure by 60%. Donning scuba gear, they visited the corals every two weeks to look for color changes and biopsy their tissues. They then analyzed those tissue samples in the lab to monitor the algae living inside.
That summer, the heat stress measured at Newfound Harbor was the second-highest on record, but the shaded corals suffered less bleaching than expected. The researchers noticed the biggest differences during the most intense periods of heat stress. The shades also appeared to reverse bleaching that had already occurred, as some of the corals started to get their color back.
Interestingly, the unshaded corals also saw less bleaching than expected, and none of the corals in either group died. Neely and her colleagues suspect that may be because the marine heatwave caused most of the corals to be occupied by an algae called Durusdinium. This species is particularly heat tolerant but is associated with reduced coral growth and perhaps greater susceptibility to disease.
What’s more, the researchers did not observe any differences in density of algae occupation or algae species between the shaded and unshaded corals. That may have been due to the fact that their permits stipulated sampling from the corals’ edges, which are less exposed to sunlight and experience less bleaching. They also believe the shades might have been more effective if they had installed them before water temperatures rose high enough to cause bleaching.
Despite these caveats, the shades still had a measurable positive impact, and Neely said her team did not see any evidence to suggest that they caused harm to other parts of the ecosystem. The experiment serves as a proof-of-concept for positive shading effects on Caribbean corals, and future designs could work for larger reef areas.
“There have already been efforts underway this summer to trial these larger efforts,” Neely said.
Next, she hopes to investigate the longer-term effects these shades have on corals. “Theoretically they should reduce subsequent disease and increase subsequent reproduction, but this needs to be tested,” she said.
