Underwater Umbrellas Show Promise in Protecting Coral Reefs From Heat Damage

In the summer of 2024, researchers from Nova Southeastern University in Florida conducted an experiment that sounds almost whimsical until you understand what was at stake. They built twenty custom-made shades—parasol-like structures of PVC pipe and nylon fabric—and anchored them a foot above ten different colonies of brain coral in Newfound Harbor, in the Florida Keys. The shades reduced incoming light by 60 percent. What they were really trying to do was buy time for corals that were suffocating in warming water.

The motivation came from catastrophe. The previous year, in 2023, a massive bleaching event had swept through Florida's reefs with devastating force. Branching corals were essentially erased from some areas. Brain corals, which are more resilient, still took a severe hit at many inshore sites. Dr. Karen Neely, the study's lead author, and her team wanted to know if they could develop a practical tool to protect vulnerable corals during future heat waves. The umbrella experiment was their answer.

Corals bleach when water temperatures spike beyond what they can tolerate. The process is a kind of starvation. Corals live in symbiosis with algae that inhabit their tissue—the algae provide nutrition, and the coral provides shelter. When heat stress arrives, the coral expels the algae, and the relationship breaks down. The coral turns white, ghostly, because it has lost the pigment the algae provide. Without the algae, the coral begins to starve. If the stress lasts long enough, the coral dies. Restoring dead reefs is possible, but it is difficult, expensive, and leaves the ecosystem vulnerable to the next heat wave.

The shades could not cool the water itself. What they did was block ultraviolet radiation and reduce the intensity of light reaching the coral. This mattered because UV and light exposure actually worsen the damage that heat stress causes. During the summer of 2024, the thermal stress at Newfound Harbor reached the second-highest level on record. The team monitored twenty control corals—identical colonies with no shade—and compared them to the shaded group. Every two weeks, divers visited the site to document color changes and collect tissue samples to examine the algae living inside each coral.

The results surprised even the researchers. The shaded corals lost significantly less color than the unshaded ones, especially during the most intense periods of thermal stress. More striking still: some of the shaded corals actually recovered color after the umbrellas were installed. The control corals showed more bleaching overall. Neither group experienced mortality during this particular event, but Neely emphasized that even bleaching that does not kill a coral can weaken it—making it more susceptible to disease and reducing its ability to reproduce.

The team found no meaningful difference in the density or species composition of algae between the shaded and control groups, which suggested that the shades were working primarily by reducing the intensity of the stress itself rather than by fundamentally altering the coral's biology. Neely noted that if the shades had been installed earlier in the heating event, they might have been even more effective. The study, published in Frontiers in Marine Science, identified the approach as having significant promise, particularly for two highly susceptible brain coral species—and likely useful for most other corals as well.

But the method carries an obvious limitation: it does not scale. You cannot shade an entire reef. What you can do is protect specific, particularly vulnerable colonies or populations of corals that are scientifically or ecologically important. The researchers believe the technique could become part of a toolkit for reef conservation during climate emergencies, a way to preserve genetic diversity and key populations while larger, systemic efforts to reduce carbon emissions continue. The work opens a question that will require more research: how much of a reef can be saved this way, and for how long, before the underlying problem—warming oceans—demands a different kind of solution.