When the water gets too hot, some corals may do better under an umbrella.

Scientists working in the Florida Keys found temporary shades helped protect vulnerable Caribbean corals during the summer of 2024, the second-hottest summer on record in those waters. The shades reduced light and helped limit bleaching, the stress response that turns corals white and can lead to colony death.

The study focused on two species of brain coral, Pseudodiploria clivosa and Colpophyllia natans, at Newfound Harbor in the Florida Keys. Dr Karen Neely of Nova Southeastern University, lead author of the article in Frontiers in Marine Science, and her team installed 20 custom-made shades over 10 coral colonies and selected 20 matching corals as controls.

The umbrella-like shades were made of pipes with nylon cloths attached. They were fixed temporarily to the seabed with cord and floated 30 centimetres over each shaded coral. The shades reduced light by 60 percent.

Divers visited every two weeks to track colour changes and take tissue biopsies to monitor the algae living inside the corals’ tissues. Corals rely on these algae for most of their nutritional needs, and the algae also give corals their colour. When heat stress breaks down that relationship, corals bleach and turn white.

The team found the shaded corals lost less colour than expected, even though heat stress at Newfound Harbor that summer was the second-highest on record. The differences were greatest during the most intense periods of thermal stress. Shaded corals also recovered some of their colour after the shades were installed.

“Heat-related coral bleaching is probably the biggest threat to reefs worldwide, and we absolutely have to address climate change to have any hope of reefs as we know them existing in the future,” Neely said.

“Solutions like shading can offer temporary protection on small scales. The 2023 coral bleaching event in Florida essentially wiped out branching corals, and at some inshore sites had a devastating impact on brain corals. This study stemmed directly from that event: we wanted to provide proof of concept for tools to save targeted corals in similar future events.”

The researchers did not record any dead coral in either the shaded or control groups. Neely said the bleaching event was less severe than expected overall, and suggested that may be because many of the corals were occupied by Durusdinium, an algae that is very heat-tolerant but is associated with lower coral growth and possibly greater susceptibility to disease.

“Shades reversed the observed bleaching, but the event wasn’t severe enough to cause mortality on control or shaded corals,” said Neely.

“However, we know that even bleaching that doesn’t cause mortality can impact corals’ future disease susceptibility and reproductive capacity. It would be interesting to quantify if and to what extent shade structures benefitted these corals after their removal by tracking disease and reproductive output.”

The study found no differences in algae density or algae species between shaded and control corals. The researchers said that may be because permit conditions required them to sample coral edges, which are less exposed to light and experience less bleaching, meaning even control samples received some shade.

The team also said similar constraints meant they could not install the shades until well after water temperatures were already high enough to bleach coral. They said earlier installation might have made the shades more effective.

Neely said the approach has limits.

“Scaling is the big limitation of this study; shading is not a cure-all for coral reefs, and its use would require hard decisions about what corals are worth saving,” said Neely.

“We found it was effective on two highly-susceptible coral species, so it is likely to be useful for most others, but additional studies would be needed to confirm that. It is also likely that thermal stress would reach a point at which no amount of shading could help a coral, but only future experimentation would be able to define the bounds of that.”

Read more from Frontier.