High sea temperatures driven by the climate crisis damage corals, bleaching them ghostly white and killing them. While restoration of dead reefs is possible, it's difficult and expensive, and restored corals are vulnerable to future heatwaves. Finding ways to stop or slow the damage to existing reefs is crucial. Now scientists think they've found a way to protect some especially vulnerable Caribbean corals: umbrella-like shades. Shading can't cool the water, but it prevents UV and irradiance damage, which makes heat damage to corals worse.
"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," said Dr Karen Neely of Nova Southeastern University, lead author of the article in Frontiers in Marine Science. "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."
Safer in the shade?
In the summer of 2024, Neely and her team set up 20 custom-made 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 selected 20 matching corals to act as controls.
The umbrella-like shades, made of pipes with nylon cloths attached, were temporarily fixed to the seabed with cord and floated 30cm over each shaded coral. They reduced light by 60%. Divers visited every two weeks to track color changes and take tissue biopsies from the corals, to monitor the algae living inside the corals' tissues. Corals have a symbiotic relationship with these algae: the coral provides the algae with a home and the algae provides the coral with most of its nutritional needs. Algae also lend corals their color, so when corals turn white under heat stress, we know that the relationship has broken down and the coral is starving.
Although the heat stress measured at Newfound Harbor that summer was the second-highest on record, the shaded corals lost less color than expected — and the differences were greatest during the most intense periods of thermal stress. Shaded corals even recovered some of their color after the shades were installed.
However, Neely and her team also saw less bleaching than they expected overall, and no dead coral. This could have been because 2023's record heatwave meant that most of the corals were occupied by an algae called Durusdinium, which 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."
Under the umbrella
There were no differences in density of algae occupation or algae species between the control and shaded groups. This could have been influenced by the sampling location: the team's permits stipulated sampling from the corals' edges, which are less exposed to the light and experience less bleaching. This means that even the samples from the controls received some shade.
Similar constraints also meant that the scientists couldn't install the shades until well after the water temperature got high enough to bleach coral; if they'd been installed earlier, the shades might have been more effective. However, there was still a significant positive impact.
"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."
