Temporary shading structures placed over vulnerable corals can reduce bleaching caused by marine heat waves and help some corals recover part of their coloration, according to a study conducted in the Florida Keys, United States.
The experiment took place in the summer of 2024, when water temperatures in the region reached the second-highest value on record. The researchers installed 20 shading structures over ten colonies of two species of brain corals and compared their progression with that of 20 similar corals that did not receive protection.
The structures, built with tubes and nylon fabric, were temporarily fixed to the seabed and positioned about 30 centimeters above the corals. They reduced the amount of light reaching the colonies by about 60%.
The researchers monitored the corals every two weeks, assessing changes in their coloration and collecting small tissue samples to analyze the algae living inside the corals.
Less light, less bleaching
Bleaching occurs when thermal stress causes corals to lose the microscopic algae with which they live in a symbiotic relationship. These algae provide a large portion of the nutrients that corals need and also contribute to their coloration.
When the water gets too warm, that relationship can be disrupted and the corals turn white. If thermal stress persists, they can die.
In the study, corals protected by the structures lost less color than expected, especially during periods of greater thermal stress. Some of the corals that already showed signs of bleaching also regained part of their coloration after the installation of the structures.
The researchers emphasize, however, that the bleaching observed during the study was less intense than they had expected and that none of the monitored corals died. One possible explanation lies in the conditions observed in the previous year.
The marine heatwave of 2023, which heavily affected Florida’s reefs, may have left a population of corals more heat-tolerant. Many of the specimens analyzed in 2024 were associated with the alga Durusdinium, known for its greater tolerance to elevated temperatures, although this association may be related to slower coral growth and greater susceptibility to some diseases.
A solution for specific situations
According to the researchers, the shading structures do not cool the water. Their aim is to reduce exposure to solar radiation, which can exacerbate heat-related damage.
The researcher Karen Neely, of Nova Southeastern University and the lead author of the study, believes that this approach could be used as a temporary measure to protect certain corals during extreme heat events.
“The shading devices reversed the observed bleaching, but the episode was not severe enough to cause mortality in either the control corals or the protected corals,” she explains.
Even when it does not lead to death, bleaching can have downstream consequences, notably in terms of disease susceptibility and reproductive capacity. The researchers therefore argue that it will be important to monitor the corals after the structures are removed to determine whether temporary protection yields lasting benefits.
The study, however, has limitations. The structures could only be installed after the water temperature had reached levels capable of causing bleaching, so earlier installation could have produced more significant effects.
Moreover, the use of shading structures is difficult to scale up to large areas. The team itself considers that this technique does not constitute a global-scale solution for coral reefs and that its use would entail selecting which corals to protect.
The research thus points to a potential emergency tool to protect particularly vulnerable colonies during marine heat waves, but it does not replace reducing ocean warming, which remains the main pressure on coral reefs.
