Combining shading and lipid-enriched diets as an adaption tool to reduce coral bleaching

Mass coral bleaching driven by climate change impacts coral reefs globally. As net zero emissions and a return to pre-industrial global temperatures are unlikely to occur in the near future, there is an urgent need to engineer intervention methods that can mitigate the risk of coral bleaching at different scales. Coral dietary enrichment and shade-based irradiance reduction have each been shown to reduce coral bleaching. Here, we tested the hypothesis that combining these two intervention methods could further reduce the risk and impact of bleaching using an outdoor experiment with fragments of the coral Hydnophora exesa. The experiment was set up over three orthogonal factors: shade (2 levels - 4 h of 30% shade and no shade), temperature (2 levels - 32.6 degrees C and 26.4 degrees C) and food type (2 levels - fatty-acid enriched and non-enriched Artemia). The provision of 30% shade for 4 h did not significantly affect any of the measured bleaching response variables, likely due to the low natural irradiance for all treatments throughout the experiment. Significant bleaching of H. exesa fragments occurred in the high-temperature treatments after 18 days of thermal stress. Feeding the corals Artemia enriched with omega-3 and omega-6 fatty acids had a minor impact on the proportion of fatty acids in the corals and resulted in a decline in chlorophyll a content and symbiont density. Overall, these results suggest that coral PUFA enrichment may have limited potential as a mitigation tool to minimise the risk of mass coral bleaching as numerous factors such as species and lipidome composition must be considered. In addition, we recommend that irradiance values higher than the natural light levels recorded during our experiment are required to effectively test the ability of shading technologies designed to reduce mass bleaching of coral reefs.