Modelling marine heatwaves impact on shallow and upper mesophotic tropical coral reefs

Coral reefs ecosystems, often compared to rain forests for their high biodiversity, are threatened by coral bleaching. Coral bleaching occurs when the symbiotic relationship between dinoflagellates and corals breaks under environmental stresses, notably high ocean temperatures. Thermal stress on coral reefs predominantly occurs during marine heatwaves, which can take place synchronously at the surface and subsurface, or asynchronously in either one of the two levels. Subsurface marine heatwaves tend to last longer with potentially higher cumulative intensities compared to their surface counterpart. However, to the best of our knowledge, no global coral bleaching model takes into account the variability between the thermal stress measured at the surface and the one experienced by coral reefs at their specific depth. Here we show that developing a marine heatwave impact model for shallow and upper mesophotic coral reefs, increased coral bleaching modelling accuracy by 4.7 ± 1.3% compared to a model using surface marine heatwaves. To define marine heatwaves at coral reef depth, we used trilinear interpolation using the GLORYS12 reanalysis temperature product. Our model provides coral bleaching values at times and locations where no record was taken, providing a global reconstructed dataset of coral bleaching with daily resolution from January 1st 1993 to December 31st 2020 in 9944 locations. Furthermore, our model indicates that since 1993 over 40% of coral reefs bleached. We anticipate this study to be a starting point for more accurate coral bleaching modelling. Observing that upper mesophotic coral reefs (30-50m) might be more threatened than shallow coral reefs, provides additional evidence to reshape our perception of upper mesophotic coral reefs as potential refugees from climate change.