The effect of small-scale morphology on thermal dynamics in coral microenvironments.

The thermal microenvironments of corals is a topic of current interest given their relationship to coral bleaching. We present computational fluid dynamics (CFD) model of corals with both smooth and rugged polyp surface topographies for two species of massive corals (Leptastrea purpurea and Platygyra sinensis) in order to predict their microscale surface warming. This study explores whether variation in polyp depth (PD) may directly effect a coral overall surface area-to-volume (A/V) ratio and consequently its surface warming. Validation of our models was made against detailed laboratory measurements of coral surface warming and thermal boundary layer thickness. Our results suggested that while differences in surface warming exist between smooth surfaces and surfaces covered in micro-polyps (5 mm depth), the variation in terms of surface warming is small (~0.18-0.19∘C) and it can be largely attributed to increasing A/V ratios. Our results demonstrated good agreement with measurements of surface temperatures on living corals and that ignoring the presence of polyps by modelling heat transfer associated with a smooth surface makes no material difference to the values obtained or the interpretation of the processes leading to surface warming.