Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching

Coral bleaching events, predicted to increase in frequency and severity as a result of climate change, are a threat to tropical coral-reef ecosystems worldwide. Although the onset of spatially extensive, or 'mass', bleaching events can be predicted using simple temperature stress metrics, no models are available for predicting coral mortality risk or sub-lethal stress associated with bleaching. Here, we develop a model that links the functional response of colony energy balance and energy-store dynamics to coral mortality risk and recovery during and following bleaching events. In a series of simulations using response functions and parameter values derived from experimental studies for two Indo-Pacific coral species (Acropora intermedia and Montipora monasteriata), we demonstrate that prior energy-costly disturbances and alternative energy sources are both important determinants of coral mortality risk during and following bleaching. The timing of the onset of coral mass mortality is determined by a combination of bleaching severity (loss rate of photopigments), duration of the bleaching event, heterotrophy and the size of energy reserves (as lipid stores) before bleaching occurs. Depending on initial energy reserves, model results showed that high rates of heterotrophy could delay the onset of coral mortality by up to three weeks. Survival following bleaching was also strongly influenced by remaining lipid reserves, rates of heterotrophy, and rates of photopigment (or symbiont) recovery. Our results indicate that energy-costly disturbances and low availability of food, before and during bleaching events, respectively, work to increase bleaching-induced coral mortality risk for acroporid corals on Indo-Pacific reefs.