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

Abstract

Summary 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.

Potential bleaching effects on coral reproduction

“…Given the extraordinary energy expenditure needed for coral spawning and the potentially terrible reproductive results before settlement (noted here), we wonder whether a coral might have a greater reproductive advantage in future years, if it does not spawn after a bleaching event (Anthony et al 2009;Schoepf et al 2013). In fact, this is the effective reproductive strategy of many species .…”

Annual coral bleaching and the long-term recovery capacity of coral

“…Consequently, lipid levels are expected to be lowest towards the end of this period, which coincides with peak SST at our study site. Such unfortunate timing likely influences resistance to heat stress [19] and may contribute to the increased bleaching C h2e , heterotrophic C is preferentially incorporated into tissues when the difference is less than 0, whereas photosynthetic C is preferentially incorporated when the difference is greater than or equal to 0. Averages are shown +1 s.e.…”

“…[44]), compromise resistance to further stress events (e.g. [19]) and diminish a coral's capacity to compete for space.…”

Tropical storm off Myanmar coast sweeps reefs in Ritchie’s Archipelago, Andaman

“…The ability of coral reefs to flourish in the unproductive environment is attributed to upstream divergence of oceanic currents encountering the island mass effect, complex currents and topographically generated gyres which induce upwelling and favourable productive conditions (Hamner and Hauri 1981). Phytoplankton blooms and subsequent zooplankton abundance are found to help in the recovery of corals bleached due to unfavourable environmental conditions, by promoting feeding Grottoli et al 2006), delaying their death and increasing the chance of survival of bleached corals (Anthony et al 2009). Island mass effect is thus considered a critical factor in determining the resilience of coral reefs to bleaching in the face of increased incidence of bleaching events caused directly or indirectly by changing climate.…”