Divergent recovery trajectories in reef-building corals following a decade of successive marine heatwaves

Increasingly frequent marine heatwaves are devastating coral reefs. Corals that survive these extreme heat stress events must rapidly recover if they are to withstand subsequent events, and long-term survival in the face of rising ocean temperatures may hinge on recovery capacity and acclimatory gains in heat tolerance over an individuals lifespan. To better understand coral recovery trajectories in the face of successive marine heatwaves, we monitored the responses of bleaching-susceptible and bleaching-resistant individuals of two dominant coral species in Hawaii, Montipora capitata and Porites compressa, over a decade that included three marine heatwaves. Bleaching-susceptible colonies of P. compressa exhibited beneficial acclimatization to heat stress (i.e., less bleaching) following repeat heatwaves, becoming indistinguishable from bleaching-resistant conspecifics during and after the third heatwave. In contrast, bleaching-susceptible M. capitata repeatedly bleached during all successive heatwaves and exhibited seasonal bleaching for up to three years following the third heatwave. Encouragingly, bleaching-resistant individuals of both species remained pigmented across the entire time series; however, pigmentation did not necessarily indicate physiological resilience. Specifically, M. capitata displayed incremental yet only partial recovery of symbiont density and tissue biomass across both bleaching phenotypes up to 35 months following the third heatwave. Conversely, P. compressa appeared to recover across most physiological metrics within two years, reverting to predictable seasonal variability. Ultimately, these results indicate that even some visually robust, bleaching-resistant corals can carry the cost of recurring heatwaves over multiple years, leading to divergent recovery trajectories that may erode coral reef resilience in the Anthropocene. Significance StatementCoral reefs are in jeopardy as climate change has led to increasingly frequent marine heatwaves. Some corals can survive these extreme heat stress events, thus acquiring environmental memory that may prime them for increased resistance and resilience in subsequent heatwaves via beneficial acclimatization. Yet, as the time between heatwaves decreases, the accumulation of stress experienced by some individuals may preclude opportunities for beneficial acclimatization. This nearly decade-long study revealed divergent recovery trajectories within and between species in response to successive marine heatwaves, ranging from costly to beneficial. As the climate continues to change, surviving corals must not only gain heat tolerance, but also rapidly recover to maintain the critically important ecosystem services that humanity relies on.