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As the frequency of extreme marine heatwaves increases, the time available for recovery between events is decreasing, opening the possibility that accumulation of heat stress may preclude acclimatization. To better understand which of these trajectories are occurring, we have been following the in situ heat stress responses of bleaching-susceptible and bleaching-resistant individuals of two dominant Hawaiian reef-building coral species (Montipora capitata and Porites compressa) over the last 8 summer seasons, encompassing 2 severe marine heatwaves (2015 and 2019). Our data indicate that there are both inter- and intraspecific differences in response to repeat heatwaves, with bleaching-susceptible colonies of P. compressa showing acclimatization to heat stress (i.e. no bleaching), whereas bleaching-susceptible M. capitata exhibited moderate bleaching during the second heatwave as well as in response to seasonal temperature maxima in non-heatwave years. Through high-frequency physiological sampling, we further explored recovery capacity over 2 years following the second heatwave. By 29 months post-heat stress, photosynthetic rates, symbiont densities and host biomass of P. compressa reached an asymptote, suggesting near-complete recovery. Conversely, M. capitata displayed incremental yet only partial recovery, with bleaching-susceptible corals continuing to exhibit reduced metabolic rates compared to bleaching-resistant conspecifics. In the absence of a sufficient recovery period, frequent marine heatwaves may compound organismal stress, threatening the maintenance of coral reef ecosystem function into the future.

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