Contrasting respiratory response to regulate resilience under marine heatwave intensities and onset rates.

Marine heatwaves (MHW) occur at varying profiles (intensity, onset, duration) and their impact depends on species-specific thermal physiology. While physiological processes may support resilience to MHW-induced thermal stress, metabolic costs may have strong post-MHW implications. Here, by assessing respiratory response, we link metabolic costs of a model marine crustacean (copepod) to MHW profiles and determine the mechanisms involved in recovery and potential vulnerability to additional thermal stress. Our results suggest that copepods potentially used metabolic depression as a response to increased temperature, but the responses were not uniform across MHW profiles. Harsher MHW (composite higher intensity and onset rate) caused an upsurge in metabolic rate but also high metabolic rates during the recovery phase. Therefore, copepods exposed to the harsher MHW scenario appeared to recover faster, which may be consistent with an 'active' recovery strategy involving elevated energetic investment in physiological repair. On the other hand, copepods exposed to lower MHW intensities showed a more gradual recovery pattern that may be consistent with a more 'passive' recovery strategy which may infer vulnerability to a subsequent secondary thermal stress. The counterintuitive and contrasting responses suggest that the vulnerability of some organisms to MHW is dependent on the MHW profiles and the type of environmental cue it emits, but especially on the regulatory mechanisms that support resilience. Our study highlights that understanding the impact of MHW on organisms requires research beyond the direct impacts of MHW-associated thermal stress and better assessment of post-MHW response, including potential cumulative impact of additional environmental stress.