Andrew R Villeneuve , Easton R White 

Marine heatwaves (MHWs) can cause thermal stress in marine ectotherms, experienced as a pulse against the press of anthropogenic warming. When thermal stress exceeds organismal capacity to maintain homeostasis, organism survival becomes time-limited and can result in mass mortality events. Current methods of detecting and categorizing MHWs rely on statistical analysis of historic climatology, and do not consider biological effects as a basis of MHW severity. The reemergence of thermal tolerance landscape models provides a physiological framework for assessing the lethal effects of MHWs by accounting for both the magnitude and duration of extreme heat events. Here, we used a simulation approach to understand the effects of a suite of MHW profiles on organism survival probability across 1) thermal tolerance adaptation strategies, 2) interannual temperature variation, and 3) seasonal timing of MHWs. We identified survival isoclines across MHW magnitude and duration broadly connecting acute (low duration-high magnitude) and chronic (long duration-low magnitude) events with equivalent lethal effects on marine organisms. While most attention has been given to chronic MHW events, we show similar lethal effects can be experienced by more common but neglected acute marine heat spikes. Critically, a fixed-baseline definition of MHWs does not accurately categorize biological mortality. By letting organism responses define the extremeness of a MHW event, we can build a mechanistic understanding of MHW effects from a physiological basis. MHW responses can then be transferred across scales of ecological organization and better predict marine ecosystem shifts to MHWs.