Cool ocean temperatures fail to buffer the impacts of heat exposure during low tide on the behaviour and physiology of a keystone predator

Air temperatures are warming at faster rates than ocean temperatures, and this "land-sea warming contrast" may create reprieves from thermal stress by providing cool underwater refugia during extreme heat events. Here we tested the impacts of the "land-sea warming contrast" on physiology (metabolism) and behaviour (feeding) in the juvenile life stage of a keystone intertidal predator, Pisaster ochraceus, by experimentally manipulating air ([~]20, 25, 30) and water ([~]15, 20) temperatures (at independent rates) representing early summer, late summer, and heatwave conditions in Barkley Sound (British Columbia, Canada). We further made observations of air temperatures, sea surface temperatures, and Pisaster moribundity at our study location to support interpretation of our results. We predicted metabolism and feeding would increase with early and late summer temperatures, but decrease during heatwave conditions as animals surpass their thermal optimum. We observed the greatest mortality and lowest feeding in juvenile Pisaster exposed to cool ocean temperatures ([~]15) and high aerial temperatures typical of extreme heat events ([~]30). Feeding rates increased with heat stress duration, indicating animals may be compensating for elevated metabolism. Metabolic rates did not differ between air temperatures, but oxygen consumption was higher in animals with access to mussels than for Pisaster that were fasted. The highest levels of experimental and field moribundity were observed in August, indicating Pisaster may have accumulated physiological stress damage following elevated air and ocean temperatures throughout the summer. Our research implicates shifts in community dynamics due to the loss of this keystone species as air temperatures warm. Summary StatementCooler ocean temperatures, rather than creating thermal refugia, may cause physiological stress for juvenile Pisaster ochraceus exposed to warm air during low tide.