Abstract
Aim To identify geographic locations where lethal temperatures and upper zonation limits of M ytilus californianus beds coincide and to determine the dominant climatic factor where lethal limits occur. Location Intertidal shores along 1500 km of the west coast of N orth A merica. Methods Lethal temperatures for M . californianus were determined using temperature‐controlled chambers in which both the magnitude and duration of exposure were manipulated. Field surveys of the shore level and solar exposure of upper zonation limits of mussel beds were conducted at 15 sites. Measurements from the lethal temperature studies and field surveys were used with a biophysical model of mussel body temperature to hindcast M . californianus mortality rates from 1997–2007. Results The critical lethal high body temperature was c . 38 ° C . Hindcasts of mortality rates predicted that lethal temperatures occurred along the upper limits of the mussel bed at only two of the sites, S anta C ruz and A legria, CA , USA , and daily maximum air temperature during low tide was the variable that distinguished these sites. S anta C ruz and A legria were not adjacent to each other and not the most equatorward of the sites examined. Main conclusions Local upper zonation limits of the mussel M . californianus are determined by lethal high body temperatures at two of 15 locations, which means that alternative environmental factors are determining local zonation at most of the study sites. Mussel body temperature is driven by many physical variables, but air temperature was higher at S anta C ruz and A legria than elsewhere making it the most likely factor driving the climatic sensitivity at local upper zonation limits. There was no significant evidence of local acclimation or local adaptation in this species. Some local zonation limits are more vulnerable to climate change than others, potentially causing the magnitude and timing of zonation shifts to differ even among closely situated sites.