| Summary: | <p> In addition to higher average temperatures, many regions of Earth are also experiencing higher variability. I investigated how prior thermal history and intensity of an acute high temperature challenge affected post-exposure photosynthetic performance of microalgae on rocky shores. I measured community-level net photosynthetic rate (NPR) on experimental plates left in the field for one or seven months before and after exposure to a simulated low tide. After one month, microalgal NPR decreased more after exposure to higher peak temperatures, although the effect was mitigated somewhat by prior exposure to thermal variability. In contrast, after seven months there was no effect of temperature on NPR. There were, however, differences due to shore height and experiment duration, with respect to overall biomass and pre-exposure NPR. Scanning electron microscopy (SEM) suggested that changes in the identity and relative abundances of microalgal taxa associated with increasing aerial exposure during low tides might be responsible.</p><p>
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