Summer temperatures can impose lethal stresses on rocky intertidal organisms and are influenced by substrate characteristics such as orientation and slope. Here, we show that rock size plays an even more important role than these other factors in determining rock temperatures and influencing the distribution of intertidal organisms.   

Results/Conclusions

Maximum summer temperatures of small cobbles were 10°C hotter than boulders or benches. This temperature difference was reflected in substrate-specific patterns of mortality of the barnacle, Semibalanus balanoides, at three sites in Narragansett Bay, RI. Annual mortality was significantly lower on boulders and benches than on stabilized cobbles. This substrate size-dependent mortality was reflected in surveys of 14 other sites in Rhode Island, where substrate size accounted for more of the variation in the upper limit of the barnacle zone than all other rock characteristics combined. In addition, experimentally increasing effective rock size lowered cobble temperatures and delayed barnacle mortality significantly. In contrast, the positive effect of substrate size on barnacle survival was not apparent in surveys at 9 sites in Maine where summer heat stress is less severe. This suggests that, boulders and benches, via their thermal buffering capacity, create an important thermal refuge for Semibalanus in thermally stressful southern New England, but these effects diminish at more thermally benign sites further north. Moreover, cobble-dominated rocky shores in southern New England may be particularly vulnerable to increased heat stress associated with global climate change.