This study investigated the effects of intertidal elevation on ecologically relevant temperature traits and the intensity of organism desiccation during low tides on a subpolar NW Atlantic shore. The goal was to relate those descriptors of environmental stress to elevational changes in the growth of dominant sessile species (algae and mussels) that influence overall community structure. In-situ measurements taken at the low, middle, and high intertidal zones revealed that daily temperature maxima, daily temperature range, and desiccation rate increased significantly from low to high elevations. Conditions were especially stressful at the high zone, as the highest temperature (47°C, recorded during aerial exposure at low tide) more than doubled seawater temperature and desiccation rate surpassed 90%. Transplant experiments showed that seaweed (Ascophyllum nodosum) and mussel (Mytilus edulis) growth rate decreased dramatically from low to high elevations. Therefore, although subpolar shores are mostly known for their harsh winter conditions, the pronounced vertical stress gradient that characterizes the intertidal zone during ice-free months also plays a major role in determining spatial changes in benthic species performance. Our study also suggests that inferring stress differences between habitats based on growth data from transplanted specimens may be a useful field method to further develop stress ecological theory.

environmental stress, intertidal, mussel, seaweed.