Benthic community composition across gradients of intertidal elevation, wave exposure, and ice scour in Atlantic Canada

Studies on community organization may focus on different properties, such as species richness (total number of species) or composition (a measure of the identity and abundance of species). In rocky intertidal habitats from Nova Scotia, Canada, we previously determined that richness varied across vertical (elevation) and horizontal (wave/ice exposure) gradients of environmental stress on 2 coasts of contrasting winter ice load: the Gulf of St. Lawrence coast and the open Atlantic coast. Across some limited ranges of stress, however, richness remained similar. Through multivariate analyses (NMDS followed by ANOSIM) of species abundance data measured for all seaweeds and benthic invertebrates, we increased our ability to detect spatial trends in community structure. The present study shows that community composition varied clearly across all studied ranges of environmental stress. Therefore, even relatively small changes in abiotic stress seem capable of altering the structure of intertidal communities. An analysis of similarity percentages (SIMPER) identified the main species that typified each environmental level and those that best discriminated between consecutive levels across the elevation and exposure gradients. Sessile organisms (barnacles and a few brown and red seaweeds) constituted the most important characterizing species, with differences in relative importance between the 2 studied coasts. Future experimental work aiming to unravel the biotic interactions underlying the observed changes in composition across the stress gradients should benefit by initially focusing on these species.