Regional consistency of intertidal elevation as a mediator of seaweed canopy effects on benthic species richness, diversity, and composition

Through a regional-scale study, we tested the hypothesis that seaweed canopies affect benthic species richness, diversity, and composition differently depending on intertidal elevation. The theory behind that prediction recognizes that macroalgal canopies (composed mainly of Ascophyllum nodosum and secondarily of Fucus spp. on the studied shores) limit temperature and desiccation stresses in understory habitats during low tides at high and intermediate elevations but have little influence at low elevations because of prolonged submergences due to tide dynamics. We measured the abundance of all benthic algae and invertebrates in 1033 quadrats covering the natural range of fucoid canopy cover at high, middle, and low elevations in wave-sheltered rocky intertidal sites spanning 350 km of coast in Atlantic Canada. Univariate and multivariate analyses indicated that benthic species richness, diversity, and composition differed significantly between low-cover (0-40%) and high-cover (60-100%) canopy treatments at high and middle elevations but not at low elevations. At high and middle elevations, high canopy cover increased benthic richness and diversity by allowing several species to occur and enhancing the abundance of most species, compared with low canopy cover. At low elevations, species generally occurred in both canopy treatments and showed a similar abundance between them. These results reveal the ecological generality of the dependency of canopy effects on elevation, which we previously found through a manipulative experiment done at one location. Overall, we conclude that environmental stress mediates seaweed canopy effects on the structure of intertidal communities.