Interactions Between Sessile Species Groups from Wave-Exposed Rocky Intertidal Habitats in Atlantic Canada Evaluated Using Multiannual Surveys

Within biogeographic regions, local communities are structured mainly by abiotic (environmental) filtering, external resource supply, and biotic interactions. In recent years, we investigated abiotic filtering and external resource supply as drivers of the latitudinal distribution of rocky intertidal species along the Atlantic Canadian coast in Nova Scotia. Here, we evaluate biotic interactions between the main sessile species groups. Specifically, we studied abundance relationships between seaweeds and filter-feeding invertebrates and between barnacles and mussels using data collected at mid-to-high intertidal elevations at eight wave-exposed locations every summer from 2014 to 2017. We assessed such relationships for each location and year through generalized additive modeling (GAM). Of the 32 relationships evaluated for seaweeds vs. filter-feeders, 31% were significant and consistently negative, suggesting competitive interactions. For barnacles vs. mussels, 25% of the relationships were significant and mostly positive, consistent with facilitation of mussel colonization by barnacles in harsh environments. The variability explained by these models was moderate, however, between around 10% and 50%. Overall, these results suggest that interactions between the studied sessile species groups are infrequent and, when present, relatively weak in these highly stressful habitats, which supports current ecological theory on community organization.