Regional differences and linkage between canopy structure and community composition of rockweed habitats in Atlantic Canada

Ascophyllum nodosum (rockweed) is a dominant, habitat-forming seaweed on intertidal rocky shores in the North Atlantic and commercially harvested in Canada, Maine and Europe. Rockweed plant structure varies regionally, and several morphotypes have been identified in Atlantic Canada alone. Yet the regionality of canopy structure, associated species communities and the link between the two have not been well understood. Using large-scale field surveys and multivariate statistical approaches, we report distinct canopy structures between sites in southwest Nova Scotia and southwest New Brunswick, the two major rockweed harvesting areas in Atlantic Canada, as well as distinct associated community composition. We then demonstrate significant links between canopy and community structure. Importantly, plant and canopy structure, including length, circumference and density, were much better predictors of associated community structure than rockweed biomass, which is often used for single-species monitoring. Adding region or site as predictors further improved model fits. Therefore, measuring plant or canopy structure would strongly improve insight regarding ecosystem changes. Moreover, incorporating information about regional differences in canopy structure and the dependence of associated communities on those canopies is critical for spatially refined, ecosystem-based management of the rockweed harvest.