Testing the relative importance of intertidal seaweeds as ecosystem engineers across tidal heights

The persistence of seaweeds as community-structuring organisms is critical for the maintenance of many ecosystem functions on rocky intertidal shores. We measured the relative importance of seaweeds in structuring the abundance, richness, and distribution of macroinvertebrates at three tidal levels on a rocky shore. Through manipulative experiments, we characterized temperature, irradiance, and desiccation underneath seaweed canopy plots made up of either Silvetia compressa or Stephanocystis dioica, assembled at three biomasses. We assessed differences in the abundance, richness, and the distribution of macroinvertebrates underneath the canopy as a function of seaweed species and biomass and evaluated differences due to the tidal height of the installed experimental seaweeds. We found that manipulated seaweed biomass significantly affected the community structure of associated macroinvertebrates at all tidal levels, especially at higher biomass treatments. Although contrary to other studies, we found no effect on richness underneath the canopies at any tidal height. Our results show that intertidal seaweeds provide protection from water loss, heat, and irradiance when compared to manipulative controls with no canopy cover. We also found that the relative importance of the experimental seaweed canopies in their ability to mitigate stressful abiotic conditions increased with prolonged emersion periods, such as at higher levels in the intertidal zone. At the higher tidal level tested, a greater abundance of macroinvertebrates was associated with the experimental canopies in comparison to the manipulation-control plots and the natural substrate. These results emphasize the role of intertidal seaweeds as bioengineers and highlight the importance of preserving and restoring high densities of leathery seaweeds on degraded shores in order to preserve the dynamics of the intertidal ecosystem overall.