The north-south divide? Macroalgal functional trait diversity and redundancy varies with intertidal aspect

Background and aims Marine macroalgae ('seaweeds') are critical to coastal ecosystem structure and function, but also vulnerable to the many environmental changes associated with Anthropogenic Climate Change (ACC). The local habitat conditions underpinning observed and predicted ACC-driven changes in intertidal macroalgal communities are complex and likely site-specific, in addition to more commonly reported regional factors like sea surface temperatures.Methods We examined how the composition and functional trait expression of macroalgal communities in SW England varied with aspect (i.e., north-south orientation) at four sites with opposing Equator- (EF) and Pole-facing (PF) surfaces. Previous work at these sites had established that average annual (low tide) temperatures vary by 1.6 degrees C and that EF surfaces experience six-fold more frequent extremes (i.e., > 30 degrees C).Key results PF macroalgal communities were consistently more taxon rich; eleven taxa were unique to PF habitats, with only one restricted to EF. Likewise, functional richness and dispersion were greater on PF surfaces (dominated by algae with traits linked to rapid resource capture and utilization, but low desiccation tolerance), although differences in both taxon and functional richness were likely driven by the fact that less diverse EF-surfaces were dominated by desiccation-tolerant fucoids.Conclusions Although we cannot disentangle the influence of temperature variation on algal ecophysiology from the indirect effects of aspect on species interactions (niche pre-emption, competition, grazing etc), our study system provides an excellent model for understanding how environmental variation at local scales affects community composition and functioning. By virtue of enhanced taxonomic diversity, PF-aspects supported higher functional diversity, and consequently, greater effective functional redundancy. These differences may imbue PF-aspects with resilience against environmental perturbation, but if predicted increases in global temperatures are realised, some PF-sites may shift to a depauperate, desiccation-tolerant seaweed community with a concomitant loss of functional diversity and redundancy.

Automated summary

The study found that polar-facing macroalgal communities had higher taxonomic diversity and functional trait expression, while equator-facing surfaces were dominated by desiccation-tolerant fucoids. These differences may impact the communities' resilience to environmental disturbances.