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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 probably site-specific and operate in addition to more commonly reported regional factors such as 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 °C and that EF-surfaces experience six-fold more frequent extremes (i.e. >30 °C). Key Results PF macroalgal communities were consistently more taxon rich; 11 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 probably 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 realized, some PF-sites may shift to a depauperate, desiccation-tolerant seaweed community with a concomitant loss of functional diversity and redundancy. [ABSTRACT FROM AUTHOR]

Background Marine macroalgae ('seaweeds') are a diverse and globally distributed group of photosynthetic organisms that together generate considerable primary productivity, provide an array of different habitats for other organisms, and contribute many important ecosystem functions and services. As a result of continued anthropogenic stress on marine systems, many macroalgal species and habitats face an uncertain future, risking their vital contribution to global productivity and ecosystem service provision. Scope After briefly considering the remarkable taxonomy and ecological distribution of marine macroalgae, we review how the threats posed by a combination of anthropogenically induced stressors affect seaweed species and communities. From there we highlight five critical avenues for further research to explore (long-term monitoring, use of functional traits, focus on early ontogeny, biotic interactions and impact of marine litter on coastal vegetation). Conclusions Although there are considerable parallels with terrestrial vascular plant responses to the many threats posed by anthropogenic stressors, we note that the impacts of some (e.g. habitat loss) are much less keenly felt in the oceans than on land. Nevertheless, and in common with terrestrial plant communities, the impact of climate change will inevitably be the most pernicious threat to the future persistence of seaweed species, communities and service provision. While understanding macroalgal responses to simultaneous environmental stressors is inevitably a complex exercise, our attempt to highlight synergies with terrestrial systems, and provide five future research priorities to elucidate some of the important trends and mechanisms of response, may yet offer some small contribution to this goal. [ABSTRACT FROM AUTHOR]

Background and Aims Worldwide, invasive species are spreading through marine systems at an unprecedented rate with both positive and negative consequences for ecosystems and the biological functioning of organisms. Human activities from shipping to habitat damage and modification are known vectors of spread, although biological interactions including epibiosis are increasingly recognized as potentially important to introduction into susceptible habitats. Methods We assessed a novel mechanism of spread – limpets as transporters of an invasive alga, Sargassum muticum , into beds of the seagrass Zostera marina – and the physiological impact of its invasion. The association of S. muticum with three limpet species and other habitats was assessed using intertidal surveys on rocky shores and snorkelling at two seagrass sites in the UK. A 4-year field study tested the effect of S. muticum on Z. marina shoot density, dry weight and phenolic compounds (caffeic and tannic acid) content, and a laboratory experiment tested the impact of S. muticum on nutrient partitioning (C/H/N/P/Si), photosynthetic efficiency (F v/ F m) and growth of Z. marina. Results On rocky shores 15 % of S. muticum occurrences were attached to the shells of live limpets. In seagrass beds 5 % of S. muticum occurrences were attached to the shells of dead limpets. The remainder were attached to rock, to cobblestones, to the seagrass matrix or embedded within the sand. Z. marina density and phenolics content was lower when S. muticum co-occurred with it. Over 3 years, photosynthetic responses of Z. marina to S. muticum were idiosyncratic, and S. muticum had no effect on nutrient partitioning in Z. marina. Conclusions Our results show limpets support S. muticum as an epibiont and may act as a previously unreported transport mechanism introducing invaders into sensitive habitats. S. muticum reduced production of phenolics in Z. marina , which may weaken its defensive capabilities and facilitate proliferation of S. muticum. The effect of S. muticum on Z. marina photosynthesis requires further work but having no effect on the capacity of Z. marina to sequester nutrients suggests a degree of resilience to this invader. [ABSTRACT FROM AUTHOR]