The influence of subtidal Laminaria canopies on local environmental conditions and the structure of understorey communities.

Understanding the role of species interactions (e.g. competition and facilitation) in structuring communities is a fundamental goal of ecology. It is well established that large canopy-forming seaweeds (e.g. kelps and fucoids) exert a strong influence on community structure, by offering biogenic habitat, altering environmental conditions and interacting with other species. While many studies have manipulated the density of seaweeds to causatively examine their effects on the local environment and associated communities, they are biased towards certain regions and canopy-forming species. We conducted a manipulative experiment at two subtidal sites characterised by mixed Laminaria canopies, in southwest England, UK. Three treatments were established in multiple replicate circular plots (area of 7.1 m): 0% kelp removal (i.e. unmanipulated control), 50% kelp removal (i.e. thinning), and 100% removal. Within each plot, temperature, light levels and sedimentation rates were monitored over 3 months, and after 5 months the density of juvenile kelp recruits, the biomass of understorey macroalgae and the abundance of recruiting fauna were quantified. We found kelp canopies had no impact on temperature or sedimentation rates but their removal led to marked increases in light availability, juvenile kelp recruitment and understorey macroalgal biomass. Overall, our study shows that physical disturbance to Laminaria canopies alters resource availability (i.e. light and space), leading to increased abundances of kelp recruits and understory algae. Significant reductions in kelp canopy density, driven by storm disturbance, harvesting or decreased water quality for example, would lead to shifts in community structure and ecological functioning.