Patterns and drivers of understory macroalgal assemblage structure within subtidal kelp forests

Kelp species are found along ~ 25% of the world’s coastlines, where they provide myriad of ecological goods and services. However, compared with many other terrestrial and marine vegetated ecosystems, kelp forests have been critically understudied in many regions, leading to pressing knowledge gaps that hinder management and conservation efforts. We conducted a large-scale survey of understorey macroalgae within subtidal kelp forests dominated by Laminaria hyperborea at 12 sites, nested within 4 regions in the United Kingdom (UK). Regions spanned ~ 9° in latitude and encompassed a gradient in average sea surface temperature of ~ 2.5 °C. We employed a combination of traditional hand-harvesting of quadrat samples and a novel 3D photogrammetry technique to quantify crustose coralline algae. The structure of understorey macroalgal assemblages was highly variable but showed clear partitioning at the spatial scales of both regions and sites. At the regional-scale, we observed a general increase in richness, diversity and biomass from north to south, most likely due to biogeographical context, ocean climate and the structure and composition of overlying kelp canopies. Site level variation was most likely driven by concurrent variability in wave exposure and kelp canopy structure. Our study shows that understorey macroalgal assemblages represent a rich and abundant component of kelp forests in the northeast Atlantic, with high biodiversity value that warrant conservation measures. As these kelp forest ecosystems are structured by multiple physical and biological processes, current and predicted environmental change will likely alter the diversity and composition of understorey macroalgal assemblages.