Spatial and temporal variation in the diversity and structure of understorey macrofaunal assemblages within Laminaria hyperborea forests in the northeast Atlantic

Kelp species function as foundation organisms along almost one third of the world's coastlines, where they underpin diverse and productive forest habitats. The three-dimensional structure of the forest modifies local environmental conditions, which in some regions allows rich understorey macroalgal and macrofaunal assemblages to develop. These understorey assemblages, however, represent somewhat overlooked components of kelp forest ecosystems, with fundamental information on their structure and the processes that shape them often lacking. Here, we used an underwater suction sampler to characterize understorey macroalgal and macrofaunal assemblages within Laminaria hyperborea forests across the mid northeast Atlantic (United Kingdom). Specifically, we investigated spatial and temporal variation in the structure, abundance, diversity, and evenness of assemblages at twelve sites nested in four regions of the UK distributed along a 9 degrees latitudinal gradient that encompassed a similar to 2.5 degrees C gradient in mean sea surface temperature. Collectively, the samples comprised of 2.4 kg of understorey macroalgae from five functional groups, and 65,323 macrofaunal individuals from 179 taxa, demonstrating that understorey assemblages represent important repositories of biodiversity within coastal marine systems. Univariate assemblage metrics exhibited significant site-level variation, while the multivariate structure of assemblages exhibited both site and regional-level variation, and all metrics exhibited significant temporal variation. The observed variation was attributed in part to complex interactions between sea surface temperature, wave exposure and habitat structure, while the high site-level variation suggests that unquantified local-scale processes are also important influencers of understorey assemblages. Given that kelp forests are becoming increasingly threatened by multiple stressors, our results provide important evidence to support enhanced management of kelp forests, as well as baseline information that can serve as a benchmark to monitor, restore, and ultimately conserve these forests and their associated ecosystem services.