Drivers of the fine-scale distribution of a canopy-forming seaweed at the southern edge of its range

There is growing emphasis on using fine-grained scales to identify the drivers of species' geographic range edges, which is essential for predicting the response of species to climate change. This is of particular relevance at the 'rear-edge' of species distributions, where higher spatial resolution may also help in the detection of potential refugia for conservation. The southern edge of the range of several canopy-forming algae falls in the NW Iberian Peninsula, where large embayments (rias), influenced by strong upwelling events, may act as contemporary climatic refugia for these key coastal organisms. We investigated the factors driving the fine-scale occupancy patterns of the seaweed Fucus serratus, employing a combination of transplant experiments and a fine-grain species distribution model (SDM). Our study revealed that habitat suitability for this species is restricted to particular sites within rias. Transplant experiments showed that germling survival was significantly reduced outside the distribution range. Grazing may limit the species distribution towards the outer sections of rias, where we found the highest densities of grazing gastropods, but not towards the innermost sections. Both winter salinity and autumn seawater temperature were important predictors in the SDM. Our model projections indicate the potential future extirpation of F. serratus in rias with an increase of 1.5 degrees C in maximum autumn temperature, below the predicted average warming of 2oC for this upwelling area by the end of the century under the SSP5-8.5 scenario. The results highlight the importance of the autumn season for the performance of this cold-temperate seaweed at the southern edge of its range.