Upwelling control of positive interactions over mesoscales: a new link between bottom-up and top-down processes on rocky shores

The dependence of positive species interactions on the traits of individuals has rarely been explored, particularly in marine communities. Therefore, we have little idea about their generality, scales of variability, or reliance on local conditions. Transplant experiments and surveys conducted at 14 sites spanning across similar to 900 km of the central Chilean coast demonstrate that alongshore variation in upwelling, occurring over 10s to 100s of kilometers, explains among-site differences in growth rates of the turf-forming alga Gelidium chilense, which drives predictable landscape patterns in turf height. At upwelling centers, turf algae grew faster and attained taller heights than at warmer downstream locations, where the algal turf remained short because it grew slower and was pruned-back in winter. Replicated, reciprocal transplants of tall and short algal turf, combined with manipulations that controlled predator access to turf, revealed that recruitment of the mussel Perumytilus purpuratus was differentially enhanced by the turf; benefits were strong when the turf was short, but disappeared when the turf was tall. By determining mussel prey supply to higher trophic levels, upwelling-controlled facilitation has important consequences for community regulation. This is the first study to identify upwelling as a physical mechanism generating environmental conditions that (1) control trait-dependent habitat modification and (2) regulate the relative importance of facilitation in marine rocky intertidal communities.