Bioturbating megafauna limits the expansion of Cymodocea nodosa seagrass meadows

In seagrass systems, there is a realization that biotic interactions affecting sediment stability might be as important as abiotic stressors in shaping the growth and distribution patterns of seagrass vegetation. However, few studies have looked at the interaction between seagrass and large (mega) bioturbating species, although through sediment reworking activity they may have the capacity to cause large scale impact on seagrass establishment. We assessed the effect of bioturbation by mobile megabioturbators (species lager than 5 cm) on subtidal seagrass (Cymodocea nodosa) using a manipulative field experiment where access by megabioturbators was restricted for 4 months during the growth season. There was a negative effect of megabioturbators on both the biomass and density of young seagrass ramets extending outwards from a mature seagrass meadow, but megabioturbators did not have a notable effect on seagrass growth in the meadows. Interestingly, the exclusion of megabioturbators in all tested habitats significantly modified the infaunal community composition, indicating the presence of complexity in biotic interactions and the possibility of indirect effects caused by the megabioturbators. Overall, megabioturbators have direct and indirect impact in seagrass habitats, playing a significant role in seagrass distribution patterns.