The Role of Fauna in Seagrass Habitats (original) (raw)

Biotic interactions influence ecosystems directly, through processes such as consumption and disturbance, and indirectly, by modifying environmental conditions or resources. In coastal marine systems, biotic interactions can, alongside abiotic factors, shape the growth and distribution of seagrass. Large (mega) bioturbating species, through their sediment reworking activity, are able to cause large scale impact on seagrass expansion into adjacent sand. We assessed the extent of this impact caused by mobile megabioturbators in two seagrass Cymodocea nodosa meadows of the Canary Islands, through: (1) a manipulative field experiment where access by megabioturbators to the middle and edges of seagrass beds, and adjacent sand was restricted for 4 months during the growth season; and (2) unattended video cameras where the presence and interactions of megafauna were recorded, in each of the three habitats. With the exception of herbivorous fish, which were more abundant in seagrass, other trophic groups of fish did not differ in their occupation of the three habitats (edge, middle, sand). By contrast, the results of the manipulative field experiment demonstrated a negative effect of megabioturbators on both the density and biomass of young seagrass ramets in the sand, just outside a mature seagrass meadow, but not on seagrass biomass in either edge or middle habitats of seagrass meadows. Interestingly, the exclusion of megabioturbators significantly modified the infaunal community composition, indicating the potential for indirect effects of the megabioturbators if the infaunal community composition in turn influences seagrass growth. Overall, these results indicate that megabioturbators play a direct and, possibly also indirect role in shaping the growth and patch expansion of seagrass at shallow depths. The failure of megabioturbators to modify the biomass of established seagrass also indicates an important role of selffacilitation in stabilizing seagrass.