The roles of habitat heterogeneity in generating and maintaining biodiversity on continental margins

The continental margins, regarded here as the ribbon of sea floor from 200 to 4000 m water depth between the shelf and the abyss, offer some of the most variable terrain in the ocean. This relatively narrow zone, which accounts for about 15% of the seabed, exhibits extreme topographical heterogeneity, sharp environmental gradients, and tectonic activity that together create habitat for a vast assortment of biological communities. Canyons, vertical walls, banks, ridges, mounds, swales and gulleys criss-cross broad flat slopes. Some of these features receive exceptional inputs of floodwaters, macrophytic detritus, suspended organic matter and debris from massive river inflows and long-shore transport. Others are squeezed by subduction or jolted by tectonic activity, triggering turbidity flows and forcing the efflux of reduced fluids that fuel chemosynthetic (seep) ecosystems. Multiple water masses with distinct hydrographic characteristics flow over and across the margins, often stratifying the water column like a layer cake. On some margins, naturally hypoxic waters smother the seabed at mid-bathyal depths. Where flow, food, or chemical conditions are suitable, structure-forming biological assemblages thrive, generating additional heterogeneity. In combination, the margins comprise some of the most heterogeneous and dynamic real estate on the planet.