Flow-topography interactions, particle transport and plankton dynamics at the Flower Garden Banks: a modeling study

Flow disruption resulting from interactions between currents and abrupt topography can have important consequences for biological processes in the ocean. A highresolution three-dimensional hydrodynamic model is used to study topographically influenced flow at the Flower Garden Banks, two small but thriving coral reef ecosystems in the northwest Gulf of Mexico. Flow past the modeled banks is characterized by vortex shedding, turbulent wake formation and strong return velocities in the near-wake regions. The speed of the oncoming current, strength of water-column stratification, and level of topographic detail used in the model each serve to modulate these basic flow characteristics. Larval retention and dispersal processes at the Flower Garden Banks, and specifically the dependence of these processes on the nature of flow disruption, are explored by coupling a Lagrangian particle-tracking algorithm to the hydrodynamic model. Passive particles released from the tops of the modeled banks as mimics of coral larvae can remain trapped in the wake regions very close to the banks on time scales of hours to days, depending primarily on the speed of the free-stream current. Most particles are swept quickly downstream, however, where their trajectories are most strongly influenced by the topography of the continental shelf. Modeled dispersal patterns suggest that there is an ample supply of larvae from the Flower Garden Banks to nearby oil and gas platforms, which can provide suitable benthic habitat for corals. The flow disturbances generated by the modeled banks result in the mixing of nutrients from deeper water into shallower, nutrient-depleted layers in the wakes of the banks. The ability of the planktonic system to respond to such an injection of nutrients is tested by embedding a simple nutrient-phytoplankton-zooplankton ecosystem model into the hydrodynamic model. Plankton biomass in the flow-disturbed wakes is shown to increase in response to the additional nutrients. This study shows how flow-topography interactions at the Flower Garden Banks can exert critical control over local larval transport processes and plankton dynamics. More generally, it demonstrates the usefulness and feasibility of using numerical models as tools to uncover important mechanisms of physical-biological interaction in the ocean.