| Boundary layer dynamics over coral reef communities control many biologically important processes, including food capture by benthic organisms, mass transfer of dissolved nutrients, larval dispersal and waste removal. At small scales, the shear at the benthic-water interface interacts with the morphology of coral to control mass exchange, while at large scales, the extreme roughness of the topography influences reef scale circulation patterns and can also control horizontal exchanges with the open ocean. In order to study these processes, I performed detailed laboratory measurements in a recirculating wave-current water flume, as well as a series of field measurements, conducted along a fringing type coral reef in the Gulf of Aqaba, Red Sea.; Laboratory experiments obtained fine scale measurements of turbulent shear stresses and rates of mixing and mass transfer over a reef canopy. Results show that wave action, interacting with the structure of the reef, has dramatic effects on intermittent shear and mixing events. Maximal shear and turbulence events are timed with flow separation of the boundary layer and the ejection of vortices into the flow. These dynamics induce large vertical fluxes of water from within the coral structure. Wave forcing thus increases mass flux from the reef 2--3 times versus comparable unidirectional currents.; In the field, values of the drag coefficient for the coral reef were three to five times greater than values over a sandy bottom, which was characterized by the canonical value of 0.003. Reynolds stresses, mean velocities, and the dissipation rate of turbulent kinetic energy were used to predict friction velocities from three independent methods. Estimates from all three measures agreed well, indicating that existing turbulent boundary layer flow theory can be applied to flow over the rough topography of coral reefs. Results also indicate that the rough bottom topography and reef variability dramatically affects both reef scale and local turbulence levels. Biological measurements show that this enhanced turbulence enables high exchange rates of dissolved and particulate matter to the reef to be sustained. Thus, hydrodynamics control a key mechanism for the health and sustainability of this important marine ecosystem. |
