Observations of spatial flow patterns at the coral colony scale on a shallow reef flat.
Journal of Geophysical Research: Oceans
Although small-scale spatial flow variability can affect both larger-scale circulation patterns and biological processes on coral reefs, there are few direct measurements of spatial flow patterns across horizontal scales <100 m. Here, flow patterns on a shallow reef flat were measured at scales from a single colony to several adjacent colonies using an array of acoustic Doppler velocimeters on a diver-operated traverse. We observed recirculation zones immediately behind colonies, reduced currents and elevated dissipation rates in turbulent wakes up to 2 colony diameters downstream, and enhanced Reynolds stresses in shear layers around wake peripheries. Flow acceleration zones were observed above and between colonies. Coherent flow structures varied with incident flow speeds; recirculation zones were stronger and wakes were more turbulent in faster flows. Low frequency (<0.03 Hz) flow variations, for which water excursions were large compared with the colony diameters (Keulegan-Carpenter number, KC>1), had similar spatial patterns to wakes, while higher frequency variations (0.05-0.1 Hz, KC<1) had no observable spatial structure. On the reef flat, both drag and inertial forces exerted by coral colonies could have significant effects on flow, but within different frequency ranges; drag dominates for low frequency flow variations and inertial forces dominate for higher frequency variations, including the wave band.Our scaling analyses suggest that spatial flow patterns at colony- and patch-scales could have important implications for both physical and biological processes at larger reef scales through their effects on forces exerted on the flow, turbulent mixing, and dispersion.