Observations of spatial flow patterns at the coral colony scale on a shallow reef flat

Abstract

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.