Observations of spatial variability in flow over coral reefs

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 flow patterns at 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. Flow around individual colonies had similar characteristics to flows around hemispheres in laboratory flumes. We observed recirculation zones immediately behind colonies, reduced currents and elevated dissipation rates in turbulent wakes, 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; wakes were more turbulent and recirculation zones stronger in faster flows. Low frequency (<0.03 Hz) flow variations were reduced downstream of colonies and intermediate frequency (0.05-0.1 Hz) variations were similar upstream and downstream, consistent with a transition from advective to local acceleration dominated behavior at Keulegan-Carpenter number of O(1). This coincided with a transition from inertial force exceeding drag for f>0.03 Hz. Flow variations at frequencies >0.1 Hz were turbulence and were greatest immediately downstream of the colony. Since flow on reef flats is typically driven by remote forcing, small scale flow structures can also vary at seasonal time-scales. Collectively, our results indicate spatial flow variability at colony scales can exceed those at larger scales. Small-scale coherent flow structures may have implications for microhabitats, localized trapping of particulate materials, and increased horizontal dispersion.