Detailed velocity underwater measurements obtained inside five coral reefs

The flow of water through a coral reef creates a turbulent flow field which controls the mass, momentum and energy transport between the reef and its surroundings. Biological functions such as feeding, respiration and reproduction depend on this transport phenomenon, and yet, most coral reef flow studies treat the flow using depth averaged variables, avoiding the need to present the spatial flow variations within the reef. To overcome this limitation we present a combination of detailed laboratory and field measurements of turbulent flow properties within reefs composed of two different types of branching corals in three different, naturally common, spatial configurations. Together with detailed geometrical mapping of coral skeletons using CT scanning, our laboratory measurements suggest the existence of an additional source of mixing deep within the reef that is unique to the coral canopy geometry. Using a custom-built submersible Particle Image Velocimetry (PIV) system, our field measurements demonstrate the effect of colony type and spatial arrangement on mean flow properties, estimated particle retention time and distribution of large coherent structures within the reef under natural flow conditions.