The impact of branched coral morphological variation on downstream wake development and its influence on recruitment

Mixing and transport are important for the survival and health of the reef and the organisms that inhabit it. Wake generation behind individual coral colonies plays an important role in facilitating mixing and transport with the reef. While all coral colonies interact with the surrounding flow and generate wakes, the complex geometry of branched coral colonies make them interesting specimens to study when examining wake generation and evolution. Any flow entering a branched coral colony encounters one branch after another developing various separation points and regions of flow reversal before it exits the colony forming a wake. Previous examination of the drag force experienced by such colonies has found it to be closer to that of a wiffle ball®. Thus, we examined and compared the changes in flow velocity and turbulent statistics in the wakes generated by branched coral colonies from three species, a sphere, and wiffle ball®. We show that the wakes produced by flow exiting and passing around branched coral colonies are influenced by not only overall colony geometry but also branch density and vary from those produced by the sphere, and wiffle ball®. While time-averaged velocity profiles between different species and the spherical objects are similar, further examination of instantaneous images highlights the impact of varying branch density. Both branch density and interstitial space influence the size and shape of flow recirculation regions found in the wakes. This in turn determines where larval recruitment occurs, with larvae trying to strike a balance protection from large forces and nutritional requirements.