Meeting Abstract
Metabolic processes like photosynthesis and the transfer of nutrients from the overlying water column to the interior of a coral colony are primarily controlled by the concentration gradients and velocity profiles around the coral. Numerous reef-scale studies have been performed to understand mass transport mechanisms in corals, but smaller scale flow dynamics within branching coral colonies has largely remained unexplored. Measurements have shown that the flow velocities in the interior of densely branched coral colonies can be reduced by up to 90%. In spite of this drastic reduction in flow magnitude, the polyps at the interior of these densely branched corals continue their biological activities normally, pointing to an unknown mechanism for preserving mass transport rates. In this study, we uncovered the mechanism for preserving mass transport rates through a single Pocillopora meandrina colony. We performed three-dimensional simulations of the flow field through the colony (obtained via CT scans of a P. meandrina skeleton) using the immersed boundary method for a realistic Reynolds number of 20,000. The computed flow fields in the interior of the colony are highly vortical because of vortex shedding from the colony’s branches, which facilitates mixing and mass transfer. We calculated the advection time scale throughout the interior of the colony in order to characterize the rate of mass transport there. Though average flow speeds were reduced by up to 64% in the interior of the colony, the advection time scale was roughly constant throughout the colony. Thus, the complex, branched geometry of the colony was shown to serve as a passive mass transport enhancement mechanism which compensates almost exactly for drastic velocity reductions in the coral’s interior.
