Hydrodynamic interactions with coral topography and its impact on larval settlement

Abstract

Many benthic larvae rely on ambient flow and turbulence for their dispersal to settlement sites. After reaching the seafloor, larvae must prevent predation as well as overcome flow forces which act to dislodge them in order to achieve successful settlement. Most oceanic benthic habitats are topographically complex and are characterized by a combination of tidally driven currents and wave-driven oscillatory flows, which can exert substantial forces along settlement surfaces. In this study, computational fluid dynamics was used to numerically model both wave-dominated and unidirectional flows over surfaces of varying topography designed to mimic the surface roughness of corals. Near-surface hydrodynamic parameters, including velocities, turbulence statistics, and surface shear stresses, were computed, along with forces on simulated larvae settled on surfaces. It was found that widely spaced surface roughness (characterized as a k-type roughness with roughness width/height > 1) yielded 50–100% higher surface shear stress than tightly spaced roughness (characterized as d-type with width/height < 1), owing to eddy vortex ejection from k-type topographies. Results also found that a larva experiences up to two orders of magnitude greater mean fluid forces when settled on top of a roughness element rather than between roughness elements. Maximum drag and lift forces on a larva were two to three times greater in wave conditions compared to unidirectional flows. Our findings demonstrate that larvae have a higher probability to remain attached to surfaces when in less wavy environments and sheltered within coral crevices of d-type roughness.