Wave-dominated flow interactions with coral topography and its impact on larval settlement

Many benthic larvae rely on ambient flow for dispersal and transport to suitable settlement sites. After reaching benthic surfaces, larvae must overcome flow forces which act to dislodge them in order to undergo 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, which mimic the surface roughness of corals. Near-surface hydrodynamic parameters, including velocities, turbulence statistics and wall shear stresses were computed, along with forces on simulated larvae settled on surfaces. Results found that a larva experiences 16 times greater mean force when settled on top of a roughness element rather than between roughness elements. In addition,maximum forces were 5-9 times greater in wave conditions compared to unidirectional flows. It was also found that wide spaced roughness on a surface (characterized as a k-type roughness) yielded 2-4 times higher maximum forces than tightly spaced roughness (characterized as d-type).In k-type topographies, eddies are shed into the flow from the roughness elements’ crests while in d-type flow behavior, the vortices stably remain within the grooves of the roughness elements.Larvae have a higher probability of settlement when in less wavy environments and sheltered within coral crevices of d-type roughness.