  The effect of substrate topography on the settlement of coral larvae in
wave-driven oscillatory flow is investigated using computational fluid dynamics
coupled to a 2D agent-based simulation of individual larvae. Substrate
topography modifies the boundary layer flow by generating vortices within
roughness features that can be ejected into the bulk flow, directly influencing
larval transport and settlement. In agreement with recent experimental
findings, millimeter-scale ridged topographies were found to increase
settlement compared to sub-mm feature heights. At this length scale, ridge
spacing-to-height ratios of 10 to 20, spacings of more than 30 coral larval
body lengths, resulted in the highest settlement rates. These optimal
topographies produce a high averaged vertical velocity variance in the bulk
flow, indicating that vertical larval movement to benthic surfaces is dominated
by passive transport driven by recirculatory flow structures. Indeed, larval
settlement was found to be positively correlated with mean vertical velocity
variance, and settlement results with substrates comprising complex multiscale
roughness were quantitatively similar to those with a simple rectangular model.
Our findings reveal how substrates can be designed with surface features to
promote larval settlement in natural flow conditions above shallow coral reefs
independent of biological cues and substrate material composition.
