Surface topography drives marine larval settlement through boundary layer flow interactions

As invertebrate larvae approach marine surfaces, their motion is affected by interactions with the benthic boundary layer flow and surface topography. We performed 2D agent-based simulations of larval transport to investigate these effects on the movement and settlement of larvae. Surface features on the millimeter scale were examined using a ridged substrate as a model topography and by systematically varying the height and spacing of the ridges. Our simulations show that certain substrate topographies modify the boundary layer flow by creating vortices and ejecting them into the bulk flow. These flow structures increase larval transport towards the surface and enhance settlement. Interestingly, the optimal settlement substrates produced the highest averaged turbulent kinetic energy (TKE), indicating that passive larval transport due to recirculatory flow structures is a critical factor in settlement. This result is supported by simulated larval concentration profiles along the water column, which reveal clear differences in the efficiency of larval transport to the substrate arising from the different flow structures. Our findings thus demonstrate that TKE can be used as a predictor of larval settlement to inform substrate design.