Three-Way Interaction Between Larval Swimming Behavior, Internal Waves, and the Mean Flow Enhances Cross-Shore Transport

In vertically sheared cross-shore currents, larval dispersion and retention are closely linked to vertical position and swimming behavior. As they propagate, internal waves will deform the vertical structure of local horizontal velocities and modulate the cross-shore transport experienced by depth-regulating larvae. During ocean experiments, a swarm of subsurface, robotic larval mimics, the Mini-Autonomous Underwater Explorers, recorded a sudden net onshore transport of 30-70 m over 15-20 min. This sudden transport was associated with a weakly nonlinear internal wave that propagated through the swarm, deformed surface-intensified velocities downward, and accelerated the mimics shoreward. Models suggest that larval swimming speeds on the order of 1 mm/s were sufficient to double the larval cross-shore transport associated with this wave, compared to passive organisms. Combining nonlinear wave theory and vertical swimming reproduced observations well; larval cross-shore transport estimates spanning the entire water column were then generated for >500 high-frequency, weakly nonlinear internal waves isolated from a 14-day, high-frequency (2 Hz) mooring time series. Overall, depth-keeping promoted onshore transport and/or retention at all depths. This was especially true closer to the ocean surface, where nearly 20% of weakly nonlinear internal waves increased onshore transport of depth-keeping organisms by ≥50 m, compared to only 1% of waves for passive organisms. Results show that depth-keeping in weakly nonlinear internal waves can induce significant larval cross-shore transport – comparable to that previously estimated for passive organisms in highly nonlinear internal waves.