Larval cross‐shore transport estimated from internal waves with a background flow: The effects of larval vertical position and depth regulation

Abstract

Cross-shore velocities in the coastal ocean typically vary with depth. The direction and magnitude of transport experienced by meroplanktonic larvae will therefore be influenced by their vertical position. To quantify how swimming behavior and vertical position in internal waves influence larval cross-shore transport in the shallow ($ 20 m), stratified coastal waters off Southern California, we deployed swarms of novel, subsurface larval mimics, the Mini-Autonomous Underwater Explorers (M-AUEs). The M-AUEs were programmed to maintain a specified depth, and were deployed near a mooring. Transport of the M-AUEs was predominantly onshore, with average velocities up to 14 cm s −1. To put the M-AUE deployments into a broader context, we simulated > 500 individual high-frequency internal waves observed at the mooring over a 14-d deployment; in each internal wave, we released both depth-keeping and passive virtual larvae every meter in the vertical. After the waves' passage, depth-keeping virtual larvae were usually found closer to shore than passive larvae released at the same depth. Near the top of the water column (3-5-m depth), $ 20% of internal waves enhanced onshore transport of depth-keeping virtual larvae by ≥ 50 m, whereas only 1% of waves gave similar enhancements to passive larvae. Our observations and simulations showed that depth-keeping behavior in high-frequency internal waves resulted in enhanced onshore transport at the top of the water column, and reduced offshore dispersal at the bottom, compared to being passive. Thus, even weak depth-keeping may allow larvae to reach nearshore adult habitats more reliably than drifting passively.

Larval transport pathways from three prominent sand lance habitats in the Gulf of Maine

“…Numerous studies have shown that the inclusion of larval behaviors alter dispersal pathways (Faillettaz et al, 2018; Levin, 2006; Rypina et al, 2014). This is most notable for larvae that exhibit vertical movements in areas with high vertical velocity shear (Garwood et al, 2021; Tapia & Pineda, 2007). The Gulf of Maine in winter is well mixed and thus vertical velocity shears are low on banks for much of the larval duration of sand lance (Mountain & Manning, 1994), though waters begin to stratify in the spring and thus vertical gradients in velocity begin to increase as sand lance larvae approach settlement (Mountain & Manning, 1994; Naimie et al, 1994).…”

Influence of Kelp Forest Biomass on Nearshore Currents

“…To properly represent water particle motions, the velocity used at each time step should be that appropriate to the particle's location at any time. Observationally, this would mean that either one deploys many instruments (as did Gaylord et al., 2007), or one should track Lagrangian particle displacements with neutrally buoyant drifters (e.g., Garwood et al., 2021). Nonetheless, we argue that our estimates of $$\zeta $$ and T r based on data from a single ADP are sufficient to show the effects of the presence or absence of kelp.…”