Numerical simulations of larval transport into a rip‐channeled surf zone

Abstract

Competent larvae of intertidal invertebrates have to migrate toward shore for settlement; however, their migration through the surf zone is not understood. We investigated larval transport mechanisms at a ripchanneled beach. Because tracking larvae in the surf zone is infeasible, we used a three-dimensional biophysical model to simulate the processes. The coupled model consists of a physical module for currents and waves, and a biological module for adding larval traits and behaviors as well as Stokes drift to Lagrangian particles. Model calculations were performed with and without onshore wind forcing. Without wind, wave-driven onshore streaming occurs in the bottom boundary layer outside the surf zone. With onshore wind, onshore currents occur near the surface. In the surf zone, offshore-directed rip currents and compensating onshore-directed currents over shoals are formed in both no-wind and wind cases. In the biological module, neutral, negative, and positive buoyant particles were released offshore. Additionally, particles either sank in the presence of turbulence or not. Two scenarios achieved successful onshore migration: Negatively buoyant larvae without wind forcing sink in the turbulent bottom boundary layer and are carried onshore by streaming; positively buoyant larvae drift toward shore in wind-driven surface currents to the surf zone, then sink in the turbulent surf zone and remain near the bottom while transported shoreward. In both cases, the larval concentration is highest in the rip channel, consistent with field data. This successful result is only obtained if turbulence-dependent sinking behavior and Stokes drift are included in the transport of larvae.

Variation in the abundance of Pseudo-nitzschia and domoic acid with surf zone type

“…We hypothesize that the very high concentrations of Pseudonitzschia in the rip current samples is due to the interaction of the rip current flow system with floating cells caught by bubbles. Results from a bio-physical model of this surf zone are consistent with this hypothesis (Fujimura et al, 2014).…”

“…The physical oceanographic results and models have been described in several previous papers (Fujimura et al, 2013(Fujimura et al, , 2014MacMahan et al, 2009;Reniers et al, 2009Reniers et al, , 2010Shanks et al, 2015). Here we compare Pseudo-nitzschia spp.…”

“…A person walking out into the surf zone collected samples from the shoals. The Sand City surf zone and especially the rip current flow regime have been extensively studied (Fujimura et al, 2013(Fujimura et al, , 2014MacMahan et al, 2009;Reniers et al, 2009Reniers et al, , 2010. At this site, rip currents are apparent as deeper channels oriented perpendicular to shore through which flows a strong current directed offshore.…”

“…Bathymetry, offshore waves, wind, tidal elevation and currents were measured throughout the field experiment. A detailed description of the physical oceanographic measurements and observations and a model of the hydrodynamics of this surf zone are reported in MacMahan et al (2009) andFujimura et al (2014), respectively.…”

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

“…Although numerical simulations were based on light winds, our companion study demonstrated that simulated larvae also enter surf zones during windy conditions [34]. Simulated larvae near the surface were transported towards the surf zone by surface flow generated by wind stress and wave transport (Stokes drift).…”