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

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. Larval recruitment is an important element in the dynamics and structure of marine populations and communities. Larvae of many intertidal invertebrates cross the surf zone, develop in the open ocean, and migrate back to the shore at the end of the larval stage (Morgan et al. 2009c; Shanks and Shearman 2009). Most invertebrate larvae are slow swimmers that regulate depth and likely depend on currents and other physical forcing to transport them onshore for settlement (Queiroga and Blanton 2005). In upwelling regimes along the western margins of continents, a widely accepted hypothesis is that larvae of intertidal invertebrates are swept offshore during winddriven upwelling events (Connolly et al. 2001). However, recent studies conducted in northern California (Morgan et al. 2009b, c; Morgan and Fisher 2010) and southern Oregon (Shanks and Shearman 2009) revealed that the onshore transport of larvae of many invertebrates is not limited by upwelling. Additionally, larvae of most species were not carried far offshore by upwelling nor onshore by downwelling, but were found at all times within several kilometers of shore; competent larvae were abundant within a kilometer from shore during the summer (Shanks and Shearman 2009; Morgan and Fisher 2010). More importantly, onshore recruitment of these competent larvae was spatially and temporally variable, suggesting the hypothesis that the surf zone may represent a semipermeable barrier to cross-shore exchange (Rilov et al. 2008; Shanks et al. 2010). Local processes within the surf zone are important for the migration of the larvae of intertidal invertebrate; however, the mechanism of larval delivery across this barrier is not understood. There are a number of possible physical transport mechanisms that need to be considered. At a heterogeneous shore with alongshore-sandbars, shoals and rip channels produce rip currents, which can enhance cross-shore exchange (MacMahan et al. 2010). Onshore transport mainly occurs over shoals where wave breaking occurs, driving onshore flows that diverge toward the shore and subsequently feed strong offshore-directed rip currents. Rip-channeled beaches are common and observed around the world. This rip-channeled system of alongshore variability-induced exchange is generally found at intermediate (gradual beach slope) beaches and not at reflective (steep beach slope) beaches (Wright and Short 1984), which is consistent with observations by Shanks et al. (2010), who showed that recruitment was higher on mildly sloping beaches than on steep beaches. Stokes drift (Stokes 1847) is a time-averaged volume transport current in the direction of wave propagation, and it may slowly transport larvae toward shore. This mechanism may be active at dissipative beaches, but might not or only partially be supported at reflective beaches because Stokes drift is associated with progressive surface gravity waves only, and steep beaches reflect waves with a wide range of frequencies resulting in (partially) standing waves. Stokes drift is explained further in the Methods section. * Corresponding author: [email protected] Limnol. Oceanogr., 59(4), 2014, 1434–1447 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.4.1434