Award: OCE-0926923

We tested the hypothesis that surfzone hydrodynamics affects larval and plankton subsidies to nearshore communities. Physics: Year 1. Cross-shore exchange between the surf zone and the inner shelf was investigated using Lagrangian and Eulerian field measurements of rip current flows on a rip-channeled beach in Sand City, California. Surface drifters released on the inner shelf during weak winds moved seaward due to rip current pulses, then returned shoreward in an arcing pattern, re-entering the surf zone over shoals. The cross-shore velocities of the seaward and shoreward moving drifters were approximately equal in magnitude and decreased as a function of distance offshore. The drifters carried seaward by the rip current had maximum cross-shore velocities as they exited the surf zone then decelerated as they moved offshore. The drifters moving shoreward accelerated as they approached the surfzone boundary with maximum cross-shore velocities as they re-entered the surf zone over shoals. It was found that Stokes drift was not solely responsible for the onshore transport across the surfzone boundary. This resulted in surface material being contained within the nearshore region. Year 2. Surf zone mixing and transport, and cross-shore exchange across the surfzone boundary on a sandy, steep, reflective beach at Carmel River State Beach, California, were described for a range of wave and alongshore flow conditions. Depth-limited wave breaking occurred close to the shore creating a narrow surf/swash zone (10 m wide). Fluorescent Rhodamine dye was released as a slug in the surf zone, and the temporal and spatial evolution was measured using an alongshore array of dye sensors and by people with dye sensors swimming repeated cross-shore transects. The measurements indicate two stages of mixing and transport occur inside the surf zone on the steep beach. 1) In the near-field (< 50 m downstream of the dye release location), visual observations indicate the dye fully mixed throughout the water column after a few incident waves then continued to disperse in two dimensions, with both advection and diffusion processes being important. The cross-shore spreading was attributed to turbulence generated by intense wave breaking, with a surfzone cross-shore diffusion coefficient of κx = 0.2 to 0.5 m2 s-1, and the alongshore spreading was attributed to shear in the alongshore current, with a surfzone alongshore diffusion coefficient of κy = 0.5 m2 s-1. Overall, a net offshore material transport does exist on the steep beach. Biology: Year 1. We studied cross-shore exchange between the inner shelf and surf zone at a more dissipative surf zone with rip currents. 1) Concentrations of coastal phytoplankton and zooplankton were one to several orders of magnitude higher in rip currents than over shoals between rips or seaward of the surf zone. 2) Overall surfzone phytoplankton concentrations (rip plus shoal concentrations) were similar to that seaward. 3) The surf zone did not act as a barrier to shoreward movement of phytoplankton or zooplankton. Year 2. We studied surfzone hydrodynamics at a more reflective shore characterized by a steep beach and very narrow surf zone. 1) Surfzone concentrations of phytoplankton and zooplankton were one or more orders of magnitude lower than in waters just beyond the breakers (20 to 125 m offshore) and they were even lower at rocky shores to either side of the beach. The surf zone clearly acted as a barrier, retarding shoreward movement of phytoplankton and zooplankton. We hypothesize that surfzone undertow holds the coastal plankton community offshore. 2) When waves were small, surfzone concentrations of detritus (tiny algal pieces) and competent larvae ready to settle were higher than in waters offshore. We hypothesize that these components of the plankton enter the surf zone near the bottom via near-bed streaming flow coupled with variations in the location of the breaker line due to wave groups. ...