Award: OCE-0927196

Scientific Merit Many intertidal invertebrates and fishes have complex life cycles that include a planktonic larval phase. The pelagic phase is often weeks long, and larval development occurs in waters over the continental shelf. At the end of their development, larvae must return to shore and cross the surf zone. On steep shores (reflective), water in the surf zone may be slowly exchanged with the inner shelf and the surf zone may be a barrier to shoreward migration. On gently sloping shores (dissipative), water in the surf zone may be flushed permitting rapid transport across the surf zone. Hence, the type of surf zone and its hydrodynamics may strongly affect larval delivery to shore. Our goal was to determine how larval behavior interacts with variation in surfzone hydrodynamics affecting the ability of larvae to cross the surf zone and replenish adult populations. In the first two years of the project, we conducted intensive one-month investigations of the physical and biological processes at dissipative and reflective surf zones, respectively. At each site we sampled the plankton community daily on the inner-shelf, just outside the surf zone, and within the surf zone while measuring settlement on plates in the intertidal zone. At the same time we collected physical oceanographic data with instruments, dye and GPS-equipped surface drifters to describe the hydrodynamics of the surf zone. We modeled the physical and behavioral processes regulating larval delivery to shore to simulate the hydrodynamic processes responsible for the transport of larvae. During both years, we also measured weekly barnacle recruitment and daily settlement of barnacles for two months at other reflective and dissipative beaches in central California and southern Oregon. During the third year, we surveyed new recruits and population densities of barnacles at many beaches along the West Coast to determine if a latitudinal gradient in wave energy (greater in the north) increases recruitment. We found that surf zone hydrodynamics is a key determinant of spatial variation in marine communities. Opposite cross-shore distributions of plankton occurred at the two beach types: all plankters were more abundant inside the surf zone at the dissipative beach and were more abundant outside the surf zone at the reflective beach, clearly indicating that spatial variation in surf zone hydrodynamics determines the ability of plankters to enter and remain in the surf zone. Water exchange was far less at the reflective beach limiting onshore transport. The postlarval stage of barnacles and two bottom-dwelling zooplankters (copepods and parasitic isopods) as well as passively sinking detritus were able to enter the surf zone at the reflective beach, suggesting that only plankters that stay near the bottom are able to cross the surf zone albeit in limited numbers. Water flowed seaward except at the bottom where it flowed landward, enabling larvae to recruit onshore. The process (benthic streaming) appears to be disrupted when waves were large reducing the ability of larvae to cross the surf zone. Modeling confirmed that sinking facilitates cross the surf zone at the reflective beach, although waves (Stokes drift) also may transport plankton at the surface onshore. Modeling also revealed that sinking and waves plays a role in transporting plankton onshore at the dissipative beach. Our latitudinal survey indicated that limited larval recruitment in California may result from changes in surf zone dynamics along the coast. Temporal variation in physical conditions also affected concentrations of plankton in the surf zone at the reflective beach. Phytoplankton increased in the surf zone when prevailing northwesterly winds (upwelling favorable) weakened and large waves increased water exchange. In contrast, barnacles settled biweekly in cold bottom waters indicating that internal tides may have transported them onshore. Thus, spatial and temporal variation in physical p...