Many intertidal invertebrates and fishes have complex life cycles that include a planktonic larval phase. At the end of their pelagic development, larvae must return to shore and cross the surf zone. The purpose of this study is to investigate for the first time the role of surf zone hydrodynamics in the rate of delivery of cyprids of intertidal barnacles to the shore. To exploit the greater physical oceanographic understanding of the hydrodynamics of sandy beach surf zones, this initial study will focus on cyprid settlement on hard substrates in surf zones associated with sandy beaches. In the first two years of the study, the investigators will carry out an intensive two-month physical and biological study of a reflective and dissipative surf zone, respectively. At each site they will sample cyprids in the waters of the inner-shelf, just outside the surf zone, and within the surf zone and they will measure settlement on plates in the intertidal zone. At the same time they will collect physical oceanographic data with both in-situ instruments and a fleet of GPS-equipped surface drifters to describe the hydrodynamics of the surf zone. The time series of the physical and biological data will be correlated to investigate mechanisms of delivery of cyprids to the shore. To simulate the hydrodynamic processes responsible for the transport of larvae, the investigators will use a 3D model, resolving both the horizontal and vertical structure of the unsteady nearshore flow. To evaluate potential transport of larvae through the surf zone, a biological module describing the spatial distribution of the larvae will be coupled to the hydrodynamic module to predict the pathways of the larvae and compare with observations. Intensive sampling will help provide insight into the actual processes transporting cyprids from the inner shelf, through the surf zone, and to the intertidal zone. During each summer, weekly barnacle recruitment and daily cyprid settlement will be measured for two months to settlement plates at reflective and dissipative beaches in central California and southern Oregon. Population densities at many beaches along the West Coast will be surveyed each year to determine if a latitudinal gradient in wave energy is correlated with adult barnacle population densities.
Because the fundamentals of surfzone dynamics are universal, results of this research will be broadly applicable not only along the West Coast, but worldwide. This project will have significant impacts on education and public outreach. It will support three graduate students and nine undergraduate students and will create new research opportunities for students of diverse backgrounds from three undergraduate institutions, local high schools and the public. The research will be included in the curriculum of intensive hands-on courses, and undergraduates will participate in the research while learning how a real-world research project addresses fundamental questions. Both a website that highlights findings and an interactive display for visitors to the Bodega Marine Laboratory will be developed. A model coupling nearshore hydrodynamics and onshore transport across the surf zone will be made available to the community to stimulate research into this emerging research topic.
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 processes affects concentrations of plankton communities in surf zones and the replenishment of intertidal communities. Broader impacts This study was the first to investigate the role of surf zone hydrodynamics on the delivery of larvae to the shore. Because the fundamentals of surfzone dynamics are universal, we anticipate that the results of our research will be broadly applicable worldwide. This project had significant impacts on education by engaging eight graduate students, four undergraduate students, two postgraduate researchers and two postgraduate volunteers. The proposed research will be included in the curriculum of intensive hands-on courses, learning how a real-world research project addresses fundamental questions. Thus far, our results have been presented at 13 scientific conferences, eight manuscripts are in the processes of being published and our model coupling nearshore hydrodynamics and onshore transport across the surf zone is available to the community to stimulate research into this emerging research topic. Thus, this collaborative study of ecological, behavioral and oceanographic approaches provided new information on larval transport and recruitment while having substantial impacts on education, research and outreach.
