The surfzone and inner-shelf are by far the most economically and ecologically important ocean regions, vital for recreation, food, and ecosystem services. Despite the importance of clean coastal waters to our economy and well-being, declining water quality threatens coastal ecosystem and human health worldwide. Healthy coasts are a significant priority to federal agencies, local government, and non-governmental organizations. This study will use dye release experiments to quantify the exchange between the inner-shelf and the surfzone, improving scientific understanding and thus allowing for accurate prediction of tracer (e.g., larvae, nutrients, pathogens) exchange from the surfzone through the inner-shelf in the San Diego South Bay. This region is a representative surfzone and inner-shelf system, allowing project results to be applied generally. This region also is home to economically valuable beaches, State Parks, a Marine Protected Area, and a National Estuarine Research Reserve, among other assets. Yet, it is often impacted by poor water quality. This study will provide local managers and citizens insight into regional tracer exchange. The investigators will train a PhD student and engage undergraduates through the Scripps Institution of Oceanography Research Experience for Undergraduates program. Additionally, they will host three SurfScience Teen Conferences for high-school students, develop collaborations with students at the underserved Kearny High School (San Diego, California), and participate in the annual Avanzamos conference which introduces young Latinas to ocean science.
During this study the analysis of observations and model results will improve the understanding of the physical processes governing tracer exchange between the surfzone and the stratified inner-shelf. By closing a tracer mass budget, dye fate can be accurately determined. Estimates of observed and modeled inner-shelf dye diffusivity will be used to test the extent to which the inner-shelf is a "material barrier". Inner-shelf lateral diffusivities (hypothesized to be smaller than the surfzone) may be comparable to or greater than surfzone diffusivities, as larger inner-shelf eddy length-scales may compensate for weaker eddy velocities. The source of stratified inner-shelf coherent structures with large length-scales (relative to surfzone generated rip currents) will be investigated. Shoreline dye observations far downstream and inner-shelf cross-shore dye flux measurements will determine the extent to which surfzone dilution mechanisms are or are not Fickian. The observations and model results will be used to determine how inner-shelf upper water column stratification is set by surfzone ejection of warm water. Lastly, the vertical mixing of tracer on the stratified inner-shelf will be diagnosed. Additional hypotheses driven modeling studies will be performed with varying stratification, wind, and rip current conditions
