Aerial image of Imperial Beach 2009 dye study with important regions labeled

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.


Photo of the Tijuana River plume in Imperial Beach, CA courtesy of WILDCOAST. Brown colors show sediment laden water exiting from the Tijuana Estuary mouth. Sediment is just one example of the type of tracer our study can apply to.

This study will improve the understanding of the physical processes governing tracer exchange between the surfzone and the stratified inner-shelf through the analysis of field observations and model results. By closing a tracer mass budget for the dye (i.e., accounting for all of the dye as it spreads), dye fate can be accurately determined. Dye observations and measurements will determine the extent to which physical processes move dye from a region of high concentration to a region of low concentration. The spreading and mixing of the dye laterally (lateral diffusion) has previously been believed to be smaller over the inner-shelf than in the surfzone (even suggesting that the inner-shelf is a barrier to exchange), yet we hypothesize it may be comparable to or greater than in the surfzone due to large-scale coherent physical structures such as rip-currents and eddies such as those seen in the above images. Shoreline dye observations far downstream and inner-shelf cross-shore dye flux measurements will determine the extent to which dilution mechanisms match traditional estimates (e.g., Fickian diffusion). In addition to investigating this lateral dispersion, we will also investigate the vertical mixing of dye on the stratified inner-shelf and the potential importance of surfzone ejections of warm water using dye observations throughout the water column depth. Additional hypothesis-driven questions will be addressed using numerical simulations with varying stratification, wind, and rip current conditions. Through a combination of techniques (observations and models) used during this study, a more complete picture of what is working to spread tracers in our coastal ecosystem can be obtained.

See some more details specific to our methods and the dye releases we have conducted on our FAQs!