Numerical modeling of onshore plankton transport

Cross-shore exchange of plankton plays an important role in marine ecosystems and coastal communities. Larvae of many intertidal invertebrate species grow offshore and come back to the shore for settlement by crossing the energetic surf zone: however, shoreward transport mechanisms are not well understood. To test the possible onshore transport mechanisms, numerical simulations are performed based on the field campaigns conducted at Sand City beach (a mildly sloping rip-channeled beach) and Carmel River State Beach (CRSB, a steep pocket beach). The model consists of the 3D hydrodynamic model Delft3D and an individual based model for larval tracking. As weak swimmers, many invertebrate larvae are greatly controlled by currents, but biological factors such as larval buoyancy and turbulent-dependent sinking behavior also play important roles. Depending on the vertical positions of larvae, wave-induced bottom boundary layer streaming or wind-driven surface currents carry larvae toward the shore. Stokes drift is necessary to achieve onshore larval migration for all cases. In addition, beach morphology and coastal configurations affect larval distribution patterns and their concentrations. Model results show that onshore larval delivery rate is higher at a mild sloping beach than at a reflective beach, consistent with literature and the field observations along the California coast. Furthermore, alongshore variability is also important for larval migration toward the shore. Rip channels induce rip currents as well as feeder currents (i.e., shoaling), and enhance cross-shore exchange. Wave group also acts as additional forcing to the cross-shore material exchange by producing transient rip currents. This model study confirms that all physical, biological, and geological regimes determine onshore larval transport patterns.