Dynamics of the bottom boundary and nepheloid layers in a costal upwelling system (NW Iberia)

The dynamics of the bottom boundary layer (BBL) and their effects on the development of nepheloid layers (NLs) on a high-energy and upwelling-affected margin, the NW Iberian continental shelf, was studied by means of ADP currents, wave time series and across-shelf hydrographic surveys covering an entire annual cycle. The BBL hydrodynamics showed that high levels of bottom shear stress in the inner shelf occurred mainly during the downwelling seasons, when there was a coupling between wave storms and intense currents. This wave-current coupling, promoting strong resuspension events, favored the generation of bottom nepheloid layers (BNLs) that were limited to the inner continental shelf inshore of a thermohaline front generated by the interaction of the Iberian Poleward Current (IPC) and the West Iberian Buoyant Plume (WIBP). The front prevents the offshore export of resuspended particles during the downwelling season. Indeed, resuspended material is directly transported northward within an inshore coastal current. In contrast, during the upwelling season only thin BNLs were developed, and surface nepheloid layers (SNLs), principally composed by biogenic particulate material, were advected offshore in the Ekman layer often as part of the development of upwelling filaments, becoming the principal offshore export pathway.