Bottom boundary layer dynamics and its effects on particle resuspension and transport in the NW Iberian coastal upwelling system

Abstract

Coastal regions are one of the most biogeochemically active areas of the biosphere, covering only < 15% of the total ocean surface, they account for approximately 23-30% of the total primary production. Intense gradients of energy and matter occur in these coastal regions as they are natural boundaries among terrestrial, oceanic and atmospheric systems. In fact, one of the intrinsic characteristics of coastal regions, in contrast to oceanic systems is the interaction/proximity with the seafloor sediments. In fact, in these coastal regions, the so-called bottom boundary layer (BBL), that water column portion directly affected by sediment – water column interface, plays a key role in the dynamics and biogeochemistry of these ecosystems. The main objective of this PhD Thesis is to study the temporal and spatial variability of the BBL in the NW Iberian coastal margin. With this aim, wave time series and current measurements from ADCPs, moored at hydrodynamically different sites in the Ría de Vigo and adjacent inner continental shelf, have been analyzed. The role of currents and waves on the BBL hydrodynamically behavior, and ultimately on particle dynamics at the near bottom water column- sediment interface has been also evaluated. This PhD Thesis evidences that there is a high temporal variability of BBL in the Ría de Vigo interior responded to different forcing factors that determined the magnitude of the bottom shear stress. In general terms, the pattern of the bottom shear stress was modulated by tides. However, regarding to the short time scales, the most important total shear stress pulses corresponded to the effect of currents and waves during the downwelling season. This dissertation also shows that there is a marked spatial variability in agreement with the spatial gradient of the hydrodynamic conditions derived from both currents and waves. The most hydrodynamically intense BBLs occurred in the outer positions along the northern margin where energy derived from both currents and waves was higher. High levels of bottom shear stress in the NW Iberian inner continental shelf occurred mainly during the downwelling season, when there was a coupling between wave storms and current intensification effects, promoting high resuspension events and thus generation of bottom boundary layers (BNLs). In fact, well developed BNLs confined over the continental shelf were registered during this season, pointing to the absence of across shore export of resuspended material to the adjacent ocean. This confinement is due to the existence of a front generated by the interaction of the Iberian Poleward Current (IPC) and the West Iberian Buoyant Plume (WIBP). This PhD Thesis confirms the spatial variability of the wave orbital velocity as a primary shorting factor controlling the sediment distribution along this margin, and explains the position of the Galician mud depocenter