Influence of an upwelling filament on the distribution of labile fraction of dissolved Zn, Cd and Pb off Cape São Vicente, SW Iberia

Abstract

Under northerly winds upwelling is recurrent at the Cape São Vicente, SW Iberia, and plays a major role on the distribution of dissolved nutrients and metals. The aim of this work was to characterize the dissolved metals distribution of zinc (Zn), cadmium (Cd) and lead (Pb), associated with a filament of upwelled water that stretches seaward from the Cape. Additionally, the relationships between labile metals and other oceanographic parameters, such as current velocity and wind field patterns, temperature and salinity, nutrients, chlorophyll a and suspended solids were evaluated. The mass transport of the dissolved metals exported offshore was estimated, after a period of relatively strong and persistent upwelling. At the end of October 2004 a total of 42 CTD Rosette casts up to 400 dbar were sampled, distributed on an almost regular grid, together with along-track Acoustic Doppler Current Profile (ADCP) velocities. Seawater samples from two transects across the filament were analysed: one closest to the shore, where upwelling was intense and phytoplankton noticeably grew; and another further offshore where the filament was still well defined, but narrower and less marked despite with the maximum velocity currents. Labile dissolved metals were determined using anodic stripping voltammetry (ASV). The range of the metals recorded at the transect closest to the coast recorded was 0.26–3.8 nM (mean: 0.8 nM) for Zn, 2–11 pM (mean: 3 pM) for Cd and 8–60 pM (mean: 13 pM) while for the offshore transect was: 0.26–5.1 nM (mean: 1.2 nM) for Zn, 2–26 pM (mean 4 pM) for Cd and 8–74 pM (mean: 15 pM). Zinc recorded the highest concentrations, similar at both transects, and like Cd the lowest concentrations were found at near-surface depths. In opposition, the highest Pb concentrations were found at the near-surface depths at the northern stations in both transects. The filament exported more material in the offshore transect than in the transect closest to the coast, corresponding to a maximum export of ~135 kmol d−1 of Zn, 276 mol d−1 of Cd and ~1365 mol d−1 of Pb. The quantification of the cross-shelf fluxes imposed by the filament did show that metals fluxes are strong enough to play a key role in the oceanographic behaviour of the transition zone between the coastal and offshore waters in the region. Considering the periods of strong upwelling events and the extent of their duration along the year, the amounts of exported water mass which include nutrients, metals and particles must be hugely increased and responsible for the high productivity of the waters.