Geochemical vs. microbial approach to the new production of the coastal upwelling system of the Ría de Vigo (NW Spain)

Abstract

The fate of the inorganic and organic N trapped in the coastal upwelling system of Ría de Vigo (NW Spain), accumulation/export versus production/ consumption, was studied at the short time–scale (2–4 d) during July 2002. A transient geochemical box model was applied to the measured residual currents and concentrations of inorganic (NT), dissolved (DON) and particulate (PON) organic N to obtain the i) net balance of inputs minus outputs (i – o); ii) the net accumulation (V·dN/dt); and iii) the net ecosystem production (NEP) of NT, DON and PON. The average NEP during July (107 mg N m-2 d-1) indicates an autotrophic metabolism of the ría. About 25% of this material was exported to the shelf and the remaining 75% was transferred to the sediments or promoted to higher trophic levels. Measurements of oxygen production (Pg) and respiration (R) were performed in a single site twice a week at five depths. In addition, microzooplankton grazing and sedimentation rates were measured for first time in the Ría de Vigo. The high grazing rates observed reduces the efficiency of the ría to transfer organic matter directly from phytoplankton to the metazoans Comparison of the metabolic state of the Ría de Vigo derived from these in vitro measurements (Pg, R, grazing and sedimentation) and the in situ geochemical budget shows that they agree in 2 of the 3 study cases. Both methods are complementary and their simultaneous application allows obtaining a better knowledge of coastal upwelling ecosystems functioning