Physical–biological coupling in the coastal upwelling system of the Ría de Vigo (NW Spain). I: In situ approach

The fate of the inorganic and organic nitrogen trapped in the coastal upwelling system of Ría de Vigo (NW Spain) was studied at the 2 to 4 d time-scale during July and September 2002. A transient geochemical model was applied to the measured residual currents and concentrations of inorganic nitrogen (NT), and dissolved (DON) and particulate (PON) organic N to obtain (1) the net balance of inputs minus outputs (i – o); (2) the net accumulation; and (3) the net ecosystem production (NEP) of NT, DON and PON. Previously unaccounted lateral variations in residual currents and N species concentrations in the ría were considered. The ría was autotrophic during July (average NEP, 107 mg N m–2 d–1). 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. During summer upwelling episodes, 30 to 70% of the organic N exported to the shelf came from materials previously accumulated in the ría. By contrast, during summer relaxation, 60 to 70% of the accumulated organic N came from in situ conversion of NT and the remaining 30 to 40% was allochthonous. As shown by other authors, NEP was rather low during intense upwelling and increased to high values during its subsequent relaxation. In September, the metabolism changed from heterotrophic to slightly autotrophic (average NEP, 26 mg N m–2 d–1). The DON and PON imported from the shelf during autumn downwelling experienced a different fate: DON was consumed and PON accumulated in the ría. By contrast, DON and PON were produced in situ at the expense of N nutrients previously accumulated into the system during autumn relaxation. The roles of vertical convection and turbulent mixing in the fertilization of the photic layer were also assessed. Mixing was the most important fertilization mechanism, ensuring transport of nutrients under upwelling and downwelling conditions.