MEPS 176:215-230 (1999)  -  doi:10.3354/meps176215

Carbon cycling in a large coastal embayment, affected by wind-driven upwelling: short-time-scale variability and spatial differences

Gabriel Rosón1,*, X. A. Álvarez-Salgado2, Fiz F. Pérez2

1Facultad de Ciencias del Mar, Universidad de Vigo, Apartado 874, E-36200 Vigo, Spain
2Instituto de Investigacións Mariñas, C.S.I.C., Apartado 2026, E-36208 Vigo, Spain

ABSTRACT: Carbon species fluxes and net budgets were studied with a 2-D box model in the Ría de Arousa (Spain), a large indentation in the NW Iberian upwelling system. The embayment acted as a CO2 source to the atmosphere during the upwelling season (average CO2 flux across the seawater-air interface, FCO2 = +16 mg C m-2 d-1), despite the elevated net community production (NCP) rates (average NCP = 0.84 g C m-2 d-1). The high pCO2 levels in source Eastern North Atlantic Central Water (ENACW) and the reduced residence times within the Ría (~6 d) seem to be the reasons for the observed FCO2. The CO2 increase associated with CaCO3 fixation by the intensive culture of mussels on hanging ropes did not affect FCO2 significantly. High FCO2 values were observed during strong upwelling, which was 3 times larger than the average during the upwelling season. Under these conditions the CO2-rich source ENACW reached the surface and this coincided with low phytoplankton production (NCP = -0.01 g C m-2 d-1). Source ENACW was enriched within the Ría with carbon released from the sediments. FCO2 was reduced to 1/3 of its value compared to the average upwelling season under conditions of moderate upwelling. Phytoplankton grew rapidly (average NCP = 1.49 g C m-2 d-1) at the expense of the gentle injection of new nutrients at the base of the pycnocline. Negligible FCO2 values were observed during upwelling relaxations, when ENACW into the Ría was replaced by the warm and CO2-equilibrated shelf surface waters. FCO2 reached a maximum during a strong autumn downwelling event because of the concurrence of warm shelf surface water flow into the Ría with strong carbon release from the sediments. CO2 fluxes per unit area always increased coastwards, because of the progressive CO2 enrichment of the source ENACW as it enters the embayment and the decrease of residence times.

KEY WORDS: Carbon fluxes · Biogeochemical cycling · Upwelling stress-relaxation · Downwelling · Rías Baixas (NW Spain)

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