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

Abstract

Carbon species fluxes and net budgets were studied with a 2-D box model in the Ria de Arouse (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, P(CO2) = +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 Ria (~6 d) seem to be the reasons for the observed F(CO2). The CO2 increase associated with CaCO3 fixation by the intensive culture of mussels on hanging ropes did not affect F(CO2) significantly. High F(CO2) 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 Ria with carbon released from the sediments. F(CO2) 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 flew nutrients at the base of the pycnocline. Negligible F(CO2) values were observed during upwelling relaxations, when ENACW into the Ria was replaced by the warm and CO2-equilibrated shelf surface waters, F(CO2) reached a maximum during a strong autumn downwelling event because of the concurrence of warm shelf surface water flow into the Ria 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.