Air-sea CO2 fluxes in a coastal embayment affected by upwelling:: physical versus biological control

Water column pCO(2) and air-sea CO2 fluxes were studied during an 18-month period (May 1994-September 1995) in a coastal embayment affected by upwelling, located in the northwestern Iberian Peninsula (Ria de Vigo and adjacent shelf). Overall, the region acted as a net annual atmospheric CO, sink, with magnitude ranging from 0.54 mgC m(-2) d(-1) in the Ria estuary to 22 mgC m(-2) d(-1) offshore. During moderate upwelling and upwelling relaxation conditions the sampling area was a sink for atmospheric CO2. By contrast, during winter conditions and during intense upwelling the flux reversed towards the atmosphere. The relative influence of physical and biological processes on pCO(2) was evaluated using two different approaches: firstly, statistical analysis of physico-chemical correlations, and secondly, a thermodynamic analysis in the oceanic CO2 system. Both methods yielded consistent results, showing that the main processes controlling seasonal and spatial pCO(2) variability were the production and remineralization of organic matter, explaining ca. 70 % of the total variability. In the inner part of the embayment, air-sea CO2 exchange was mainly modulated by CO2 partial pressure gradient, whereas in the adjacent shelf, wind speed largely contributed to CO2 fluxes between the ocean and the atmosphere. (C) 1999 Ifremer / CNRS I IRD / Editions scientifiques et medicales Elsevier SAS.