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 upwelli ng, located in the northwestern Iberian Peninsula (Ria de Vigo and adjacent shelf). Overall, the region acted as a net annual atmospheric CO, sink, wi th 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 c onditions the sampling area was a sink for atmospheric CO2. By contrast, du ring winter conditions and during intense upwelling the flux reversed towar ds the atmosphere. The relative influence of physical and biological proces ses on pCO(2) was evaluated using two different approaches: firstly, statis tical analysis of physico-chemical correlations, and secondly, a thermodyna mic analysis in the oceanic CO2 system. Both methods yielded consistent res ults, 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 emba yment, air-sea CO2 exchange was mainly modulated by CO2 partial pressure gr adient, whereas in the adjacent shelf, wind speed largely contributed to CO 2 fluxes between the ocean and the atmosphere. (C) 1999 Ifremer / CNRS I IR D / Editions scientifiques et medicales Elsevier SAS.