Temporal patterns of carbon dioxide in relation to hydrological conditions and primary production in the northeastern shelf of the Gulf of Cadiz (SW Spain)

Abstract

The variations of the partial pressure of CO2 (pCO2) in surface seawaters of the northeastern shelf of the Gulf of Cadiz were studied during a 12-month period (March 2003–March 2004) in relation to hydrological conditions, chlorophyll concentrations, and primary production (PP). Monthly air–sea CO2 fluxes also were calculated. In spring, during the period of complete mixing of the water column, nutrients mainly brought to surface waters by the river discharge allowed a phytoplankton bloom to develop. PP rates estimated by remote sensing showed the highest values between March and May 2003, occurring concomitantly with the lowest values of surface pCO2. The undersaturation registered with respect to atmospheric pCO2 led the area to behave as a net sink for CO2. In late summer, water stratification, wind relaxation, and the absence of river discharge resulted in decreased chlorophyll concentrations, which were accompanied by a reduction in the productivity of the shelf. A marked increase in surface pCO2 took place, also in response to higher water temperatures and the flux reversed towards the atmosphere. As autumn progressed, nutrient concentrations rose as a consequence of fluvial inputs, which in conjunction with a complete mixing in the water column favored the appearance of a second, but less intense, phytoplankton bloom. PP increased with respect to summer and a decrease in surface pCO2 was observed. The undersaturation and the modulating influence of the wind resulted in a net capture of atmospheric CO2. Although chlorophyll concentration decreased during winter in response to the reduction in the incident irradiance, the air–sea CO2 flux remained directed towards the ocean. These preliminary results strongly suggest that, on annual basis, the northeastern shelf of the Gulf of Cadiz behaves as a net sink for atmospheric CO2.