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Variations of the partial pressure of CO2 (pCO2) and related parameters were determined in surface seawater adjacent to the Tagus Estuary during May 2000 (spring) and March 2001 (winter). The pCO2 values of 730 and 1450 µatm estimated in spring and winter, respectively, are of the same order of magnitude as those of other European coastal areas. The extremely high Tagus River discharge (1820 m3 s–1) that occurred in winter created a pronounced estuarine plume reflected in decreased surface salinity and elevated supersaturation of CO2 with respect to the atmosphere (~400%). The pCO2 distribution was mainly governed by enhanced CO2 solubility at low temperatures and by the hydrodynamics. In contrast, in spring, when the discharge was much lower (190 m3 s–1), the distribution of pCO2 depended on phytoplankton productivity. In fact, CO2 was close to equilibrium with the atmosphere and controlled by biological processes. The estimated CO2 fluxes for the entire study area reached mean values of 1 and 55 mmol CO2 m–2 d–1 in spring and winter, respectively. The CO2 emissions during the productive period exhibited negligible values due to the reduction of pCO2 in surface waters. This study reveals that the coastal area off the Tagus Estuary functions as a likely source of CO2 to the atmosphere during some periods of the year.
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