CO2 variability in surface coastal waters adjacent to the Tagus Estuary (Portugal)

Abstract

 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.