Abstract
Ten field cruises were carried out in Tagus estuary from 1999 to 2007 to study the dynamics of the inorganic carbon system. Dissolved inorganic carbon (DIC) and total alkalinity (TA) increased with salinity. DIC and TA were generally conservative in the estuarine mixing zone (salinity > 10), while a complex distribution pattern was observed at the upper estuary. DIC values peaked 1786.9 ± 155.8 µmol kg−1 at that segment. Estimated annual mean fluxes of DIC were 0.27 Tg C yr−1 from the river to the estuary, and 0.37 Tg C yr−1 from here to the coastal area. The Tagus estuary was always CO2 supersaturated, with partial pressure of CO2 (pCO2) reaching 9160 µatm in the upper estuary. An average emission of 0.11 Tg C yr−1 was estimated from the estuary to the atmosphere, corresponding to 23% of exported DIC. Only 8% of the riverine DIC was ventilated. The non-conservative behaviour of CO2 parameters in the estuary segment under freshwater influence was attributed to alternations in the relevance of riverine/terrestrial runoff, photosynthesis, aerobic respiration, organic matter mineralization and CaCO3 precipitation/dissolution.
Highlights
Estuaries rank among the most productive and dynamic aquatic ecosystems[1]
The Douro, Tagus and Sado inner estuaries and their adjacent coastal waters behave as sources of CO2 to the atmosphere[6], with fluxes ranging from 31 to 76 mol C m−2 d−1, which corresponds to an additional source of 0.1% to the CO2 emissions by the near-shore ecosystems[4]
Tagus waters environmental properties were strongly affected by the river discharge and showed a seasonal variability typical of a mid-latitude system, with most parameter values increasing from winter to spring/summer
Summary
Estuaries rank among the most productive and dynamic aquatic ecosystems[1]. They are frequently characterized by strong physical-chemical gradients, enhanced biological activity and intense sediment dynamics. Data of water-air CO2 fluxes are available for just three Portuguese estuarine systems: the Aveiro coastal Lagoon, the Douro and Sado estuaries[5]. The Douro, Tagus and Sado inner estuaries and their adjacent coastal waters behave as sources of CO2 to the atmosphere[6], with fluxes ranging from 31 to 76 mol C m−2 d−1, which corresponds to an additional source of 0.1% to the CO2 emissions by the near-shore ecosystems[4]. Spatial and temporal CO2 variability is addressed in this estuary based on information obtained from 10 surveys carried out from 1999 to 2007. Some estimates are advanced on the water-air CO2 fluxes over the estuary
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