Inorganic carbon distribution and CO 2 fluxes in a large European estuary (Tagus, Portugal)

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 CO 2 supersaturated, with partial pressure of CO 2 ( p CO 2 ) 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 CO 2 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 CaCO 3 precipitation/dissolution.