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Tidal variation of CO2 system parameters was evaluated in the Tagus Estuary (SW Portugal) based on field observations performed in 2007 in the lower estuary under low river discharge conditions, during spring and autumn. Differences were found in the distribution patterns of environmental variables along a tidal cycle and at neap and spring tides essentially due to tide amplitude effects and tidal asymmetric behavior. A dominant east-westbound current was detected transporting CO2 enriched estuarine water offshore mainly during the ebb, and the reverse tidal current transporting more oxygenated marine water and with high chlorophyll a concentration during the flood. Distinct seasonal features were observed in terms of CO2 water properties and productivity. Regarding the tidal variability of the CO2 system parameters, besides the key processes controlling the variability, such as tidal advection and horizontal mixing of the two water masses present (estuarine and oceanic), specific biogeochemical processes emerged as playing an important role. Data obtained indicate that photosynthesis (in spring), aerobic respiration, and CaCO3 precipitation/dissolution (mainly in autumn) processes co-occur and control short-scale variations of CO2, reflecting the complexity of CO2 chemistry in the Tagus Estuary.
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