Thermodynamic uptake of atmospheric CO2 in the oligotrophic and semiarid São Francisco estuary (NE Brazil)

Abstract

Estuarine carbonate chemistry predicts that thermodynamic equilibration during the mixing of freshwater with seawater will generate a carbon dioxide (CO2) sink for the case of warm and poorly buffered tropical rivers. The São Francisco River estuary has historically become oligotrophic, after the construction on its watershed of a series of hydroelectric dams, where organic matter and nutrients are retained. During two cruises in late winter (Aug. 2014) and early summer (Nov. 2015), dissolved inorganic carbon (DIC) and total alkalinity (TA) increased linearly with salinity in the main estuarine channel, where the mixing time of water was half a day, and showed a close to conservative behaviour. In the main channel, water partial pressure of CO2 (pCO2) recorded at 1min frequency, followed an asymmetric bell-shaped trend versus salinity, close to the curve predicted by thermodynamic conservative mixing of freshwater DIC and TA with seawater DIC and TA. The low (0-3) salinity region was always a source of atmospheric CO2, where, despite low chlorophyll concentrations, a pCO2 diurnal change of about 60 ppmv suggested the occurrence of photosynthesis in summer . At salinities above 3, under-saturated pCO2 values (down to 225 ppmv in winter and neap tides) and invasion of atmospheric CO2 of 0.38-1.70 mmol m -1 h -1 occurred because of predominating thermodynamics during estuarine mixing. In winter and neap tides, higher river discharge, intense estuarine mixing, lower temperatures and limited tidal pumping resulted in observed pCO2 different from theoretical conservative pCO2 by less than 3ppmv at salinities >3. Conversely, in summer and spring tides, recorded pCO2 values were on average +43±35 ppmv above the conservative mixing curve, when tidal pumping, CO2 invasion and surface heating were more significant in the mixing zone, but not sufficient to offset the thermodynamic uptake of atmospheric CO2. By combining carbonate chemistry with estuarine mixing modelling and gas exchange calculations, we estimate that heating contributed to about 15% and gas exchange to about 10% of the positive pCO2 deviation from conservative mixing during summer.The remaining 75% of the deviation was maximum at ebb tides and within a salinity ranges consistent with the occurrence of tidal pumping from marches and mangrove soils. Indeed, in the mangrove channel, water were supersaturated with pCO2 values of 976±314ppmv, while in the main channel, highest positive pCO2 deviations from conservative mixing (up to +100ppmv during several hours) occurred at ebb tides. An important finding was that, in the São Francisco, the thermodynamic and biological processes compete with each other to CO2 fluxes both at low salinities where evasion and autotrophy occur, and at high salinities where invasion, heterotrophy and tidal pumping occur. Our study suggests that carbonate thermodynamics during mixing is a key process that has been overlooked in estuarine studies, although it can generate important airwater CO2 exchange and contribute significantly to...

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