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 in the case of warm and poorly buffered tropical rivers. The São Francisco River estuary has historically become oligotrophic after the construction of a series of hydroelectric dams in its watershed, 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) were found to increase linearly with salinity in the main estuarine channel, where water mixing required half a day, and showed nearly conservative behaviour. In the main channel, the water partial pressure of CO2 (pCO2) recorded at a 1-min frequency followed an asymmetric bell-shaped trend versus salinity, similar to the curve predicted by the 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 approximately 60 ppmv suggested the occurrence of photosynthesis in summer. At salinities above 3, undersaturated 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 the predominating thermodynamics during estuarine mixing. In winter and neap tides, the higher river discharge, intense estuarine mixing, lower temperatures and limited tidal pumping resulted in observed pCO2 differences from the theoretical conservative pCO2 by less than 3 ppmv at salinities >3. Conversely, in summer and spring tides, the 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 approximately 15% and gas exchange contributed to approximately 10% of the positive pCO2 deviation from conservative mixing during summer. The remaining 75% of the deviation reached its maximum at ebb tides and within salinity ranges consistent with the occurrence of tidal pumping from marches and mangrove soils. Indeed, in the mangrove channel, water was supersaturated, with pCO2 values of 976 ± 314 ppmv, while in the main channel, the highest positive pCO2 deviations from conservative mixing (up to +100 ppmv for several hours) occurred at ebb tides. An important finding was that in São Francisco, the thermodynamic and biological processes compete with each other for 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 they can generate important air-water CO2 exchange and significantly contribute to the carbon budget of estuaries and river plumes.