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

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