Seasonal variability in water-air CO2 exchanges and carbon origin in a subtropical estuary

The seasonal and interannual variabilities in the water-air net carbon dioxide (CO2) flux were investigated in the lower zone of the Patos Lagoon Estuary (PLE), the largest choked lagoon worldwide. The studied region experienced periods of CO2 ingassing (outgassing) during ocean-dominated (riverine-dominated) conditions. During ocean-dominated conditions (summer/autumn), CO2 uptake was an average of -15 mmol m-2 d-1, whereas under riverine-dominated conditions (winter/spring), CO2 emissions prevailed, reaching an average of 22 mmol m-2 d-1. The magnitude of CO2 uptake in summer/autumn was comparable to the CO2 released to the atmosphere in winter/spring in the mesomixohaline zone. However, the CO2 uptake in summer/autumn was 4-fold higher than the CO2 released in winter/spring in the euhaline zone of the PLE. Unlike most estuarine systems, the PLE acted as a net CO2 sink, with a net CO2 uptake of -2 mmol m-2 d-1 during the period investigated (May 2017 to June 2021). The water-air CO2 exchanges are modulated mainly by the balance between continental freshwater discharge and the inflow/outflow of seawater. However, the combined effect of wind speed, biological activity, and water residence time adds complexity to understanding the carbon exchanges in this region. The higher concentration of organic matter entails the highest estuarine CO2 concentration during autochthonous production, which indicates heterotrophy in estuarine waters. Part of this carbon produced in the estuary is likely exported to the coast, as suggested by the high CO2 concentration in the estuary mouth. The CO2 export from the PLE to the coastal zone appears to make the lagoon resilient to regional anthropogenic CO2 build-up. Owing to the water-air net CO2 flux range associated with complex PLE biogeochemistry dynamics, further investigations as a long-term time-series program will improve knowledge of regional CO2 exchanges in the context of climate and elucidate the role of large estuaries and coastal bays in the global carbon budget.