Carbon dioxide emission and dissolved carbon transport of a large lowland river entering the Gulf of Mexico under hydrological drought

River flows have been reported to export large quantities of dissolved carbon, via both lateral transport and vertical outgassing. However, less is known about how hydrological drought years can affect transport and transformation. This study investigated dissolved carbon dynamics in North America's largest swamp river, the Atchafalaya River (AR), under the historically dry conditions during 2021–2023. Discharging on average 200 km³ of freshwater annually into the Northern Gulf of Mexico, the AR makes a substantial contribution of terrestrial carbon flux to the coastal system. From June 2021 to May 2023, we conducted river measurements and sampling in the AR at Morgan City, which included partial pressure of dissolved carbon dioxide (pCO₂), water temperature, dissolved oxygen, pH, chlorophyll-a, colored dissolved organic matter (cDOM), and turbidity. From this 24-month study, we found a significantly lower pCO₂ (850–5422 μatm) and CO₂ outgassing rates (mean: 13.06 mmol m⁻² hr⁻¹) compared to previous years of 2019–2021 (pCO₂: 1130–8650 μatm; CO₂ outgassing: 19.70 mmol m⁻² hr⁻¹). Similarly, during the 2-year dry conditions, mass export of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) to the Gulf of Mexico decreased to 4.21 Tg C yr⁻¹ and 1.78 Tg C yr⁻¹, respectively, from 6.40 Tg C yr⁻¹ and 2.40 Tg C yr⁻¹ in the previous wet years. River droughts lowered riverine carbon transport and transformation, likely due to the limited hydrological connectivity of the river channel with the floodplains. However, during both the wet and dry years, DIC concentrations were constantly higher than DOC concentrations, suggesting active organic carbon mineralization in the river water, and also in the soil organic matter.