Carbon dynamics in tropical blackwater rivers: Contrasting CO₂ drivers between coastal Southeast Asia and inland Amazonia

Blackwater rivers have a characteristic dark water color and contain high concentrations of dissolved organic carbon (DOC), while exhibiting low pH and nutrient levels. They typically act as sources of carbon dioxide (CO₂) to the atmosphere. Although blackwater rivers occur throughout the tropics, their carbon dynamics vary geographically. In this study, we compare rivers in Southeast Asia (Sumatra and Borneo) and the Amazon (Rio Negro basin), covering a range of wetland and peat influence. Using in situ measurements of CO₂ concentrations alongside associated water chemistry, we investigate riverine carbon dynamics and the key drivers of CO₂ in these two regions. In Southeast Asia, blackwater rivers show elevated CO₂ concentrations and fluxes, driven by labile DOC from thick coastal peat deposits, with levels generally increasing with wetland coverage. In contrast, the Rio Negro exhibits more moderate CO₂ concentrations despite similar levels of soil-derived organic carbon. Measurements in 2023 reveal no positive correlation between wetland extent and CO₂, indicating a weaker dependence on peatland-derived DOC. Comparative analyses of DOC, pH, and O₂ suggest that wetland influences on water chemistry are broadly similar across the regions, yet CO₂ concentrations in the Rio Negro remain substantially lower than in Southeast Asian rivers with comparable wetland coverage. These intercontinental contrasts highlight the role of peatland origin and biogeochemistry in regulating tropical river CO₂ emissions and are essential for improving global estimates of carbon fluxes and their sensitivity to environmental change.