Reassessing riverine carbon dioxide emissions from the Indian subcontinent

Anthropogenic perturbations are increasing uncertainties in estimating CO₂ emissions via air-water CO₂ flux (F_CO2) from large rivers of the Indian subcontinent. This study aimed to provide an improved estimate of the total F_CO2 from the subcontinental rivers by combining calculations of the partial pressure of CO₂ (pCO₂) in eight major rivers with new measurements in the Ganges and Godavari. The average pCO₂ in the two newly surveyed rivers, including tributaries, wastewater drains, and impoundments, were 3–6 times greater than the previously reported values. In some highly polluted urban tributaries and middle reaches of the Ganges that drain metropolitan areas, the measured pCO₂ exceeded 20,000 μatm, ~40 times the background levels of the headwaters originating in the carbonate-rich Himalayas. The high pCO₂ above 28,000 μatm in the lower reach of the Godavari was seven times the moderate levels of pCO₂ in the headwaters of the volcanic Deccan Traps, indicating enhanced CO₂ production in soils and anthropogenic sources under favorable conditions for organic matter degradation. Across the northern rivers, pCO₂ exhibited a significant negative relationship with dissolved oxygen, but a positive relationship with inorganic N or P concentrations. The strong influence of water pollution on riverine pCO₂ suggests that CO₂ emissions from hypoxic, eutrophic reaches can greatly exceed phytoplanktonic CO₂ uptake. Spatially resolved pCO₂ data, combined with three gas transfer velocity estimates, provided a higher range of F_CO2 from the subcontinental rivers (100.9–130.2 Tg CO₂ yr⁻¹) than the previous estimates (7.5–61.2 Tg CO₂ yr⁻¹). The revised estimates representing 2–5% of the global riverine F_CO2 illustrate the importance of the Indian subcontinental rivers under increasing anthropogenic pressures in constraining global inland waters F_CO2.