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.