Reassessing riverine carbon dioxide emissions from the Indian subcontinent.

Anthropogenic perturbations are increasing uncertainties in estimating CO emissions via air-water CO flux (F) from large rivers of the Indian subcontinent. This study aimed to provide an improved estimate of the total F from the subcontinental rivers by combining calculations of the partial pressure of CO (pCO) in eight major rivers with new measurements in the Ganges and Godavari. The average pCO 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 pCO exceeded 20,000 μatm, ~40 times the background levels of the headwaters originating in the carbonate-rich Himalayas. The high pCO above 28,000 μatm in the lower reach of the Godavari was seven times the moderate levels of pCO in the headwaters of the volcanic Deccan Traps, indicating enhanced CO production in soils and anthropogenic sources under favorable conditions for organic matter degradation. Across the northern rivers, pCO 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 pCO suggests that CO emissions from hypoxic, eutrophic reaches can greatly exceed phytoplanktonic CO uptake. Spatially resolved pCO data, combined with three gas transfer velocity estimates, provided a higher range of F from the subcontinental rivers (100.9-130.2 Tg CO yr) than the previous estimates (7.5-61.2 Tg CO yr). The revised estimates representing 2-5% of the global riverine F illustrate the importance of the Indian subcontinental rivers under increasing anthropogenic pressures in constraining global inland waters F.