Shifting flow regimes and water quality regulate CH₄ dynamics in anthropogenically perturbed tropical rivers and...

Rivers transport and process carbon along the land-ocean continuum and emit CH₄ to the atmosphere. Widespread human perturbation of river networks has disrupted the natural flow regime, regulating the supply of allochthonous matter and subsequently affecting the in-stream biogeochemical processes. The spatiotemporal variability of CH₄ in human-impacted rivers remains less explored, especially in the highly populated regions of South Asia, resulting in uncertainties in the global flux estimates. This study investigates the evolution of CH₄ in tropical rivers and engineered canals along a gradient of increasing anthropogenic disturbances. Sampling was conducted along different sections (selected based on the level of human perturbations) of the Sabarmati River, Mahi River, and Narmada Canal, located in western India, during high and low flow conditions. It was observed that water stagnancy resulted in higher dissolved CH₄ in the lined and stagnant Sabarmati riverfront (a discontinuity in the free-flowing river continuum) compared to the lined and flowing Narmada Canal. Furthermore, the release of wastewater resulted in a ∼70 km low-oxygen environment accompanied by extremely high CH₄ concentrations in the downstream section of the Sabarmati. CH₄ exhibited positive correlations with temperature, dissolved inorganic nitrogen, and particulate organic carbon. δ¹³C_CH4 indicated that acetoclastic methanogenesis was the major pathway of CH₄ production, and evidence of CH₄ oxidation was observed in the stagnant riverfront of the Sabarmati. This study provides estimates of CH₄ fluxes from the region and highlights the climate implications of river development projects, along with the dominant role of wastewater in rivers with reduced natural flow.