Carbon Degradation-Driven Hypoxia Enhances Phosphorus Mobilization and Nutrient Stoichiometric Imbalance Under Climate Warming and Atmospheric Nutrient Loading in the Ganga River

ABSTRACT: Considering the 518 km middle stretch of the Ganga River, we conducted in-situ and controlled incubation studies from 2021 to 2024 to investigate carbon degradation-driven biogeochemical changes in the river constrained by climate warming and atmospheric deposition of nutrients. High concentrations of labile (415.70 µg·C·g⁻¹) and recalcitrant (231.30 µg·C·g⁻¹) carbon fractions coupled with extracellular enzyme activities and CO₂ emissions (up to 270 mg·m⁻²·h⁻¹) indicated microbial degradation and, consequently, dissolved oxygen depletion (≤2 mg·L⁻¹) at polluted sites. The combined influence of atmospheric nutrient loading, hypoxia-driven sediment-P release, and denitrification associated nitrogen loss, shifts N:P ratios of river water and sediments. Warming altered the redox conditions at sediment-water interface, enhancing microbial extracellular enzyme activities and CO₂ emission (up to 24% at +2 °C). However, it caused a decline in microbial apparent carbon use efficiency. The study shows that the multi-stressors synergy and associated feedbacks progressively destabilizes riverine stoichiometric balance with a likely switch over to N or P limitation and ecological functioning. Our study highlights that the watershed-scale nutrient loading and climate-sensitive biogeochemical transformations jointly regulate oxygen-dependent nutrient cycling in large rivers, emphasizing the need for integrated river-watershed management under a changing climate scenario.