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 ℃). 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.

Keywords

Atmospheric deposition / Benthic hypoxia / Carbon degradation / Climate warming / Ganga River / N:P stoichiometry

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