Water quality–carbon coupling in a human-impacted river network: Contrasting CO2 supersaturation and CH4 sink strength along oxidizable loading and seasonal gradients

Abstract

Routine river monitoring often evaluates water quality separately from greenhouse-gas disequilibrium, limiting its use for integrated carbon assessment. We investigated dissolved CO₂ and CH₄ at 16 sites across the Liao River during July and December 2024, a wetter-than-normal sampling year, and related their patterns to hydrochemistry, EEM-PARAFAC-derived dissolved organic matter (DOM) descriptors, and 16S rRNA-based microbial attributes. CO₂ was supersaturated at all sites, indicating persistent source potential across the river network, whereas CH₄ was predominantly undersaturated, with 27 of 32 observations below atmospheric equilibrium and only localized positive patches. CO₂ supersaturation was most closely associated with an oxidizable loading–DOM optical quality gradient: COD_Mn provided the clearest water-quality signal, while HIX, BIX, and ureolysis further reflected DOM processing conditions. The CO₂ response increased most strongly within the observed COD_Mn range of approximately 2.4–4.5 mg L⁻¹ and remained elevated above ∼5 mg L⁻¹. In contrast, CH₄ sink strength was most strongly associated with seasonal temperature structure, with BIX and dissolved oxygen representing related DOM freshness and oxygen-background dimensions. Most microbial diversity and FAPROTAX-inferred functional signals covaried with the same thermal–DOM–oxygen background, except for the selective CO₂-related signal of ureolysis. These results indicate that routine COD_Mn monitoring and DOM optical diagnostics can help identify reaches associated with elevated CO₂ supersaturation, whereas CH₄ assessment requires seasonally resolved monitoring and targeted evaluation of localized source-like conditions.