Enhanced regulation of carbon emissions and DOM transport by cascade reservoirs in the upper yellow river during the spring thaw.

Research on how cascade reservoirs modulate carbon cycling in climate-sensitive alpine rivers remains limited. This study investigates greenhouse gas (GHG) emissions and dissolved organic matter (DOM) transport regulated by three cascade reservoirs in the upper Yellow River: Longyangxia (LYX), Lijiaxia (LJX), and Liujiaxia (UJX). Field measurements during spring thaw revealed that reservoirs significantly enhance CO2 and CH4 diffusion fluxes (FCO2 and FCH4) compared to natural river sections. FCO2 showed a decreasing trend along the flow direction (LYX: 14.6 ± 1.11 > LJX: 10.0 ± 0.51 > UJX: 9.06 ± 0.66 mmol m-2·d-1), while FCH4 exhibited an increasing trend (UJX: 0.31 ± 0.04 > LJX: 0.26 ± 0.01 > LYX: 0.12 ± 0.03 mmol m-2·d-1). Optical spectroscopy showed that DOM composition transitioned from terrestrial, glacial signatures upstream to more humified, anthropogenic characteristics downstream. The vertical distribution of DOM was closely linked to reservoir depth. Shallow reservoirs (≤40 m) exhibit high DOM, the humic acid content, and humification levels in subsurface layers (peaking at 7-10 m depth); deep reservoirs (≥70 m) show these traits in benthic layers (70-100 m). Critically, correlation analysis linked CO2 emissions to microbial degradation of labile DOM components, whereas CH4 emissions were associated with both fresh autochthonous and recalcitrant terrestrial humic substances, differing between reservoirs. Our findings demonstrate that cascade reservoirs can function as interconnected biogeochemical processors during the thaw period, with potential to enhance regulation of carbon emissions and DOM transport, thereby playing a critical role in understanding the carbon cycle in reservoirs across the fragile Qinghai-Tibet Plateau region.