Water-Carbon Coupling Processes on the Third Pole: Differential Carbon Source-Sink Effects and Mechanisms in Lakes, Reservoirs, and Rivers of The Yarlung Tsangpo River Basin

Amid global warming and expanding high-altitude hydropower development, characterizing the carbon source-sink dynamics of cryospheric waters is pivotal for sustainable water resource management. However, water-carbon coupling across natural and artificial water bodies in the Third Pole remains poorly understood. This study deeply explored the coupling influence mechanism of hydrological, climate, biogeochemical, land cover, and human factors on water-air carbon dioxide (CO₂) flux across lakes, reservoirs, and rivers in the Yarlung Tsangpo basin (YL). Results indicate that compared with inland waters, lakes and reservoirs in the YL were obvious carbon sinks (4.91 ± 86.92 and 21.23 ± 95.67 mmol m^-2 d^-1, respectively), especially during normal and flood periods, whereas rivers predominantly acted as CO₂ sources (103.04 ± 194.88 mmol m^-2 d^-1). The key drivers of CO₂ flux were pH, air temperature, and runoff etc. Runoff showed obvious spatial heterogeneity that correlated negatively with CO₂ flux in upstream lakes/reservoirs but positively in downstream rivers. Structural equation modeling identified pCO_2water (pH-controlled), climate (rainfall, temperature), and runoff as direct drivers for CO₂ flux. Increased rainfall and temperature facilitate CO₂ uptake of YL waters, further retained by lentic systems but re-released by rivers via runoff. Runoff effects on carbon sequestration in reservoirs were weaker than in lakes, indicating that while damming converts rivers from sources to sinks, operations may weaken the sink capacity. Operation ratios range from -0.24 ~ 0.56, and less than -0.36 might be an ideal regulation range. Our findings highlight the critical role of lentic systems in CO₂ uptake within the YL. While impoundment can reduce carbon emissions from rivers, careful operational regulation is essential.