CO₂ oversaturation and degassing using chambers and a new gas transfer velocity model from the Three Gorges Reservoir...

Reservoirs are considered as important carbon source of the atmosphere, whilst, regional and global reservoir CO₂ quantification is hampered by data limitation and bias in spatial and temporal sampling. By deploying chamber measurements and employing the newly developed model of gas transfer velocity, CO₂ partial pressure (pCO₂) and evasion in the main stem of the Three Gorges Reservoir (TGR) were investigated. The pCO₂ ranged from 429 to 8668 μatm with an average of 2511.6 ± 1721.3 μatm, 6.1-fold higher than the ambient air pCO₂ (mean: 410 μatm). All the samples were net CO₂ sources via water-air interface, displaying pronounced spatial and monthly variability. The CO₂ areal flux averaged 212.5 ± 120.1 mmol/m²/d in June, 123.3 ± 78.5 mmol/m²/d in July in the lotic TGR main stream, much higher than its lentic system, i.e., 79.6 ± 41.3 mmol/m²/d in November, and 76.3 ± 88.1 mmol/m²/d in March. Much lower k levels in the lentic reservoir surface resulted in lower CO₂ evasion rates. Furthermore, dam impoundment considerably altered the riverine carbon cycle, as reflected by the changing magnitude of CO₂ efflux and environmental controls of dissolved CO₂. Precipitation and concurrent soil CO₂ influx exhibited a central role in controlling riverine pCO₂, and respiration of allochthonous organic carbon was a secondary factor in the TGR lotic system, whilst, both in-stream metabolism and terrestrial inputs played crucial roles in controlling aqueous CO₂ in the TGR lentic system. In comparison, we provided key findings of k model and more reliable CO₂ quantification with a consideration of water level shifts and a complete coverage of spatial sampling. Our higher CO₂ emission (1.47 (1.16-2.13) Tg CO₂/y) than previous studies called more field measurements to assess the resulting changes in CO₂ flux owing to dam operation and changing environment, and their implications for regional carbon budgets should be warranted.