Mechanisms controlling dissolved CO2 over-saturation in the Three Gorges Reservoir area

The emission of CO2 to the atmosphere from inland waters is an important part of the global carbon cycle. In this study, we made spatial and temporal measurements of CO2 partial pressure (pCO2) along the Three Gorges Dam system. The pCO2 ranged from 619 to 2383 μatm and was supersaturated relative to atmospheric CO2. Further, pCO2 showed obvious spatial and temporal variations: pCO2 at the high-flow season was much lower than that at the low-flow season near the upstream part of the reservoir, whereas pCO2 in the reservoir water and after the dam showed an opposite seasonal trend. Organic matter mineralization produced more CO2 in the surface water of the reservoir area at the high-flow season and should be responsible for the δ13C-depleted dissolved inorganic carbon (δ13CDIC). In addition, organic carbon mineralization is sensitive to temperature variability, which is expected to be an important driver of the dissolved CO2 over-saturation in the reservoir. This study suggested that the construction of Three Gorges Reservoir increased the water transit time and accelerated the organic carbon mineralization in the Changjiang River. The results indicate that carbon cycling changes markedly in large impounded rivers.