Carbon dioxide (CO 2 ) partial pressure and emission from the river-reservoir system in the upper Yellow River, northwest China

The exchange of carbon dioxide (CO 2 ) flux between rivers and the atmosphere is an important part of the global carbon cycle. Reservoir development and environmental changes gradually transform rivers into river-reservoir systems. However, the current estimates of CO 2 exchange flux at the water–air interface in river-reservoir systems, especially in ecologically fragile regions, are still largely uncertain. In this study, the CO 2 partial pressure (ρCO 2 ) and exchange flux (FCO 2 ) from river-reservoir systems in the upper reaches of the Yellow River (YeUR) were investigated using the CO 2 SYS system and a boundary layer approach. The spatiotemporal dynamics and driving factors of the partial pressure of ρCO 2 and FCO 2 were revealed. Our results demonstrated that, driven by the freeze–thaw cycle of the permafrost active layer and the development of cascade reservoirs, the average ρCO 2 in the two water periods was higher in the cascade reservoir section (CR) than in the source region section (SR) and higher in the flood period than in the dry period. Driven by water temperature stratification and light conditions, the ρCO 2 of each reservoir in the CR exhibited seasonal variations along with water depth. The environmental factors TN, TP, T, DO, and DOC were the main influencing factors of ρCO 2 distribution and could be used as predictors of ρCO 2 in the dry period ( R 2 = 0.40 P < 0.01). In the dry period, the FCO 2 in the SR was − 112.91 ± 165.94 mmol/(m 2 ·d), which was a sink of CO 2 , and the FCO 2 in the CR was 131.02 ± 156.77 mmol/(m 2 ·d), which was a source of CO 2 . In the flood period, the FCO 2 in the SR was 686.54 ± 624.33 mmol/(m 2 ·d), and the FCO 2 in the CR was 466.10 ± 366.67 mmol/(m 2 ·d). Both the SR and the CR were sinks of CO 2 . Our results contribute to the understanding of CO 2 exchange in river-reservoir systems and carbon cycle processes.