Carbon dioxide (CO2) partial pressure and emission from the river-reservoir system in the upper Yellow River, northwest China.

Abstract

The exchange of carbon dioxide (CO2) 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 CO2 exchange flux at the water-air interface in river-reservoir systems, especially in ecologically fragile regions, are still largely uncertain. In this study, the CO2 partial pressure (ρCO2) and exchange flux (FCO2) from river-reservoir systems in the upper reaches of the Yellow River (YeUR) were investigated using the CO2SYS system and a boundary layer approach. The spatiotemporal dynamics and driving factors of the partial pressure of ρCO2 and FCO2 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 ρCO2 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 ρCO2 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 ρCO2 distribution and could be used as predictors of ρCO2 in the dry period (R2 = 0.40 P < 0.01). In the dry period, the FCO2 in the SR was - 112.91 ± 165.94mmol/(m2·d), which was a sink of CO2, and the FCO2 in the CR was 131.02 ± 156.77mmol/(m2·d), which was a source of CO2. In the flood period, the FCO2 in the SR was 686.54 ± 624.33mmol/(m2·d), and the FCO2 in the CR was 466.10 ± 366.67mmol/(m2·d). Both the SR and the CR were sinks of CO2. Our results contribute to the understanding of CO2 exchange in river-reservoir systems and carbon cycle processes.