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 ± 165.94 mmol/(m²·d), which was a sink of CO₂, and the FCO₂ in the CR was 131.02 ± 156.77 mmol/(m²·d), which was a source of CO₂. In the flood period, the FCO₂ in the SR was 686.54 ± 624.33 mmol/(m²·d), and the FCO₂ in the CR was 466.10 ± 366.67 mmol/(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.

Keywords: CO2 exchange flux; Partial pressure of CO2; Spatial–temporal variation; “River-reservoir” system of the upper Yellow River.