Quantification of carbopeaking and CO2 fluxes in a regulated Alpine river

Carbon dioxide (CO2) fluxes in regulated Alpine rivers are driven by multiple biogeochemical and anthropogenic processes, acting on different spatiotemporal scales. We quantified the relative importance of these drivers and their effects on the dynamics of CO2 concentration and atmospheric exchange fluxes in a representative Alpine river segment regulated by a cascading hydropower system with diversion, which includes two residual flow reaches and a reach subject to hydropeaking. We combined instantaneous and time-resolved water chemistry and hydraulic measurements at different times of the year identifying the main CO2 pathways through a one-dimensional transport-reaction model. The spatiotemporal distribution and drivers of CO2 fluxes depended on hydropower operations. Along the residual flow reaches, CO2 fluxes were directly affected by the upstream dams only in the first 2 km downstream of each dam, where the supply of supersaturated water from the reservoirs was predominant. Downstream of the hydropower diversion outlets, the magnitude and dynamics of CO2 fluxes were dominated by systematic sub-daily peaks in CO2 transport and evasion fluxes (‘carbopeaking’) driven by hydropeaking. The additional input of CO2 released locally into the river at the hydropower diversion outlet during hydropeaking matched the amount of CO2 transported, metabolised, and exchanged with the atmosphere along the whole upstream reach. Hydropower operational patterns and regulation approaches in Alpine rivers significantly affect CO2 fluxes and their response to biogeochemical drivers across different temporal scales. This work contributes to understanding and quantifying these processes to clarify the role of natural and anthropogenic drivers in global carbon cycling.