Abstract:

Riverine carbon emissions play a crucial role in the global carbon cycle, where the hyporheic zone (HZ) serves as a key component that significantly influences CO₂ emission. The production of CO₂ within the HZ is strongly affected by dynamic factors such as river temperature (T) and dissolved oxygen (DO). However, most existing models assumed that T and DO were time-invariant, leading to an inaccurate capture of the temporal variability of these processes, which ultimately caused significant uncertainty in riverine carbon emission estimation. To address this problem, this study developed a coupled model integrating physical and biogeochemical processes by including the periodic variations of T and DO for a representative bedform-induced HZ. The model was numerically solved using COMSOL Multiphysics, and the dynamic impacts of river T and DO on CO₂ production within the HZ were analyzed using the Damköhler number (Da) and correlation coefficients. Our results indicate that both T and DO competitively regulate CO₂ production rate. Moreover, when the mean residence time of HZ is less than 15.7 h, a critical temperature threshold (T_c) exists, above or below which the dominant role in regulating CO₂ production rate shifts between T and DO. Specifically, when the mean river temperature is below T_c, the CO₂ production is primarily controlled by T fluctuations, whereas above T_c, it is dominated by DO fluctuations. Notably, this shift in dominant role disappears when the mean residence time exceeds 15.7 h. Our study reveals how T and DO fluctuations impact dynamics of CO₂ production in the HZ, and could improve the assessment of the HZ’s contribution to the global carbon cycle, particularly for predicting how the river ecosystem responds to climate change.

Key words: hyporheic zone, aerobic respiration, temperature threshold, CO₂, biogeochemical processes, coupled model