Effects of stream DO and DOC on streambed respiration under varying groundwater upwelling conditions

Streambed respiration comprises aerobic respiration (AR) and denitrification (DN). It plays an important role in maintaining stream water quality and stream carbon and nitrogen balances. Under gaining stream conditions, dissolved oxygen (DO) and dissolved organic carbon (DOC) in a stream can still enter streambed to fuel respiration through hyporheic flow and diffusion. In this study, we established theoretical and field-based reactive transport models to quantify the effect of upwelling groundwater, stream DO and DOC on biogeochemical reactions. Modeling results show that enhanced dispersion of DO and DOC reinforces streambed respiration mostly within the thickness of 0.4 m. Stream DO affects AR and DN in opposite manners. As stream DO concentration increases from 3.2 to 16 mg L^-1, CO₂ efflux increases by 120%, while N₂ efflux decreases by 12%. As stream DOC concentration increases by 5 times, both the CO₂ efflux and N₂ efflux increase by 17% and 3%, respectively. Groundwater flux affects the extent and efficiency of streambed respiration. As groundwater flux increases, both the supply of substrates for AR and the CO₂ efflux increase. Meanwhile, the substrate residence time decreases, leading to retarded DO consumption by AR and so the decrease in N₂ efflux. Field-based modeling indicates that CO₂ efflux of streambed varied from 5.08 to 8.26 g m^-2 d^-1, contributing to stream CO₂ emission. Overall, this research sheds new light on controls of stream chemistry and groundwater upwelling on streambed respiration.