Inferring the role of fluvial metabolism in CO2 emissions from O2-CO2 paired measurements across contrasting hydrological conditions

Headwater streams are critical control points for carbon dioxide (CO₂) emissions to the atmosphere, traditionally assumed to originate primarily from terrestrial sources. However, in-stream metabolic activity can become a substantial CO₂ source, particularly in water-scarce regions characterized by net heterotrophic streams and low groundwater inputs. To explore this idea, we analyzed patterns of CO₂ and oxygen (O₂) concentrations at high-temporal resolution alongside stream aerobic metabolic rates to identify CO₂ sources under contrasting hydrological conditions in an intermittent, oligotrophic Mediterranean stream. During high-discharge periods, there was no correlation between O₂ and CO₂ concentrations, and O₂-CO₂ patterns indicated CO₂ oversaturation. These results indicate that despite ecosystem respiration (ER) predominated over gross primary production (GPP) during high-discharge periods, lateral groundwater inputs were likely the dominant source of CO₂ emissions within the stream. Under low-flow conditions, GPP was still low in this net heterotrophic stream. Yet. a negative relationship between O₂ and CO₂ concentrations emerged, suggesting a major role of in-stream metabolic activity in driving O₂-CO₂ dynamics. These findings reinforce the concept of headwater streams as key CO₂ emitters, while emphasizing the influence of hydrological conditions on their dual role: acting as chimneys for terrestrially-derived CO₂ during high-flow periods and as active carbon biogeochemical reactors during low-flow periods.