Diel Variability of pCO₂ and CO₂ Outgassing from the Lower Mississippi River: Implications for Riverine CO₂ Outgassing Estimation

Carbon dioxide (CO₂) outgassing from river surface waters is an important component of the global carbon cycle currently not well constrained. To test the hypothesis that riverine partial pressure of CO₂ (pCO₂) and CO₂ outgassing rates differ between daylight and darkness, we conducted in-situ pCO₂ and ambient water measurements over four 24-h periods in the spring and summer of 2018 in the Lower Mississippi River under varying flow regimes. We hypothesized that diel pCO₂ variation will correlate inversely with solar radiation due to light-induced photosynthesis. Despite differing ambient conditions between seasons, we found a consistent diel cycle of riverine pCO₂, with highest values before sunset and lowest values during peak daylight. Recorded pCO₂ measurements varied by 206–607 µatm in spring and 344–377 µatm in summer, with significantly lower records during daylight in summer. CO₂ outgassing was significantly lower during daylight in both seasons, with diel variation ranging between 1.5–4.4 mmol m⁻² h⁻¹ in spring and 1.9–2.1 mmol m⁻² h⁻¹ in summer. Daily outgassing rates calculated incorporating diel variation resulted in significantly greater rates (26.2 ± std. 12.7 mmol m⁻² d⁻¹) than calculations using a single daily pCO₂ value. This study suggests a likely substantial underestimation of carbon outgassed from higher order rivers that make up a majority of the global river water surface. The findings highlight the need for high temporal resolution data and further research on diel CO₂ outgassing in different climate regions to constrain uncertainties in riverine flux estimation. View Full-Text