Abstract
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.
Highlights
Outgassing of carbon dioxide (CO2 ) from rivers has been identified as a significant piece of the carbon cycle [1,2,3]
The primary goal of this study was to identify whether a diel shift in pressure of CO2 (pCO2) driven by solar radiation during the day results in significantly different pCO2 and CO2 outgassing rates between daylight and darkness hours
On a daily basis in the two seasons, both riverine pCO2 and CO2 outgassing showed a distinct diel cycle, with levels decreasing from sunrise to peak daylight hours, followed by a gradual increase during hours of darkness. pCO2 measurements varied by 206–607 μatm over 24-h periods with significantly lower pCO2 values during daylight hours in summer
Summary
Outgassing of carbon dioxide (CO2 ) from rivers has been identified as a significant piece of the carbon cycle [1,2,3]. The process has been attributed to an oversaturation of dissolved CO2 in the water column, resulting in diffusion of CO2 into the atmosphere. The two primary inputs of CO2 into the water column of large river systems are (1) in-situ respiration of organic material and (2) flushing of CO2 produced in soil pores from terrestrial and wetland environments via decomposition of organic material and root respiration [4]. Many of the world’s largest rivers are supersaturated in CO2 with respect to the atmosphere, resulting in a large flux of CO2 from the water surface [1,5,6]. Current estimates of global CO2 flux from rivers to the atmosphere vary largely from 230 to
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