## 5 Conclusions  
This study monitored in-stream \(p\)CO\({}_{2}\) and ambient water and weather conditions in spring and summer of 2018 in the Lower Mississippi River under varying river discharge and temperature regimes. Based on the field measurements at 3-h intervals, hourly CO\({}_{2}\) outgassing rates for this 10 th-order, large river system were estimated. The ultimate goal of the study was to assess the diel riverine \(p\)CO\({}_{2}\) cycle and its influence on CO\({}_{2}\) outgassing calculations. To our best knowledge, this is the first field assessment on diel \(p\)CO\({}_{2}\) and CO\({}_{2}\) emissions from a large river, and the findings may have important implications for constraining the uncertainty of river CO\({}_{2}\) outgassing estimates. Seasonally, we found significantly higher \(p\)CO\({}_{2}\) values and CO\({}_{2}\) outgassing rates in spring than in summer, likely due to the higher input of carbon sources from river discharge. On a daily basis in the two seasons, both riverine \(p\)CO\({}_{2}\) and CO\({}_{2}\) outgassing showed a distinct diel cycle, with levels decreasing from sunrise to peak daylight hours, followed by a gradual increase during hours of darkness. \(p\)CO\({}_{2}\) measurements varied by 206-607 uatm over 24-h periods with significantly lower \(p\)CO\({}_{2}\) values during daylight hours in summer. CO\({}_{2}\) flux rates ranged by 1.5-4.4 mmol m\({}^{-2}\) h\({}^{-1}\) over the 24-h periods, with outgassing rates significantly lower during daylight hours in both seasons. Incorporating diel variation in daily CO\({}_{2}\) outgassing calculations resulted in outgassing rates 26.2 mmol m\({}^{-2}\) d\({}^{-1}\) (\(\pm\) std. 12.7) greater than calculations using a single daily \(p\)CO\({}_{2}\) measurement. Diel decreases in \(p\)CO\({}_{2}\) corresponded closely with increasing solar radiation and pH, suggesting strong autotrophic processes regulating CO\({}_{2}\) levels in this large river system as we initially hypothesized. As many outgassing calculations are based on low-resolution samples collected during daylight hours, we postulate that many river CO\({}_{2}\) emission estimates are likely underestimated. In order to constrain this uncertainty, future research utilizing high resolution \(p\)CO\({}_{2}\) measurements in streams spanning differing orders, geographic regions, and biological communities is needed.  
[PERSON] conceived and designed the study; [PERSON] performed the field sampling, measurements and data analysis; [PERSON] and [PERSON] wrote and revised the manuscript.  
This study was primarily supported through a National Fish and Wildlife Foundation grant (Project#: 8004.12.036402). The study was also supported from a U.S. Department of Agriculture Hatch Fund grant (Project#: LAB94230).  
The authors are grateful to the U.S. Geological Survey for providing river discharge data and the Louisiana State University Agricultural Center Ben Hur Research Station for providing meteorological records. The authors also extend a thank you to undergraduate research assistant [PERSON] for her assistance with field data collection, as well as to six anonymous reviewers for their helpful comments and suggestion.  
The authors declare no conflict of interest.