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Open AccessArticle

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

1
School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA
2
Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA
*
Author to whom correspondence should be addressed.
Water 2019, 11(1), 43; https://doi.org/10.3390/w11010043
Received: 19 October 2018 / Revised: 21 November 2018 / Accepted: 21 December 2018 / Published: 27 December 2018
(This article belongs to the Special Issue Recent Progress in River Biogeochemistry Research)
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PDF [2755 KB, uploaded 27 December 2018]
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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. View Full-Text
Keywords: aquatic carbon cycle; freshwater CO2; diel river metabolism; river carbon accounting; Mississippi River aquatic carbon cycle; freshwater CO2; diel river metabolism; river carbon accounting; Mississippi River
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Reiman, J.H.; Xu, Y.J. Diel Variability of pCO2 and CO2 Outgassing from the Lower Mississippi River: Implications for Riverine CO2 Outgassing Estimation. Water 2019, 11, 43.

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