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Freshwater-Saltwater Mixing Effects on Dissolved Carbon and CO2 Outgassing of a Coastal River Entering the Northern Gulf of Mexico

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Abstract

The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO2 outgassing at six locations along an 88-km-long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river’s mouth. We found that throughout the sampling period, all six sites exhibited CO2 supersaturation with respect to the atmospheric CO2 pressure during most of the sampling trips. The average CO2 outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m−2 year−1, respectively, with a mean of 140 mol m−2 year−1 for the entire river reach. In the short freshwater river reach before a saltwater barrier, 0.079 × 108 kg carbon was emitted to the atmosphere during the study year. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 × 108 kg) was emitted to the atmosphere during the same period. For the entire study period, the river’s freshwater discharged 0.25 × 109 mol dissolved inorganic carbon (DIC) and 1.77 × 109 mol dissolved organic carbon (DOC) into the mixing zone. DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 to 0.56 mM). These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world’s estuarine systems.

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Acknowledgements

This study was financially supported through grants from the National Fish and Wildlife Foundation (Project # 8004.12.036402) and the US Department of Agriculture Hatch Funds (Project # LAB94230). The data used are listed in the tables, figures, and supporting information of the paper. Thanks go to the US Geological Survey for making the river discharge and gage height data available for this study and to Syam K. Dodla and Manoch Kongchum for laboratory carbon analysis at the Central Analytical Instruments Research Laboratory, Louisiana State University Agricultural Center. Thanks also go to Yuyan Zhou and Bo Wang for drainage area and water surface area calculations and to Daniel Cohen for proofreading the manuscript. The authors are grateful to many students including, among others, Kaci Fisher, Paula Castello Blindt, Sanjeev Joshi, and Zhen Xu for their outstanding field assistance. Finally, the authors thank the editor and anonymous reviewers for their careful readings of the manuscript and constructive suggestions.

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The authors declare that they have no conflict of interest.

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    Songjie He & Y. Jun Xu

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He, S., Xu, Y.J. Freshwater-Saltwater Mixing Effects on Dissolved Carbon and CO2 Outgassing of a Coastal River Entering the Northern Gulf of Mexico. Estuaries and Coasts 41, 734–750 (2018). https://doi.org/10.1007/s12237-017-0320-4

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