Freshwater-Saltwater Mixing Effects on Dissolved Carbon and CO 2 Outgassing of a Coastal River Entering the Northern Gulf of Mexico

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.