Resumen
The Atchafalaya River (AR), North America?s largest swamp river, annually discharges a large volume of freshwater (nearly 200 km3), delivering ~25% of the Mississippi River?s (MR) flow and the entire Red River?s (RR) flow into the Gulf of Mexico. Studies have reported higher levels of organic carbon in the AR?s outlets compared to the MR?s outlet, raising questions about local carbon sources. In this study, we investigated dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) inputs into the AR from the RR and MR using DOC and DIC concentrations, mass loading, and isotopic signature (d13C) analyses. Monthly river water sampling was conducted in the MR and RR near their confluence where the AR is formed from May 2015 to May 2016. DIC concentrations in the RR were found to be only half of those found in the MR, while the RR?s DOC concentrations were on average 1.8 times higher than those found in the MR. Based on the models developed for this study period, the RR?s contribution to DIC mass loading in the AR represented 1.41 teragrams (Tg) (or, 29.7%) of the total 4.76 Tg DIC transported by both tributaries, while its contribution to DOC mass loading was disproportionately high, accounting for 1.74 Tg of the 2.75 Tg DOC (or, 63.2% of total DOC) entering the AR. Both d13CDIC and d13CDOC showed significantly more negative values in the RR than those found in the MR. Significant correlation between d13CDIC and d13CDOC isotope values in the RR indicated interrelation of dissolved carbon processing, which was not observable in the MR. These results strongly suggest that the RR is an extremely significant source of DOC to the AR, and thus the Gulf of Mexico, and additionally plays an important role in diluting the anthropogenically enhanced DIC fluxes of the MR.