Abstract
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 (δ13C) analyses. Monthly river water sampling was conducted in the MR and RR near their confluence where the AR is formed from May 2015–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 δ13CDIC and δ13CDOC showed significantly more negative values in the RR than those found in the MR. Significant correlation between δ13CDIC and δ13CDOC 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.
Highlights
The quantity and quality of carbon exported by rivers to coastal margins has important impacts on coastal and marine ecosystem health and functioning, and are important to the development of global carbon budgets [1,2]
This study provides insights into the role the Red River plays in tributary input of dissolved carbon to the Atchafalaya River, the North America’s largest swamp river, filling in a knowledge gap that is important to understanding carbon dynamics in the Mississippi-Atchafalaya River System
While the Mississippi River delivered the vast majority of dissolved inorganic carbon (DIC) to the Atchafalaya River (AR), the Red River was found to contribute to over half of the total dissolved organic carbon (DOC) entering the AR, which is greater than previously expected
Summary
The quantity and quality of carbon exported by rivers to coastal margins has important impacts on coastal and marine ecosystem health and functioning, and are important to the development of global carbon budgets [1,2]. Over the past three decades, on average, the MR annually discharged 474 km3 [7] and the AR discharged 199 km3 [8] of freshwater into the Northern Gulf of Mexico (NGOM), carrying large quantities of carbon to Louisiana’s coastal margins. Due to these large inputs, the river plumes from the Mississippi-Atchafalaya River System (MARS) dominate biogeochemical cycling in the NGOM [9,10]. Inputs of anthropogenic riverine carbon may play a role in coastal ecosystem disruption; for example, organic matter exported by the MARS may contribute up
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