Connectivity of floodplain influences riverine carbon outgassing and dissolved carbon transport.

Rivers not only function as a conduit for the delivery of terrestrial constituents to oceans, but they also serve as an essential medium for biogeochemical processing of the constituents. While extensive research has been conducted on carbon transport in many rivers, little is known about carbon transformation in engineered rivers reconnected with their floodplain network. Being the largest distributary of the levee-confined Mississippi River (MR), the Atchafalaya River (AR) carries 25 % of the MR water, flowing through North America's largest freshwater swamp basin and emptying into the Gulf of Mexico. Previous studies reported that this 200-km long, 5-30-km wide river basin can remove a substantial amount of riverine nutrients and carbon. This study aimed to test the hypothesis that the AR emits significantly higher CO2 into the atmosphere as it flows through its extensive floodplain network than the levee-confined MR does. From January 2019 to December 2021, we conducted biweekly - monthly in-situ measurements in the lower AR at Morgan City and in the lower Mississippi River at Baton Rouge. Field measurements included partial pressure of dissolved CO2 (pCO2 ), water temperature, chlorophyll a, colored dissolved organic matter, dissolved oxygen, pH, and turbidity. During each field sampling, water samples were collected and analyzed for concentrations of dissolved organic and inorganic carbon (DOC and DIC). Mass transport of DOC and DIC and outgassing of CO2 were quantified for the two rivers. We found that pCO2 levels were significantly higher in the AR (mean: 3614 μatm; min-max: 1200-8650 μatm) than those in the MR (1817 μatm, 695-3359 μatm), resulting in a doubled CO2 outgassing rate in the AR (486 mmol m2 d-1 ) than in the MR (241 mmol m2 d-1 ). The AR had higher DOC (8.0 mg L-1 ) but lower chlorophyll a (154.6 AFU) when compared with the MR (7.1 mg L-1 and 172.4 AFU). Water temperature was constantly higher in the AR than in the MR, especially during the wintertime. Since the Mississippi-Atchafalaya River system is among the world's largest and most engineered river systems, our assessment offers a field case study to inform on the potential implications of reconnecting rivers with their floodplains networks.