Impacts of 2021 Extreme Winter Weather on Carbon Transport and Evasion in the Lower Mississippi- Atchafalaya River System

During the 2021 winter season, near-record low water and air temperatures occurred in much of the Mississippi River Basin due to disturbances in the polar vortex. Little is known about the impact of extreme winter weather on carbon dioxide (CO2) evasion and dissolved carbon export in large rivers. As extreme weather events become more frequent due to global climate change, a better understanding of their impact on the seasonality of river carbon transport is increasingly significant. In this study, we hypothesize that extreme cold would result in a rapid decrease in river CO2 outgassing and that the effect is less prolonged in rivers with large floodplains. During the polar vortex period in 2021, we measured CO2 outgassing in the Lowermost Mississippi River (MR) at Baton Rouge and in its largest distributary Atchafalaya River (AR) at Morgan City that flows through a large swamp area. We analyzed the data in conjunction with previous CO2 outgassing records at the two locations. In situ measurements included partial pressure CO2 (pCO2), water temperature, dissolved oxygen (DO), fluorescence, colored dissolved organic matter (CDOM), and dissolved organic and inorganic carbon. Water samples were also collected for determining the level of dissolved organic (DOC) and dissolved inorganic carbon concentrations (DIC). We found that during the polar vortex winter (2020-2021), the water temperature dropped drastically to a near-record low of 2.7 C and remained lower than the previous year's average of 8 C for two months. Mirroring the decline in temperature, outgassing rates dropped in 2021, ranging from 95-150 mmol m-2d-1 in comparison to the previous years range of 120-250 mmol m-2d-1. Outgassing rates were higher in the AR compared to the MR across all sample trips. DOC/DIC concentrations were significantly affected by temperature, with DOC increasing from 6.39mg/L in 2020 to 11.9 mg/L during the polar vortex and DIC decreasing from 25.17mg/L in 2020 to 19.28 mg/L during the polar vortex. The findings validate our initial hypotheses, and we offer this as a case study to inform about the carbon transport and transformation in large rivers under sudden temperature drops.