Degree

Doctor of Philosophy (PhD)

Department

Renewable Natural Resources

Document Type

Dissertation

Abstract

Rivers and lakes play a key role in the global carbon cycle by transporting and transforming terrestrial carbon between the land, atmosphere, and ocean. However, the combined effects of hydrological variability, climate change, and human activity on riverine carbon dynamics remain unclear. This dissertation research explores dissolved carbon transport and CO₂ emissions in the Mississippi-Atchafalaya River System (MARS), a river-coastal lake continuum near the Gulf of Mexico, and effluent-derived dissolved carbon from wastewater treatment plants. The research comprises four studies that investigated riverine carbon transport and transformation, carbon dynamics in an estuarine lake, and carbon contribution of two wastewater treatment plants to the Mississippi River. The research was accomplished with biweekly to monthly field measurements, lab analysis for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) concentrations, and CO2 flux calculation. The research found large variability of dissolved carbon in the Mississippi River, with DIC (25.21 ± 5.51 mg L⁻¹), DOC (7.14 ± 2.50 mg L⁻¹), and pCO₂ (1,687 ± 828 μatm) responding to changes in discharge and temperature. On average, the river exported 13.00 Tg DIC and 3.80 Tg DOC year, and remained consistently CO₂ supersaturated (1,687 ± 828 μatm), resulting in an average CO2 emission rate of 8.65 ± 4.5 mmol m⁻² h⁻¹. During the 2021-2023 drought period, the Atchafalaya River also showed a significantly reduced pCO2 (850-5422 μatm) and CO2 outgassing rates (mean: 13.06 mmol m− 2 hr− 1) compared to previous wet years of 2019-2021 with lower DIC and DOC mass transport, likely due to reduced hydrological connectivity with floodplains. These findings suggest that temperature and hydrology largely influenced carbon transport, with higher exports during flood years and lower exports during drought conditions. As global temperatures rise, river systems are predicted to carry more carbon to coastal waters and produce more CO₂. In contrast, DOC were found always higher than DIC in a wetland adjacent brackish lake, Lake Maurepas (up to 13.85 ± 3.42 mg L⁻¹), compared to the upstream Amite River (7.92 ± 2.94 mg L⁻¹). The higher pCO₂ and CO₂ fluxes at the lake's south shore (1736 ± 1472 µatm; 11.29 ± 15.05 mmol m⁻² h⁻¹) where the river water enters than the side where the gulf water enters suggests that coastal wetlands not only supply significant DOC but also enhance carbon transformation and CO₂ release in freshwater-saltwater mixing zones. Additionally, effluent from two municipal wastewater treatment plants contributed significant human-derived carbon to the Mississippi River. The WWTP North had higher DIC and DOC concentrations (56.80 ± 16.51 mg L⁻¹; 29.52 ± 8.68 mg L⁻¹) than the upgraded WWTP South (42.64 ± 10.50 mg L⁻¹; 12.93 ± 3.68 mg L⁻¹). Annual discharges of about 3808 MT DIC and 1459 MT DOC highlight wastewater as a significant yet frequently overlooked source of carbon in riverine systems. Therefore, anthropogenic inputs, temperature, hydrological conditions, and wetland interactions all have a substantial effect on riverine carbon dynamics, indicating that future climate change will make carbon transport and CO2 emissions uncertain and intense.

Date

7-30-2026

Committee Chair

Yi-Jun Xu

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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Available for download on Tuesday, July 10, 2029

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