ENVIRONMENTAL CONTROLS ON CARBON BIOGEOCHEMISTRY IN AQUATIC SYSTEMS

Abstract

Carbon dioxide (CO2) outgassing from inland waters is a small but critical component of the global carbon cycle that is currently not well constrained. This thesis is composed of three interrelated studies with an overarching goal to further determine environmental factors impacting carbon dynamics in rivers and lakes. These studies utilized water samples and in situ measurements of pCO2 and ambient water parameters from the Lower Mississippi River at Baton Rouge and University Lake in Baton Rouge Louisiana, USA collected between January 2021 and August 2022. Additional Meteorological and hydrological data were download from public sources. Results showed the seasonality of DOC and DIC exports was affected by their concentrations, water temperature, and discharge. DIC concentrations were significantly higher in the fall (32.0 mg L-1) than those during the winter (20.4 mg L-1), while DOC concentrations were highest during the winter months (11.3 mg L-1) and varied seasonally, however, not significantly. pCO2 in the MR averaged 1703 ± 646 µatm peaking in the summer at 2594 µatm and reaching a low in the winter at 836 µatm. Extreme winter weather in the Lower Mississippi River basin resulted in near record low water temperatures likely slowing biogeochemical cycling of carbon leading to increased DOC concentrations and lower CO2 evasion. The impact of the short-term weather event was seen on a seasonal scale with overall export of DIC nearly half that of the previous two years. Findings also showed that there was a clear diurnal fluctuation of pCO2 in the University Lake with the peak in the early morning and the lowest in the later afternoon in both seasons. In the winter, the lake water was mainly CO2-unsaturated in reference the atmospheric CO2 of 420 ppm, ranging from a low of 128 μatm in the mid-afternoon when solar radiation was highest to a high of 557 μatm in the early morning hours just before solar radiation was detected. Together these studies suggest environmental controls have a significant effect on the transport and cycling of carbon within river and lake systems.