Semester of Graduation

Spring 2019

Degree

Master of Science (MS)

Department

Renewable Natural Resources

Document Type

Thesis

Abstract

Rivers serve as an important medium for the exchange of elements between land, ocean, and atmosphere. This thesis consists of three interconnected studies with the overarching goal of analyzing the environmental factors influencing dissolved carbon dynamics and river geochemistry in large rivers. These studies utilized river water samples and in-stream measurements collected from the Mississippi and Atchafalaya Rivers at hourly to monthly intervals between 2013 to 2018, along with ambient river and meteorological data downloaded from public-access databases. Results indicate substantially higher dissolved organic carbon concentrations (DOC, 611 ±181 µmol L-1) but lower concentrations of dissolved inorganic carbon in the Mississippi River than those reported more than a decade ago, likely attributed to an increase in air temperatures and river discharge in recent years. Consequently, combined total dissolved carbon exports (16.2 Tg C) to the Northern Gulf of were greater than previously estimated. Mississippi River partial pressure of carbon dioxide (pCO2) measurements were strongly predicted by river water temperature and discharge, ranging from 526 to 3961 µatm, resulting in an average annual outgassing rate of 721 g C m-2 yr-1. Results also found Mississippi River pCO2 measurements followed a distinct diel cycle, with variation ranging by 206-607 µatm over a 24-hr period, due to the solar radiation-driven diel photosynthesis cycle. This cycle produced evening CO2 outgassing rates during spring and summer months greater than daylight rates (10 ±8% and 25 ±10%, respectively), suggesting a significant under-estimation of CO2 outgassing calculations in many large rivers. Of the thirty-one non-carbon elements analyzed in this study, Al, Ba, B, Ca, Fe, Mg, Mn, Ag, Si, Na, and Ti, demonstrated drastically different concentrations between the Mississippi and Atchafalaya Rivers due to differences in lithology, land use, and anthropogenic inputs. Loads of Al, Ba, B, Fe, Li, Mn, P, K, Si, Ag, Ti, V, and Zn regularly decreased from upstream to downstream of the Atchafalaya’s natural wetland floodplain system, suggesting the river’s corridor wetlands function as a sink for many riverine metals. Collectively, these studies demonstrate a strong influence of physical and biogeochemical controls on transport and transformation of terrigenous elements in large river systems.

Committee Chair

Dr. Y. Jun xu

DOI

10.31390/gradschool_theses.4871

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