Doctor of Philosophy (PhD)
School of Renewable Natural Resources
Studying the biogeochemical connectivity between rivers and lakes can help us understand their ecological and environmental impacts within a drainage basin, which is especially true for forest watersheds that play a vital role in provisioning freshwater services to ecosystems and downstream communities. This dissertation research consists of three interconnected studies with the overarching goal of discerning the connectivity of elements in a river-lake continuum across forest-dominated landscapes. These studies utilized water samples and in situ measurements collected from the Little River-Catahoula Lake continuum in the subtropical Louisiana, USA at monthly intervals during 2015-2016 and 1978-2008 historical water quality, hydrological and meteorological data downloaded from public-access databases to determine whether the forestry best management practices (BMPs) were effective in reducing levels and loads of sediment and nutrients from forest-dominated river headwaters and investigate the transport of dissolved carbon and metals from a river upstream to the outlet of its downstream receiving lake. Results show that forestry BMPs were effective in reducing sediment runoff from the intensively managed forested headwaters but less effective in controlling stream nitrogen concentrations and loading. Phosphorus loading at the basin outlet was significantly increased, which was probably caused by a drastic increase in the application of phosphorus fertilizer after extensive BMPs implementations. The dissolved organic carbon (DOC) pool in the studied watershed was dominantly terrestrially derived, while autochthonous DOC production derived from aquatic phytoplankton during the warm productive period was also important. In contrast, dissolved inorganic carbon (DIC) in the studied river-lake continuum was mainly from carbon-13 isotope (13C) depleted sources such as soil respired CO2 and in situ organic matter, and the combined effect of metabolism and carbon dioxide outgassing controlled the DIC dynamics in the in-network lake. For all metals analyzed in this study, the river functioned as a sink for Al, Ca, Fe, K, Mg, Na, B, Ba, Mn, Sr and Ti due to sedimentation and biological immobilization, while the lake acted as a source for Al, Mg, K and Ti due to their enrichments in the lakebed, a greater weathering intensity at the lake and backwater effects.
Xu, Zhen, "Element Transport in A River-lake Continuum across Forest-dominated Landscapes: A Case Study in Central Louisiana" (2020). LSU Doctoral Dissertations. 5181.