Watershed Export of Dissolved Organic Matter in Response to Anthropogenic and Hydroclimatic Drivers in Subtropical Streams and Rivers

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Dissolved organic matter (DOM) is an assemblage of heterogeneous organic compounds that play important roles in terrestrial and aquatic ecosystems. In this dissertation, I investigated changes in the amount, source, composition, lability, and ecological functions of stream water DOM in response to agricultural land use, hydrological events, and downstream transport and transformation in the southeastern United States. The dissertation includes three stand-alone studies presented in Chapters 2, 3 and 4, respectively. In Chapter 2, I evaluated the effects of hurricane-induced storm events on the quantity and quality of DOM exported from ten watersheds of various physical and land-use characteristics within five Gulf and South Atlantic states. We found that large storms can significantly enhance the concentrations and yields of terrestrially-derived dissolved organic carbon (DOC) and nutrients in streams and rivers but decrease the percentage bioreactive DOC. This study demonstrates that extreme weather and climate events can lead to rapid, ecosystem-level disturbances that significantly shift energy and nutrient availability within drainage networks. The objective of Chapter 3 was to quantify the relative importance of agricultural land use and natural hydroclimatic drivers in affecting the quality and quantity of DOM in a group of 15 streams draining watersheds of a gradient of agricultural land use. The partial least square path modeling (PLS-PM) identified that agricultural land use increased stream water DOM quantity primarily through increasing allochthonous carbon sources. This study demonstrates that structural equation modeling is a powerful tool that should be more widely adopted to distinguish among multiple drivers and mechanisms regulating freshwater biogeochemistry. Chapter 4 investigated the longitudinal transformations of DOM in relation to ecosystem metabolism along a fluvial section including 3rd order, 7th and 8th order streams. From upstream to downstream, DOC concentrations and the relative contributions of freshly-produced DOM increased. The gross primary productivity was positively correlated with the contributions of autochthonous DOM, yet the ecosystem respiration did not vary with the quantity or quality of DOM. This study highlights the complexity of DOM transformations in relation to stream metabolism along the river continuum. Collectively, the three independent but connected studies reveal the complexity and sensitivity of inland water DOM in response to hydroclimatic and anthropogenic drivers. The findings provide new insights into potential shifts in energy and substrates exported across the terrestrial-aquatic boundary due to human activities and climate change and how these shifts can alter water quality and fluvial biogeochemical functions.

Electronic Thesis or Dissertation
Dissolved Organic Matter (DOM), EEM-PARAFAC, Extreme Weather and Climate Events, Freshwater Ecology, Land Cover and Land Use, Watershed Study