Felix, JosephQiu, Yixi2023-08-282023-08-282023-5https://hdl.handle.net/1969.6/97223A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Coastal and Marine Systems Science.Dissolved organic nitrogen (DON) profiles and contributing sources are poorly characterized in estuarine systems despite studies showing it constitutes a significant portion of the total nitrogen pool and inputs. The characterization of dissolved inorganic nitrogen (DIN) and DON processing along the pathways of surface/subsurface flow and wet deposition is also not well constrained. This dissertation work used stable isotope techniques to complete a comprehensive investigation of DIN and DON profiles, transformations, and sources across the hydrosphere (ground, surface, pore and rainwater) of a semi-arid estuary system (Baffin Bay, TX). Rainwater directly deposited 5.2 kg N/(ha*yr) to the watershed and had relatively low concentrations and varied nitrogen isotopic compositions of NH4+ (~27 µM; -10-7‰), NO3- (~17 µM; -7-10‰), and DON (~11 µM; -7-18‰). Isotope mixing model results implied agriculture and vehicle emissions as the two primary competing sources to ambient NH3 in the atmosphere. Vehicles were the dominant contributors to NOx emissions, however intermittent sources (e.g., lightning and biomass burning) rivaled vehicle emission during different seasons. Vehicle, fertilizer, and marine emissions were primary contributors to atmospheric DON and higher secondary DON formation contributions were observed when agriculture and/or biomass burning emissions were more prevalent. Tributaries in the watershed had varied NH4+ (0-300 µM), NO3- (0-272 µM), and DON (1-302 µM) concentrations that can reach the bay through riverine discharge. Several DIN hotspots were found at downstream WWTP outfalls, indicating wastewater contamination, which was consistent with the DON and NO3- dual isotope mixing model results. Groundwater had high NO3- (~1064 µM) and DON (~65 µM) concentrations, which could reach the bay through subsurface discharge. The varied ?15N-NO3- (7-56‰) and ?18O-NO3- (4-25‰) values in groundwater evidenced varying processes (i.e., anaerobic denitrification, aerobic denitrification and nitrification) affecting the NO3- isotopic composition. Isotope mixing model results suggested septic effluent and agriculture as the main NO3- and DON sources in the groundwater. As a result of source loadings and subsequent processing along the surface and subsurface runoff transport pathways, Baffin Bay surface water had high DON concentrations but low NO3- (~0.3 µM) and NH4+ (~2.3 µM) concentrations. The high NO3- and/or NH4+ concentrations in the groundwater and tributaries might be further processed by dissimilatory NO3- reduction to NH4+, denitrification, nitrification, and assimilation along the pathways and/or in the bay. Surface water had ?15N-DON values of 8‰ and significant negative ?15N-DON: ln[DON] correlations were intermittently observed, suggesting DON consumption. Isotope mixing model results suggested wastewater and manure as the primary allochthonous sources and the autochthonous source can contribute 20-40% of DON to Baffin Bay. High NH4+ concentrations (~325 µM) and varied ?15N-NH4+ values (-9-20‰) were found in porewater. Porewaters displayed a negative NH4+: DON and a positive temperature: ln[NH4+] correlation and decreased ?15N-NH4+ values in the summer. This presumably indicated a temperature- or/and solar irradiance-dependent process (e.g., remineralization and photo-ammonification) producing NH4+ from DON/PON in the porewater. This study provides a comprehensive DIN and DON assessment within a coastal watershed and its associated nutrient delivery pathways. It provided insights to future nutrient control management and watershed restoration plans in the region. This isotopic approach and assessment to investigate N sources and processing can be applied to similar systems worldwide and our findings add to the body of work needed to enhance global understanding of N biogeochemical cycles.291 pagesen-USThis material is made available for use in research, teaching, and private study, pursuant to U.S. Copyright law. The user assumes full responsibility for any use of the materials, including but not limited to, infringement of copyright and publication rights of reproduced materials. Any materials used should be fully credited with its source. All rights are reserved and retained regardless of current or future development or laws that may apply to fair use standards. Permission for publication of this material, in part or in full, must be secured with the author and/or publisher.This material is made available for use in research, teaching, and private study, pursuant to U.S. Copyright law. The user assumes full responsibility for any use of the materials, including but not limited to, infringement of copyright and publication rights of reproduced materials. Any materials used should be fully credited with its source. All rights are reserved and retained regardless of current or future development or laws that may apply to fair use standards. Permission for publication of this material, in part or in full, must be secured with the author and/or publisher.Text