Dissertations
Permanent URI for this collectionhttps://hdl.handle.net/1969.6/1139
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Browsing Dissertations by Author "Abdulla, Hussain"
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Item Determining source apportionment of Din and Don in a Gulf of Mexico watershed and airshed(2023-5) Qiu, Yixi; Felix, Joseph; Murgulet, Dorina; Abdulla, Hussain; Wetz, MichaelDissolved 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.Item Hydrological and biogeochemical controls on estuarine carbonate chemistry along a climate gradient(2022-12) Dias, Larissa Marie; Hu, Xinping; Abdulla, Hussain; Murgulet, Dorina; Felix, JosephIncreasing global atmospheric CO2 concentrations drive a net flux of CO2 into the oceans, mitigating the impacts of anthropogenic greenhouse gas emissions on the climate. This results in a reduction in pH and carbonate saturation state, a.k.a. ocean acidification, of marine waters. The acidified ocean water may advect into estuaries, leading to estuarine acidification. Many estuaries are highly sensitive to this acidification due to low buffer capacity. Because estuaries provide many important ecosystem services, alterations in their carbonate systems may have significant consequences on ecosystems and the economy. Despite the current understanding that estuaries may play a disproportionately important role in global air-sea CO2 flux, little is known about carbonate systems in subtropical estuaries. Further comprehension of estuarine carbonate systems is vital for quantification of the global carbon cycle. Specifically, subtropical estuaries in the northwestern Gulf of Mexico (nwGOM) exhibit a general long-term decrease in pH and total alkalinity (TA), with lower latitudes experiencing more extreme acidification than higher latitudes. In Chapter II, sediment cores and slurries from the semiarid Mission-Aransas Estuary of the nwGOM were incubated and surface waters were analyzed for contributions of biogeochemical processes to TA change. Changes in total TA as well as calcium and sulfate ion concentration were examined following known reaction stoichiometry. Ratio of TA: ion changes suggested that carbonate dissolution co-occurred with oxidation of reduced sulfur species, and the latter consumed TA during drought periods in Mission-Aransas Estuary. This biogeochemical (sulfide oxidation) TA consumption has been poorly studied yet may affect TA budget in other semiarid estuaries worldwide. In Chapter III, river alkalinity total load and concentration were calculated using the United States Geological Survey’s Fortran Load Estimator Program (LOADEST) and long-term trends in alkalinity and discharge of six major nwGOM rivers were determined. Stepwise multiple linear regression methods were used to generate models for predicting estuarine TA based on river alkalinity, year, and net evaporation (evaporation-precipitation). Some rivers were found to have long-term (multidecadal) declines in freshwater discharge, area-weighted alkalinity yield, of alkalinity flow-weighted concentration, with most declines occurring in the southern end of the study region. Freshwater flow-weighted alkalinity concentration (annual alkalinity load for an area divided by discharge) appeared in many of the predictive models for estuarine TA and may play a major role in regulating estuarine TA of the nwGOM. Methods for linking freshwater and estuarine carbonate dynamics are lacking in the scientific literature; this study provides a potentially useful approach for predicting estuarine carbonate chemistry based on freshwater quality and input. In Chapter IV, CO2 flux of the Trinity-San Jacinto Estuary (Galveston Bay) was calculated and compared to results from discrete samples for carbonate parameters. Inferences about spatial and temporal patterns in CO2 flux as well as ecosystem metabolism were made based on results. The Trinity-San Jacinto Estuary was found to be a net sink for atmospheric CO2, but with high seasonal and spatial variability. Specifically, large freshwater inflows in spring stimulated photosynthesis in the estuary, which increased the sink behavior. Seasons with less freshwater inflow resulted in higher heterotrophy and CO2 emission in some regions of the estuary. This research increases knowledge and research capacity in the nwGOM region on estuarine acidification and carbonate chemistry. Causes of acidification in major estuaries within the region were addressed along a latitudinal climatic gradient. This will aid with better management of fresh and estuarine water resources in the nwGOM. The results of this research will also clarify the role of semiarid, subtropical estuaries in the global carbon cycle and expand our range of knowledge on carbonate system analyses of estuaries.Item Photo-transformation of dissolved organic nitrogen in Baffin and Lavaca Bays, Texas, USA(2022-12) Shrestha, Sagar; Abdulla, Hussain; Murgulet, Dorina; Felix, Joseph; Silliman, JimDissolved organic matter (DOM) in the largest pool of reduced carbon represents one of the ocean's largest exchangeable reservoirs of organic matter. The estuarine system is considered a transitional zone between terrestrial and marine ecosystems where the consumption and transformation of organic matter through microbial and photochemical degradation. Apart from other DON sinks, photochemical degradation is one of the abiotic processes that could enhance the bioavailability of DON. For example, photo-ammonification via DON degradation can occur throughout the euphotic zone of the water column in coastal and marine waters and could link DON to significant NH4+ sources for primary production. However, despite this link between DON and the production of NH4+ fueling primary production, DON and its photoproducts are often overlooked in studies of the nitrogen budget in coastal areas. In this study, I investigate photo-transformation of DON in two semi-arid bays (Baffin Bay and Lavaca Bay) that received different DOM sources. Photo-ammonification rate was quantified in both bays and related to the change in the DOM optical properties and the transformation of DOM chemical composition that analyzed by state-of-the-art Orbitrap Fusion tripled mass spectrometer (OT-FTMS) coupled with both ion chromatography (IC) and ultraperformance liquid chromatography (UPLC) techniques. In Chapter 1, I study the photo-ammonification of surface DOM in seven stations at Baffin Bay, Texas. Baffin Bay is an inverse and hypersaline estuary ecosystem where the evaporation exceeds the freshwater input. DOM and inorganic nitrogen sources are more dominated by phytoplankton/microbes and benthic fluxes than terrestrial sources. Urban and agriculture land use/land cover classes are the most predominant within the surrounding watershed of Baffin Bay, which could be a significant source of nutrients in the bay. I observed a strong significant correlation (P > 0.05; R2=0.94) between NH4+ photoproduction and CDOM(a300) among the stations indicating the high presence of CDOM and the photodegrading responsible for the photoproduction of NH4+ in the bay. The average NH4+ photoproduction rate in Baffin Bay was 0.0472 ± 0.02 µmol. L-1h-1. I also used an electrospray ionization coupled with Orbitrap -Fusion Tribrid Mass Spectrometer (OT-FTMS) to quantify and identify the changes in the chemical composition of surface water from Baffin Bay. The relative abundance of CHON (52%) compounds was found to be higher in the pre-irradiation sample, followed by CHO (23%) and CHONS (9%) compounds. Overall, dissolved organic nitrogen (DON) formulas accounted for 72% of the total formulas detected and represented the largest class of unique formulas detected in pre-irradiated samples. After post-irradiation, molecular composition DOM was dominated by CHON (50%) compounds, followed by CHONS (18%) and CHO (14%) compounds. A higher abundance of nitrogen-containing compounds such as peptides and deaminated peptides in the initial sample was associated with the production of NH4+ by losing terminal anime groups during photooxidation. In Chapter 2, I studied the photochemical transformation of dissolved organic nitrogen (DON) in Lavaca Bay. Lavaca Bay is a secondary bay of Matagorda Bay located North of Port Lavaca, on the Gulf of Mexico. Lavaca River represents the primary freshwater source of Lavaca Bay. The average NH4+ photoproduction rate during 72 hours of irradiation was 0.0204 ± 0.02 µmol. L-1h-1. This photo-ammonification rate is less than half of the rate at Baffin Bay. The optical properties of Lavaca Bay indicated a decrease in molecular weight and aromaticity of DOM after solar irradiation. The transformations and composition of DOM were also investigated at the molecular level using UPLC-OT-FTMS (positive mode) and IC-OT-FTMS (negative mode). The relative abundance of CHON (45%) compounds was found to be higher in the pre-irradiation sample, followed by CHO (25%). After irradiation, molecular composition DOM was dominated by CHON (51%) compounds, followed by CHONS (18%) compounds. My results also suggest that molecular composition after irradiation was more oxygenated and saturated than pre-irradiated samples. This study also showed the increasing/decreasing trend of peptides and deaminated peptides during 72 hours of irradiation which resulted in the production of NH4+ and highlighted the importance of photooxidation as a potential source of NH4+ production in the estuarine system. In Chapter 3, I investigated the effects of photooxidation on sediment porewater dissolved organic matter (DOM) concentrations and molecular composition of Baffin and Lavaca Bays. I also observed a continuous increase in the production of ammonium (NH4+) in both Baffin Bay and Lavaca Bay, but the photoproduction rate was 51% higher at Baffin Bay as compared to Lavaca Bay. Changes in the molecular composition and aromaticity of DOM in both Baffin and Lavaca Bays were also observed by analysis of slope ratio (SR) and specific UV absorbance (SUVA). A closer look at the chemical composition, indicated that Baffin Bay has more labile porewater DOM responsible for photo-ammonification than Lavaca Bay. I found that Baffin Bay higher molecular diversity as represent with higher number of compounds than Lavaca Bay in both ionization modes. Elemental ratio and DBE analysis were also used to conclude that the molecular composition of Baffin Bay was more oxygenated and less aromatic after irradiation. The analysis of molecular composition indicates that 60% of the deamination of peptides was carried out by the oxidative deamination process and was dominated by aliphatic compounds such as glycine, alanine, proline, etc. A detailed analysis of the photo-transformation of peptides and deaminated peptides in Baffin and Lavaca Bays highlights the role of photodegradation of peptides and deaminated peptides as a possible contribution to photo-ammonification in coastal water and the potential degradation to individual amino acids and small organic acids.