UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

Browse

Subject: search.filters.subject.dissolved organic matter

Search Results

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    ECOLOGICAL SIGNIFICANCE OF DISSOLVED ORGANIC MATTER COMPOSITION AND REACTIVITY IN DEPRESSIONAL FRESHWATER WETLANDS
    (2022) Armstrong, Alec William; Palmer, Margaret; Gonsior, Michael; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Dissolved organic matter (DOM) plays a central role in the biogeochemistry of aquatic ecosystems and is an important flux of carbon (C) from terrestrial to aquatic systems. Wetlands are rich sources of DOM to downstream waters, but the origins of wetland DOM and its role in biogeochemical processes in wetlands and downstream are not fully understood. To better understand the role of wetlands in mediating the movement and transformation of organic matter between terrestrial and aquatic ecosystems, I characterized the chemical composition and the microbial and photochemical reactivity of wetland DOM in a depressional wetland setting in the interior Delmarva Peninsula. I used laboratory experiments to understand DOM reactivity. I characterized sensitivity to photodegradation, concluding most wetland DOM was somewhat sensitive though site differences affected sensitivity. In another experiment, wetland DOM showed little biodegradability, but C losses to microbes were enhanced after photodegradation. This suggested photochemical and biological degradation may have interacted to influence wetland DOM composition within wetlands and in downstream waters. I also found terrestrial sources of DOM (plant and soil leachates) were more biodegradable than wetland surface water. I concluded wetland DOM was largely comprised of leftover material from previous microbial metabolism in soils or wetland water. To characterize wetland DOM and explore its environmental influences, I undertook a field sampling campaign of 22 wetlands over 18 months. Samples were characterized using a suite of DOM measurements, and variability in these data was modeled using water level, regional air temperature, a proxy for site canopy cover, estimated photosynthetically active radiation, and others. DOM varied considerably seasonally and among sites, and modeling suggested that complex seasonal and site-related interactions influenced DOM, not including water level. This research indicates that depressional freshwater wetlands accumulate and process DOM, some of it likely originating from soils and some within wetlands, but spatial and seasonal variability lead to DOM variability. Wetland DOM exported to downstream waters has intrinsically low biodegradability, though this may be enhanced by photodegradation downstream. This research may be useful for efforts to improve representation of depressional freshwater wetlands in mineral soils in C cycle models and inform policy concerned with wetland biogeochemical functions and connections with downstream waters.
  • Thumbnail Image
    Item
    A Deuterium Labeling Method for the Characterization of (Chromophoric) Dissolved Organic Matter Using Ultrahigh Resolution Electrospray Ionization Mass Spectrometry
    (2015) Baluha, Daniel Robert; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Dissolved organic matter (DOM) is a complex ensemble of naturally occurring organic compounds found in virtually all aquatic environments. The overwhelming diversity of DOM makes it extremely difficult to understand the relationship between its bulk physicochemical properties and its molecular structure and composition. This dissertation describes the development of a novel method to identify ketone/aldehyde-containing species within DOM, which are known to contribute substantially to the ultraviolet/visible (UV-vis) absorption and emission of chromophoric DOM. In this method, an aqueous sample is treated with sodium borodeuteride (NaBD4) and is analyzed via ultrahigh resolution electrospray ionization (ESI) mass spectrometry. Ketone/aldehyde-containing species (at mass m) in the untreated sample are identified by searching the mass spectrum of the reduced sample for peaks corresponding to deuterated derivatives (at mass m+3.021927n). Initial experiments demonstrated that this method reliably discriminates among mass spectral peaks in an untreated DOM sample that comprise species with zero, one, and/or two reducible moieties. The reactivity and optical properties of reducible species within Suwannee River fulvic acid (SRFA) were studied by treating an aqueous sample with several amounts of NaBD4. This study demonstrated that most species with at least one ketone/aldehyde moiety were reduced a single time under low [NaBD4], while higher [NaBD4] resulted primarily in additional reductions on multi-ketone/aldehyde species. Furthermore, the changes in UV-vis absorption and emission of the reduced aliquots relative to that of the untreated were correlated with the number of ketone/aldehyde-containing species reduced and identified by this method. The fully developed protocol was used to compare DOM extracted from several aquatic environments. Two pools of ketone/aldehyde-containing species were tentatively identified: A terrestrially-produced group of lignin/tannin-derivatives and a microbially-produced group of carboxyl-rich alicyclic molecules. While the first pool has previously been shown to contribute substantially to the absorption/emission of chromophoric DOM, the second pool most likely would not. The mass labeling method developed here revealed compositional features that are not observable by common ESI mass spectrometric analyses and may serve as a useful way to link the physicochemical properties of DOM to its structure and composition.
  • Thumbnail Image
    Item
    Plant-sediment Interactions and Biogeochemical Cycling for Seagrass Communities in Chesapeake and Florida Bays
    (2007-12-17) Nagel, Jessica; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Seagrasses are prominent, productive components of shallow coastal ecosystems worldwide. The role of seagrasses in biogeochemical cycling varies widely across ecosystems, and this is due in large part to the complex interactions and feedbacks among processes controlling dynamics of carbon, oxygen, nutrients, and dissolved organic matter (DOM). This dissertation examines the importance of the keystone seagrass species, Thalassia testudinum, to biogeochemical cycling at the community and ecosystem levels in Florida Bay. The research presented here also describes the consequence of disturbances, such as shifts in species composition and seagrass dieback, on biogeochemical processes in both Florida and Chesapeake Bays. In Florida Bay, T. testudinum was shown to stimulate sediment microbial activities and benthic production of oxygen, inorganic nitrogen, and DOM relative to adjacent benthic communities without seagrass but containing benthic microalgae. Strong diel patterns in net fluxes of these solutes in both communities underscore the importance of photosynthesis. Ecosystem-level production (P) and respiration (R) rates were also enhanced in T. testudinum communities. Clear seasonal and regional variations in P and R were evident across Florida Bay, with lowest rates reported in the northern regions. Seagrass dieback had a negative effect on sediment nitrification rates and net ecosystem production (P-R) at one site in Florida Bay, and loss of seagrass habitat may result in significant changes to biogeochemical budgets within this system. In mesohaline Chesapeake Bay, the ephemeral submersed plant species, Ruppia maritima was also shown to stimulate organic production, nutrient cycling, and sediment biogeochemical processes compared to benthic communities without seagrass; however, the more persistent native species, Potamogeton perfoliatus, had an even greater impact on these processes. Collectively, the results of this research reveal the potential significance of seagrass to biogeochemical cycling in Chesapeake and Florida Bays and suggest that disturbances, such as seagrass dieback or shifts in species composition, may substantially alter biogeochemical budgets within these systems.
  • Thumbnail Image
    Item
    THE ROLE OF ORGANIC MATTER IN THE DISSOLVED PHASE SPECIATION AND SOLID PHASE PARTITIONING OF MERCURY
    (2006-01-24) Miller, Carrie Lynn; Mason, Robert; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The interaction of mercury (Hg) and methylmercury (MeHg) with organic matter is extremely important in the dissolved phase speciation and solid phase partitioning of Hg and MeHg in aquatic systems. This study shows, that under oxic conditions Hg and MeHg will likely associated with Fe oxides through an indirect association with organic matter, while under sulfidic conditions, solid phase Fe sulfide will dominate the complexation of Hg to the solid phase. As a result of the association of Hg with Fe solids, which undergo dynamic changes at redox interfaces in aquatic systems, the distribution of Hg on particles is likely changing at redox boundries, areas that have been shown as active zones of methylation. Redox zones are also going to be important in controlling the mobility of MeHg from the site of production to areas in aquatic systems in which uptake by biota occurs. Although the dissolved phase speciation of Hg has been shown as an important factor in Hg methylation, as a result of the diffusive uptake of neutral Hg-sulfide into bacterial cells, this speciation had previously not been measured. Hg forms stronger bonds with reduced sulfide relative to dissolved organic matter (DOM), therefore, it was not previously thought that DOM was important in the speciation of Hg under sulfidic conditions. Using modified octanol-water partitioning extractions and centrifugal ultrafiltration, the speciation of Hg in sulfidic natural samples and laboratory solutions was examined. It was shown that the concentration of neutral Hg-sulfide complexes are lower than predicted by thermodynamic models, as a result of an interaction of these species with DOM. It is proposed that the interaction of Hg with DOM is not a complexation, but rather, a partitioning of neutral Hg-sulfide complexes into hydrophobic portion of the DOM. Thermodynamic constants were calculated for this interaction and applied to model the speciation of Hg in natural samples. The concentration of neutral Hg-sulfide is lower than models previously predicted, as a result of the DOM interaction. Since the concentration of neutral Hg-sulfide affects methylation, DOM could impact the rate of Hg methylation in aquatic systems.