Theses and Dissertations from UMD
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Item MECHANISMS REGULATING GREENHOUSE GAS EMISSIONS AND SOIL CARBON STORAGE IN MID-ATLANTIC COASTAL PLAIN WETLANDS(2024) Stewart, Graham; Palmer, Margaret; Williams, Michael; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wetlands are key components of the global carbon (C) cycle, storing significant amounts of C while emitting methane (CH4), a greenhouse gas. As wetland restoration emerges as a potential climate mitigation strategy, understanding the factors that influence wetland greenhouse gas exchange across land uses is essential for aligning management with ecology and biogeochemistry. This dissertation investigates variability in soil organic carbon (SOC) storage and CH4 flux in mid-Atlantic Coastal Plain wetlands, focusing on the roles of hydrology, vegetation, and land-use history in shaping underlying ecosystem processes.In Chapter 1, I surveyed SOC stocks across neighboring least-disturbed wetlands with similar vegetation and hydrogeomorphology and found substantial variation. Hydrologic regimes and relative topography partially explained variability, highlighting the importance of landscape heterogeneity in determining wetland C storage capacity. In Chapter 2, I measured CH4 fluxes across five dominant vegetation patch types in a freshwater wetland using a multi-scale approach. I found that vegetation patches had distinct CH4 signals throughout the growing season, likely driven by differences in the mechanisms that regulate fluxes. The magnitude of the CH4 source was linked to patch identity, suggesting that CH4 fluxes were properties of patch types, and that a patch-explicit representation may be needed for modeling and estimating wetland greenhouse gas exchange. In Chapter 3, I explored the temporal dynamics of CH4 flux across wetlands with different land-use histories, identifying key biophysical drivers at multiple time scales. I found that after two decades, CH4 dynamics in a restored wetland appeared to have converged with those at a natural wetland and diverged with those at a cultivated former wetland. Together, these findings demonstrate the importance of acknowledging and accounting for the inherent variability and context-specificity in wetland C dynamics and suggest that wetland management and restoration for climate mitigation requires a detailed understanding of wetland ecosystem processes.Item Integrated Geochemical Studies of the Shuram Excursion in Siberia and South China(2024) Pedersen, Matthew; Kaufman, Alan J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Ediacaran Period Shuram Excursion (SE) is a globally-distributed and highly controversial phenomenon where over millions of years, sedimentary carbonates record δ13C values of -10‰ and lower. This carbon cycle anomaly may reflect disequilibrium in the world’s oceans, driven by the oxidation of a large pool of dissolved organic carbon (DOC), with the oxidants sourced from the intense weathering of the continents, forcing major changes to ocean chemistry through the ventilation of the deep ocean, evidenced by a positive shift in carbonate uranium isotope values, and invoking the onset of early animal biomineralization. This study utilizes high-resolution carbonate Li isotopes from two SE-successions, U isotopes, REE abundances and Ce anomalies which reveal the dynamic interplay between intensified continental weathering associated with tectonic reconfiguration and the subsequent environmental and ecological response that may have been amplified by the ecosystem-engineering abilities of a newly discovered sponge-grade animal.Item Methane Biogeochemistry and Microbial Communities in Natural and Restored Freshwater Depressional Wetlands(2024) Hamovit, Nora David; Yarwood, Stephanie A; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wetlands are the largest natural source of methane (CH4), a potent greenhouse gas. Wetland CH4 emissions are dependent on rates of microbial CH4 production (methanogenesis) and consumption (methanotrophy). These processes vary spatially and temporally with environmental conditions, edaphic characteristics, and microbial community structure, making it difficult to predict wetland CH4 emissions. This high variability can be further pronounced in restored wetlands that have undergone environmental and edaphic disturbances. The following work aims to understand this variability by assessing patterns of methanogenesis and methanotrophy, and their associated microbial communities, across natural and restored freshwater depressional wetlands on the Delmarva Peninsula (USA). Sites addressed in this work were restored from agricultural land between 1986 and 2004 through multiple programs funded by the United States Department of Agriculture (USDA). In the first set of experiments, we identified a high abundance of active acetoclastic methanogens in intact core incubations from a restored wetland suggesting a higher potential for methanogenesis in situ compared to the natural wetland assessed. The co-occurrence of active methanogens and Fe-reducing bacteria in these restored wetland cores contradicted the hypothesis that loss of competition may allow methanogens to be the primary users of acetate. Following assessments across vegetative-hydrologic zones in a series of restored wetlands of varying ages, and their natural counterparts, highlighted vegetation type and extent as a driver of methanogen community abundance, composition, and activity. In turn, restored wetlands showed elevated potentials rates of methanogenesis compared to natural sites. Potential rates of methanotrophy (aerobic and anaerobic), however, were also elevated in restored wetlands, which could constrain CH4 emissions in situ. Variability of environmental conditions (ie. hydrology and vegetation) and edaphic measures (ie. soil organic matter (SOM)) across all sites sampled are reflected in distinct microbial community composition and CH4 biogeochemistry. Clear patterns of SOC accumulation and CH4 biogeochemistry with restoration age were not observed for these wetlands, and variability in environmental conditions and edaphic measures across the sites (restored and natural), emphasize the need for continued monitoring and maintenance of the wetlands. Our results suggest efforts to manage herbaceous vegetation extent and maintain regular seasonal hydrology in future restorations may help prevent high potentials for CH4 production, and thus emissions.Item LONGITUDINAL STREAM SYNOPTIC (LSS) MONITORING TO EVALUATE WATER QUALITY IN RESTORED STREAMS(2023) Malin, Joseph Thomas; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many kilometers of streams are being restored in the Chesapeake Bay watershed and elsewhere in efforts to stabilize streambanks, protect infrastructure, and improve water quality. Urban development and impervious surface cover increase peak flows, which degrade streams. Restoration strategies often employ engineering approaches to enhance stream-floodplain reconnection, dissipate erosive forces from urban runoff, and enhance contaminant retention. In this study, longitudinal stream synoptic (LSS) monitoring (sampling multiple points along flowpaths across both space and time) was conducted to assess the effectiveness of different forms of stream restoration in attenuating pollutants downstream. Spatial and temporal monitoring of carbon, nutrients, salt ions, and metals were conducted across five watersheds experiencing varying levels of stream-floodplain reconnection and stormwater management within the Chesapeake Bay region. Study sites included Sligo Creek (minimal floodplain reconnection), Paint Branch (streambank stabilization without significant reconnection), Scotts Level Branch (engineered stream-floodplain reconnection), Little Paint Branch (natural floodplain reconnection from sedimentation), and Campus Creek (regenerative stormwater conveyance with engineered floodplain reconnection). We investigated: (1) whether changes in water chemistry can be detected along longitudinal flowpaths in response to stream-floodplain reconnection, and (2) which monitoring scales across space and time can provide useful information regarding the effectiveness of restoration. Results from this work suggest that longitudinal synoptic monitoring can track the fate and transport of multiple contaminants and evaluate restoration strategies across high spatial-resolution scales. Along all five watersheds, stream water chemistry varied substantially across finer spatial scales (sometimes within hundreds of meters) in response to changes in landscapes, restoration features, or local hydrology. There were significant declining concentrations (p<0.05) or stable concentrations of nutrients, salts, and metals as streams flowed through restoration features. There were significant increasing trends in chemical concentrations (e.g. Na+, Ca2+, K+) in unrestored stream reaches with increasing impervious surface cover. Principal component analysis (PCA) also indicated that there were changes in the chemical compositions of mixtures of salts, metals, and nutrients in response to restoration projects, storm events, and seasons. Interestingly, dissolved Fe and Mn concentrations showed significant increasing trends along some stream reaches with hydrologically connected floodplains. Fe and Mn also showed significant decreasing trends along some unrestored stream reaches surrounded by increasing impervious surfaces. Increased concentrations of dissolved Fe and Mn may have been an indicator of increased hydrologic connectivity between groundwater and surface water and decreased redox potentials. Overall, longitudinal water quality changes over meters and kilometers can be useful in detecting effects of stream restoration on water quality at the watershed scale. Results suggest that water quality in urban streams can change locally in response to restoration projects for multiple chemicals, but the incremental changes associated with different forms of stream restoration and riparian conservation can also be overwhelmed across broader watershed spatial scales and during storm events.Item MOBILIZATION OF CHEMICAL COCKTAILS BY FRESHWATER SALINIZATION SYNDROME IN THE CHESAPEAKE BAY WATERSHED(2023) Galella, Joseph George; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Increasing trends in base cations, pH, and salinity of urbanizing freshwaters have been documented in U.S. streams for over 50 years. These patterns, collectively known as Freshwater Salinization Syndrome (FSS), are driven by multiple processes, including applications of road salt and human-accelerated weathering of impervious surfaces, reductions in acid rain, and other anthropogenic legacies of change. FSS mobilizes chemical cocktails of distinct elemental mixtures via ion exchange, and other biogeochemical processes. Urban streams in temperate areas experience chronic salinization throughout the year punctuated by acute salinization during winter storms with associated road salting. My research analyzed impacts of FSS on stream water chemistry in the field with routine bi-weekly and targeted high frequency sampling during road salting events. Field sites were proximal to USGS stream sensors using multiparameter datasondes, allowing for additional parameters to be monitored at 5-15 minute resolution. In the laboratory incubation analyses were also conducted using sediment and water samples to assess the function of stormwater best management practices (BMPs) during road salting events. Acute FSS associated with road salting was found to mobilize chemical cocktails of metals (Mn, Cu, Sr²⁺), base cations (Na+, Ca²⁺, Mg²⁺, K⁺), nutrients (TDN), and organic matter (NPOC). Regression relationships were developed among specific conductance and major ion and trace metal concentrations. These linear relationships were statistically significant in most of the urban streams studied (e.g., R2 = 0.62 and 0.43 for Mn and Cu, respectively), and showed that specific conductance could be used as a proxy to predict concentrations of major ions and trace metals. Principal component analysis (PCA) showed co-mobilization (i.e., correlations among combinations of specific conductance, Mn, Cu, Sr²⁺, and all base cations during certain times of year and hydrologic conditions). Co-mobilization of metals and base cations was strongest during peak snow events but could continue over 24 hours after specific conductance peaked, suggesting ongoing cation exchange in soils and stream sediments. Increased salt concentrations of all three major road salts (NaCl, CaCl₂, and MgCl₂) had profound effects on major and trace element mobilization, with all three salts showing significant positive relationships across nearly all elements analyzed. Salt type showed preferential mobilization of certain elements. NaCl mobilized Cu, a potent toxicant to aquatic biota, at rates over an order of magnitude greater than both CaCl₂ and MgCl₂. Hourly mass fluxes of TDN in streams were also found to be elevated during winter months with peaks coinciding with road salting events. Targeted winter snow event sampling and high-frequency sensor data suggested plateaus in NO₃⁻ / NO₂⁻ and TDN concentrations at the highest peak levels of SC during road salt events between 1,000 and 2,000 μS/cm, which possibly indicated source limitation of TDN after extraction and mobilization of watershed nitrogen reservoirs by road salt ions. My results may help guide future regulations on road salt usage as there are currently no federally enforceable limits. NaCl is the most commonly used deicer in the United States, largely because it is often the least expensive option. Other technologies such as brines and other more efficient deicers (CaCl₂ and MgCl₂) should be considered in order to lessen the deleterious effects of FSS.Item MANAGEMENT OPTIONS FOR FARMERS FACING SALTWATER INTRUSION ON THE EASTERN SHORE OF THE CHESAPEAKE BAY(2023) Schulenburg, Alison Nicole; Tully, Kate; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rising sea levels, storms, and perigean spring tides push saltwater into coastal agricultural fields. This phenomenon, known as saltwater intrusion, alters nutrient cycling and damages crop yields. As sea levels continue to rise, saltwater intrusion will only worsen, with devastating consequences to agroecosystems along the coast of the Chesapeake Bay. Researchers and farmers alike are looking for solutions to adapt to and mitigate the effects of saltwater intrusion. Landowners may respond by altering their management practices. Farmers may 1) adapt by planting a salt-tolerant crop, 2) attempt to remediate soils with trap crops, 3) restore native marsh grasses, or 4) abandon fields altogether. My project investigates the survival of different crops and plant treatments under saltwater-intruded conditions and the potential for these plants to survive and to remove excess nutrients (e.g. sodium and phosphorus) from the soil, with the overall goal to benefit both the farming community and water quality in the Chesapeake Bay. Results from this study will help inform new management practices to increase soil health and maintain crop yields. Finally, the goal of this work is to guide local best management practices and potential easement opportunities for landowners facing saltwater intrusion, and ultimately determine optimal strategies for climate resilience.Item QUANTIFYING NITROUS OXIDE AND METHANE FLUXES USING THE TOWER-BASED GRADIENT METHOD ON A DRAINAGE WATER MANAGED FARM ON THE EASTERN SHORE OF MARYLAND(2022) Zhu, Qiurui; Davidson, Eric A.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Excess nitrogen resulting from agricultural fertilizer and manure applications on the Eastern Shore degrades the Chesapeake Bay's water quality and causes environmental issues such as algal blooms and "dead zones". Drainage water management (DWM) is an effective best management practice (BMP) to reduce hydrological nitrate export from croplands to surface and ground water by controlling the timing and the amount of ditch discharge and retaining water within ditches and adjacent fields using drainage control structures (DCS). While promoted denitrification in the subsurface and reduction in nitrate leaching are intended consequences of maintaining higher water table level, an unintended environmental consequence is possible production of nitrous oxide (N2O) from denitrification and methane (CH4) from methanogenesis, which are both potent greenhouse gases (GHGs). Whether the application of DWM leads to a "pollution swapping" concern (i.e., trading reduction of nitrate concentrations in ditch water for increases in emissions of N2O and CH4 to the atmosphere) is a question that must be addressed before more widespread implementation of DWM can be endorsed. In this dissertation, I employed a micrometeorological method called the flux gradient (FG) method to a corn-soybean rotation agricultural system with DCS in eastern Maryland on the Delmarva Peninsula to answer this question. This method was chosen because it allows near-continuous measurements of soil trace gas exchanges at multiple locations with a single laser spectrometer at a fine temporal resolution without disturbing the microclimate between soils and the atmosphere. Soil N2O and CH4 fluxes were quantified using the FG method on this drainage water managed farm for three consecutive years when no fertilizer, synthetic fertilizer, and biosolids were applied in 2018 (soybean), 2019 (corn), and 2020 (corn), respectively. Statistical tests indicated that there were no consistent treatment effects of DWM on soil GHG emissions between DWM and non-DWM conditions, suggesting that DWM did not trade the intended consequence of reduced nitrate leaching for the unintended consequence of increased soil GHG emissions. The biosolid addition in 2020 led to the largest N2O emissions among the three years, while the lowest N2O emissions in the growing season were found in the unfertilized soybean year of 2018. In contrast, different fertilization regimes did not yield distinct differences between the three years for CH4 fluxes. In addition, some potential methodological concerns associated with this tower-based micrometeorological approach were addressed and resolved, conferring confidence that the FG method can be applied simultaneously to multiple plots for N2O and CH4 measurements. This research adds to the existing understanding of the impacts of DWM on soil GHG emissions and suggests that this BMP could be applicable in other regions of the Chesapeake Bay as well as other watersheds. This work also contributes to the efforts of studying the impacts of soil organic amendments on soil GHG emissions and deriving improved estimates of emission factors (EFs) for organic amendments.Item Freshwater salinization syndrome limits management efforts to improve water quality(2022) Maas, Carly Marcella; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Freshwater Salinization Syndrome (FSS) refers to the interactive effects of salt ions on the degradation of the natural, built, and social systems. FSS can mobilize chemical mixtures, termed ‘chemical cocktails’, in watersheds. The formation of chemical cocktails across space and time depends on the amounts and types of salt pollution, the surrounding land use including conservation and restoration areas, and the location along the flowpath in the watershed. We investigated (1) the formation of chemical cocktails temporally and spatially and (2) the natural capacity of watersheds and streams to attenuate salt ions along flowpaths with conservation and restoration efforts. We monitored high-frequency temporal and longitudinal spatial chemical changes in stream water in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management efforts (i.e., national parks, regional parks, and floodplain reconnection) in six urban watersheds in the Chesapeake Bay region. There were significant relationships between watershed impervious surface cover and mean concentrations of salt ions (Ca2+, K+, Mg2+), metals (Fe, Mn, Sr2+), and nutrients (total dissolved nitrogen) (p < 0.05). Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths in response to winter road salt applications were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails that were less enriched in salt ions and trace metals were attenuated downstream. There was also downstream attenuation of FSS ions during baseflow conditions through management efforts including a regional park, national park, and floodplain restoration. Conversely, chemical cocktails that formed in response to multiple road salt applications or prolonged road salt exposure did not show patterns of attenuation downstream. The spatial patterns were quite variable, with increasing, plateauing, or decreasing patterns based on the magnitude, timing, duration of road salt loading, and extent of management efforts. Our results suggest that FSS can mobilize multiple contaminants along watershed flowpaths, however, the capacity of current watershed management strategies such as restoration and conservation areas to attenuate FSS is limited.Item Insights into benthic macroinvertebrate ecology in the northern Bering and southern Chukchi Seas from stable isotope analysis(2022) Green, Emma Mackenzie; Cooper, Lee W; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the Pacific Arctic Region, the northern Bering Sea and southern Chukchi Sea support large and diverse communities of benthic macroinvertebrates that provide an important link to the pelagic communities and marine mammals that rely on the benthic populations for food. While the abundance and biomass of these benthic macroinvertebrates are well documented, little is known about how benthic macroinvertebrates interact with each other and how these interactions are affected by climate change. I measured the stable isotope composition (bulk δ15N and δ13C values) of similar species collected in 2014, 2016, 2017, and 2021 in the northern Bering and southern Chukchi Seas. Although there was little change over time in either δ15N or δ13C values, both stable isotope ratios were significantly different between stations with differing production phenologies. The southern Chukchi Sea (a productive set of sites with high chlorophyll concentrations throughout the summer) had lower δ15N values and higher δ13C values, while the northern Bering Sea site with production mostly associated with the period of sea ice breakup had higher δ15N values and lower δ13C values. This pattern was observed across similar species and feeding types. The higher δ15N values in the northern Bering Sea could be due to an extra step in the food chain from bacterial reworking. The contrast between these two regions in δ13C might indicate higher primary production in the southern Chukchi Sea compared to the northern Bering Sea. The differing food web dynamics between these two sites highlight the benthic diversity across small scales and similar organisms in Pacific Arctic food webs.Item EVALUATING CARBON SEQUESTRATION POTENTIAL OF NATURAL AND RESTORED TIDAL MARSHES IN CHESAPEAKE BAY THROUGH QUANTIFICATION OF METHANE FLUXES AND IDENTIFICATION OF DRIVERS(2022) Hanacek, Daniella; Staver, Lorie; Cornwell, Jeffrey; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The production of methane in brackish marshes may offset the carbon sequestered by these wetlands. Brackish tidal marshes are widespread in Chesapeake Bay and there exists a need for understanding the carbon balance of these ecosystems. This thesis presents the results of measurements of methane flux, through static flux chamber experiments, and analysis of marsh porewater to examine biogeochemical and plant-mediated drivers of methane flux in marshes of Chesapeake Bay. In addition, there is growing interest from the scientific and resource management community in how natural marshes cycle carbon and whether restored marshes show biogeochemical similarities. Therefore, I tested my hypotheses in the natural marshes of Monie Bay, part of the Chesapeake Bay National Estuarine Research Reserve – Maryland, and in restored tidal marshes created with dredged sediments at Poplar Island. Methane emissions offset annual carbon storage at Monie Bay and Poplar Island by 0.7 and 2.1 percent, respectively, based on average values of annual fluxes. However, there remains uncertainty in the accuracy of this estimate given the spatial and temporal variability in my observed fluxes, and the limited sampling frequency and spatial extent of my study. Within such uncertainty lays a justification for continued long-term monitoring of methane emissions in restored and natural marshes of Chesapeake Bay to resolve this important marsh management question.