Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
6 results
Search Results
Item A numerical investigation of variability in particulate organic matter transport and fate, phytoplankton and primary production, and denitrification in a partially mixed estuary(2020) Wang, Hao; Hood, Raleigh; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In Chesapeake Bay substantial quantities of organic matter are produced during the spring bloom, which contributes to severe chronic bottom oxygen depletion during the summertime. However, the details of this transport in the estuarine system under realistic forcing is still unclear. In this Research, a three-dimensional model was used to investigate the production, transport, and fate of organic matter in Chesapeake Bay. Analysis of a control volume in the deep channel revealed that the sinking flux of fast-sinking particulate organic nitrogen (PON) into the deep channel is comparable to the horizontal advective transport. The model analysis also revealed a pronounced east to west transport of PON during the springtime and a tendency to export mass from the eastern shore to the deep channel and from the deep channel to the western shore of the Chesapeake Bay, and also a convergence of mass transport on the western shore. This transport is consistent with the lateral estuarine circulation in Chesapeake Bay that arises due to the asymmetry of the flood-neap tidal cycle. In addition, the model revealed that seasonal variations in wind alter the magnitude and distribution of organic matter flux in the along channel and cross channel direction, with northerly winds during the springtime favoring more northward organic matter transport and more organic matter accumulation in the deep channel, however, the lateral net flux direction remains the same. In Chesapeake Bay, phytoplankton biomass typically peaks in spring whereas primary production peaks in summer. For this to happen, phytoplankton growth rates must be low in spring and high in summer and very likely there must be low grazing losses in spring and high grazing losses in summer as well. In this research, a three dimensional coupled physical-biological model is used to explore how these seasonal patterns in phytoplankton and primary production arise during the year from 2000 to 2005. It is shown that with the seasonal variation of maximum carbon to chlorophyll ratio, temperature control on phytoplankton growth, and temperature-dependent zooplankton grazing effects, my model can capture the spring peak in phytoplankton biomass and the summer peak in the primary production, agreeing well with the observations. The model simulates high phytoplankton growth rates in the summer, with the maximum growth rates occurring in late summer. The model also reveals that nutrient supply shifts from river-derived nitrate in the springtime to organic matter- derived ammonium during summer. The simulation results also reveal that a substantial fraction of the ammonium that supports the high summer production is derived from allochthonous transport rather than autochthonous ammonium production. The transport process provides as large as 50% ammonium needed for uptake during summertime in the mesohaline Chesapeake Bay. My research also confirms the importance of nutrient recycling in supporting high summer production in Chesapeake Bay. Denitrification is an essential process in the marine nitrogen cycle because it removes bioavailable nitrogen from the aquatic system. Current understanding of denitrification variability in Chesapeake Bay is severely constrained by the sparse observations that provide insufficient coverage in both space and time. In this research, denitrification variability is examined in the Chesapeake Bay using a three dimensional coupled physical-biogeochemical model based on the Regional Ocean Modelling System (ROMS). Model simulations indicate that denitrification occurs not only in the sediment but also in the water column at significant, though somewhat lower rates. Model results indicated that the water column accounts for around 7.5% of the total denitrification amount that occurred in the system during the 2001 and 2002 period of this study. This conflicts with the historical assumption that water column denitrification in Chesapeake Bay is negligible. The model also reveals the spatial patterns in denitrification with more denitrification occurring in the upper to middle bay due to higher availability of organic matter in these areas compared to the lower bay. In terms of temporal variability, denitrification peaks in the sediment in spring while in the water column it peaks in the summer. The reason for this difference in the timing is related to the availability of oxygen: In the spring oxygen levels in the water column are too high to allow denitrification so it happens only in the sediment where low oxygen levels persist all year around. In summer low oxygen and depletion of nitrate below the pycnocline completely shuts down denitrification in the sediment in the mesohaline and polyhaline region of the by. However, water column denitrification continues at the interface between oxygenated waters near the surface and oxygen-depleted waters below where coupled nitrification-denitrification happens. The model also reveals that denitrification removes significant quantities of biologically available nitrogen, meaning that without this process, more summertime primary production would occur in the form of more surface chlorophyll, increasing as much as 10ug/L in the middle bay region, which would, in turn, lead to more oxygen depletion.Item Nitrogen Uptake and Denitrification in Restored and Degraded-Urban Streams: Impacts of Organic Carbon and Integrated Stormwater Management(2015) Newcomer Johnson, Tamara Ann; Kaushal, Sujay S.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Managing the N cycle and restoring urban infrastructure are major challenges especially in urban ecosystems. Organic carbon is important in regulating ecosystem function and its source and abundance may be altered by urbanization. My research focused on urban-degraded, restored, and forested watersheds at the Baltimore LTER in the Chesapeake Bay watershed. In Chapter 2, I investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian-stream interface. Denitrification enzyme assay experiments showed carbon was limiting in hyporheic sediments and variable carbon sources (grass clippings, decomposing leaves, and periphyton) stimulated denitrification differently. Evidence from stable isotopes, molar C:N ratios, and lipid biomarkers suggested that urbanization can influence organic carbon sources and quality in streams, which may have substantial downstream impacts on ecosystem services such as denitrification. In Chapter 3, I investigated whether stormwater best management practices (BMPs) integrated into restored and degraded urban stream networks can influence watershed N loads. I hypothesized that hydrologically connected floodplains and stormwater BMPs are “hot spots” for N retention through denitrification because they have ample organic carbon, low dissolved oxygen levels, and high residence time. I used reach-scale nitrogen mass balances, in-stream tracer injection studies, and 15N in situ denitrification to measure N retention in stormwater BMPs and their larger stream networks. There were high rates of in situ denitrification in both stormwater BMPs and floodplain features. Hydrologically connected floodplains can be important “hot spots” for N retention at a watershed and stream network scale because these areas likely receive perennial flow through the groundwater-surface water interface during both baseflow and storm events, while BMPs only receive intermittent flow associated with storm events. In Chapter 4, I conducted a literature review of N retention within hydrologically reconnected streams and floodplains. I reviewed 79 stream and floodplain restoration empirical studies from North America, Europe, and Asia and found that methods for measuring N retention varied considerably. I found many diverse strategies for promoting the ecosystem function of N retention in urban and agricultural watersheds.Item Advanced Denitrification in Bioretention Systems Usinging Woodchips as a Primary Organic Carbon Source(2013) Peterson, Ian James; Davis, Allen P; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bioretention systems still lack the ability to effectively mitigate nitrogen concentrations from urban stormwater. Column tests were conducted to evaluate the effect of nitrate concentration, stormwater retention time, limestone addition, and woodchip species, size, and mass percentage on the bioretention denitrification process. Denitrification of artificial stormwater appeared to follow pseudo-first-order kinetics. A 0.8 day average retention time showed the highest nitrate removal percentage of 82.4 + 0.4%. Longer retention times correspond to greater removal efficiency. Willow Oak and Red Maple woodchips resulted in the highest total nitrogen removal efficiencies at 61.9 + 0.8% and 61.8%, respectively. Smaller woodchips and higher woodchip mass percentage corresponded to greater nitrate removal efficiencies, but also higher organic nitrogen leaching. Media containing 4.5% 5 mm Willow Oak woodchips by mass represented optimum conditions with a pseudo-first-order denitrification rate of 4.1 + 4.6 day-1 with nitrate concentrations of 1.5 to 4.5 mg/L N.Item DYNAMICS OF METABOLIC GASES IN GROUNDWATER AND THE VADOSE ZONE OF SOILS ON DELMARVA(2011) Fox, Rebecca Jane; Fisher, Thomas R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Denitrification removes nitrogen from watersheds under reducing conditions, but N2O and CH4, both greenhouse gases, can also be produced. The overarching hypothesis of my thesis was that hydric environments accumulate N2O and CH4 in groundwater and the vadose zone. To test the hypothesis, groundwater samples were taken monthly during 2007-2009 at 64 piezometers in 10 wetlands for analysis of excess N2, N2O, CH4, and CO2. Vadose zone gas and groundwater samples were taken during 2008-2010 at two riparian buffers and a hydrologically restored wetland. The hydrology of the 10 locations was complex. A hydrologic connection across a transect was determined at one location where NO3- significantly decreased, excess N2 significantly increased, and moderate concentrations of N2O and CH4 accumulated. Within these 10 locations, three N2O and four CH4 hot spots were identified, and hot moments accounted for a large percentage of total accumulated N2O and CH4. I found evidence of CH4 ebullition, the production of CH4 bubbles in the vadose zone that strip other dissolved gases. The locations that accumulated the most dissolved CH4 and N2O were natural wetlands and riparian areas, respectively. I measured both positive and negative excess N2 concentrations in the vadose zone. Flux estimates ranged from -600 to 880 kg N ha-1 yr-1, which brackets missing N estimates at the watershed scale. These concentrations were calculated using N2/Ar, and both gases are affected by physical processes. These calculated excess N2 profiles could have been produced through either biological and/or physical mechanisms, and these processes currently cannot be distinguished. Less than 1% of the missing N on the transect scale, measured as the difference in N concentration between two piezometers, was accounted for by calculated diffusional fluxes from groundwater to the vadose zone. The primary mechanism transporting gases from the vadose zone to the atmosphere was diffusion, but convection transported 20% of the calculated median CO2 yearly flux. Increased production of N2O and CO2 was observed in the vadose zone after rainfall events. Overall, large concentrations of N2O, CH4, CO2, and excess N2 accumulated in the groundwater and vadose zone of these locations, supporting the overarching hypothesis.Item NITROGEN CYCLING AND CONTROLS ON DENITRIFICATION IN MESOHALINE SEDIMENTS OF CHESAPEAKE BAY(2009) Owens, Michael Sean; Cornwell, Jeffrey C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrogen is a key nutrient in the eutrophication of coastal and estuarine systems. In shallow water systems, sediment recycling can be an important source of nutrients for phytoplankton growth. The balance between nitrogen recycling and denitrification regulates the importance of sediments as a nitrogen source. To assess controls on denitrification, we conducted intensive seasonal measurements of sediment water exchange and denitrification using sediment core incubations. Peak rates of denitrification were observed in fall and spring (>100 μmol N-N m-2 h) followed by a decrease to 10 μmol N m-2 h in summer. Although denitrification rates were stimulated by labile organic carbon additions from the water column, the overall efficiency of the process sharply declined as temperature increased and bottom water O2 declined. Macrofauna activity was shown to enhance sediment transport of O2 by >5 fold, increase organic matter decomposition and maintain a high rate of denitrification efficiency.Item Nutrient Removal by Tidal Fresh and Oligohaline Marshes in a Chesapeake Bay Tributary(2005-11-22) Greene, Sarah E; Boynton, Walter R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Located at the interface between estuaries and surrounding uplands, tidal marshes are in position to receive and transform material from both adjacent systems. Of particular importance in eutrophic estuarine systems, tidal marshes permanently remove nutrients via two mechanisms - denitrification and long-term burial. Denitrification was measured (monthly) in two marshes in a Chesapeake Bay tributary for 7 months, using the MIMS technique. Burial of nitrogen (N) and phosphorus (P) was measured using 210Pb techniques. Strong spatial and temporal patterns emerged, and there was a Michaelis-Menten type response in denitrification rates to experimentally elevated nitrate levels. Denitrification rates measured may account for removal of 22% of N inputs to the upper estuary on an annual basis. Burial rates could account for 30% of N inputs and 60% of P inputs. Based on the cost of nutrient control technologies, Patuxent marsh nutrient removal may be valued at $10 to 30 million yr-1.