Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item INVESTIGATING WATERSHED-SCALE CONTROLS ON STREAMWATER NITRATE EXPORT USING STABLE ISOTOPES(2022) Bostic, Joel; Nelson, David M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dramatic increases in anthropogenic nitrogen inputs to watersheds over the past century have elevated riverine nitrate (NO3¯) loads, impairing downstream ecosystems. Impacts to receiving waters are largely determined by the amount and timing of streamwater NO3¯ export, and knowledge of the watershed-scale controls on spatiotemporal patterns of NO3¯ export is thus critical for effective mitigation. Land-use activities produce generalizable patterns of streamwater NO3¯ in specific watersheds but it remains unclear how land use might modulate more widespread nitrogen inputs, such as atmospheric deposition, and regulate temporal dynamics of streamwater NO3¯ export. To address these questions, I quantified nitrogen sources and inferred watershed-scale nitrogen cycling processes using stable nitrogen and oxygen isotopes and concentrations of NO3¯ in Chesapeake Bay watersheds. In my first chapter, I quantified streamwater export of atmospheric NO3¯ using triple oxygen isotopes (Δ17O) of NO3¯ in 832 streamwater samples collected from 14 sub-watersheds of diverse land use, nitrogen input rates, size, and lithology across two years during a range of hydrologic conditions. Results indicate that watersheds with either greater impervious surface areas or higher terrestrial nitrogen input rates associated with agricultural practices retain less unprocessed atmospheric NO3¯. I use these results to extend the kinetic nitrogen saturation conceptual model to atmospheric NO3¯ streamwater export and from forested to non-forested systems. In my second chapter, I used seasonal patterns of, and relationships between, NO3¯ concentrations, δ15N of NO3¯, and discharge in the same 832 samples to assess the relative importance of watershed-scale controls on spatiotemporal patterns of streamwater NO3¯ export. Surprisingly, similar seasonal patterns of δ15N-NO3¯ were measured across all watersheds. Similar seasonality of δ15N-NO3¯ suggests consistent temporal variation in biological processes, such as denitrification and/or assimilation, across diverse watersheds. In my third chapter, I used δ15N and Δ17O of NO3¯, as well as isotopes of water, to investigate NO3¯ source export in storm events relative to baseflow in two Baltimore County, Maryland, watersheds with contrasting land use. In the more developed watershed I found that storms had a disproportionate impact on atmospheric NO3¯ export, and the amount of NO3¯ deposited on impervious surfaces was approximately equivalent to the amount of atmospheric NO3¯ streamwater export during storms, while atmospheric NO3¯ exhibited approximately chemostatic behavior in the less developed watershed. These results highlight the importance of reducing hydrologic effects of impervious surfaces to limit atmospheric NO3¯ export, especially given predictions that increasing precipitation intensity will be associated with future climate change. In conclusion, my results demonstrate that land use modulates the retention of atmospheric NO3¯, but biological processes impart a consistent seasonal signal on streamwater NO3¯ irrespective of land use.Item EVALUATING RESTORATION POTENTIAL AND STORM SURGE ATTENUATION IN DITCHED AND UNDITCHED COASTAL MARSHES(2017) Lundberg, Dorothea June; Prestegaard, Karen; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effects of ditching on the hydrological regime and ecosystem services of ditched coastal marshes—as well as the effects of hydrologic restoration of these systems—have yet to be extensively studied. The goals of this project were (1) to determine differences between ecohydrological processes in Ditched and Unditched coastal marshes, (2) to determine the effects of ditch plugging restoration projects on Atlantic Coast and Chesapeake Bay marsh hydrology, and (3) to evaluate Hurricane Sandy storm surge in the coastal marshes. Two separate pairs of Ditched and Unditched marshes were used in this study. The paired sites were adjacent, with similar topography, vegetation, and tidal patterns. Data collection included hydrological properties such as ditch density, tidal stage, water table fluctuations; as well as soil properties. Soil properties were similar in Ditched and Unditched marshes, while ditched marshes had lower water table elevations than Unditched marshes. Ditch plugging restoration partially restored the hydrological regime. A comparison of Chesapeake and Atlantic coastal marshes during Hurricane Sandy indicated similar storm surge elevations, but shorter durations of inundation at the Chesapeake Bay marshes when compared with the Atlantic marshes.Item Effects of urbanization and infiltration-based watershed restoration on the hydro-ecology of headwater streams(2016) Fanelli, Rosemary Margaret; Palmer, Margaret; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Urbanization profoundly alters the hydrologic routing of a landscape resulting in the degradation of downstream aquatic ecosystems. To mitigate these effects, watershed managers implement infiltration-based storm water control measures (SCMs), designed to convert stormwater runoff into groundwater recharge. However, the ability of infiltration-based SCMs to restore hydrological processes, and to reverse damage to the downstream ecosystem, remains poorly understood. To address this research gap, I examined the hydro-ecological effects of urbanization and SCM implementation in 11 headwater watersheds spanning an urbanization-restoration gradient (4 forest, 4 urban-degraded, and 3 urban watersheds restored with SCMs) near Annapolis, Maryland, USA. Regenerative stormwater conveyances (RSCs) were the type of SCM examined in the study. I used high-frequency precipitation, stream stage, and baseflow discharge collected at the watershed outlets to develop metrics characterizing watershed storage and stream responses to precipitation. I then conducted water quality sampling, temperature monitoring, and quantified aquatic insect community composition in the downstream ecosystems. Finally, I employed high-frequency groundwater monitoring in one of the SCMs to identify potential mechanisms controlling their hydrological function. The hydrological effects of urbanization were clearly observed across the study watersheds, but only one of the three restored watersheds modulated hydrology (e.g., a larger minimum runoff threshold relative to the other urban watersheds). However, baseflow in this stream was low compared to the forested streams, suggesting that enhanced infiltration of stormwater runoff did not recharge storage zones that support stream baseflow. Aquatic insect diversity and the percentage of sensitive taxa declined with increasing urbanization, with no significant effect of restoration. Water quality remained poor in both urban-degraded and urban-restored streams, with higher conductivity values, lower dissolved oxygen, and warmer stream temperatures than in forested streams. These water quality issues likely hampered recovery of sensitive taxa in downstream ecosystems. Groundwater monitoring in one of the SCMs indicated that high runoff delivery rates from the watershed limited infiltration within the SCM, which allowed the conveyance of untreated runoff to the downstream channel. The centralized design of RSCs, and their placement in areas of topographic convergence above channel heads, likely limits their effectiveness for restoring hydrological processes to urban watersheds.Item Comparison of Hydrologic and Hydraulic Characteristics of the Anacostia River to Non-Urban Coastal Streams(2016) McDowell, Mallori; Prestegaard, Karen L; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Streams in urban areas often utilize channelization and other bank erosion control measures to improve flood conveyance, reduce channel migration, and overbank flooding. This leads to reductions in evapotranspiration and sediment storage on floodplains. The purpose of this study is to quantify the evapotranspiration and sediment transport capacity in the Anacostia Watershed, a large Coastal Plain urban watershed, and to compare these processes to a similar sized non-urban watershed. Times series data of hydrologic and hydraulic changes in the Anacostia, as urbanization progressed between 1939-2014, were also analyzed. The data indicates lower values of warm season runoff in the non-urban stream, suggesting a shift from evapotranspiration to runoff in urban streams. Channelization in the Anacostia also increased flow velocities and decreased high flow width. The high velocities associated with channelization and the removal of floodplain storage sites allows for the continued downstream transport of sediment despite stream bank stabilization.