Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item The long-term change of Chesapeake Bay hypoxia: impacts of eutrophication, nutrient management and climate change(2019) Ni, Wenfei; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Eutrophication-induced coastal hypoxia can result in stressful habitat for marine living resources and cause great economic losses. Nutrient management strategies have been implemented in many coastal systems to improve water quality. However, the outcomes to mitigate hypoxia have been mixed and usually small when only modest nutrient load reduction was achieved. Meanwhile, there has been increasing recognition of climate change impacts on estuarine hypoxia, given estuaries are especially vulnerable to climate change with multiple influences from river, ocean and the atmosphere. Due to the limitation of observational studies and the lack of continuous historical data, long-term oxygen dynamics in response to the changes of external forces are still not well understood. This study utilized a numerical model to quantitatively investigate a century of change of Chesapeake Bay hypoxia in response to varying external forces in nutrient inputs and climate. With intensifying eutrophication since 1950, model results suggest an abrupt increase in volume and duration of hypoxia from 1950s-1960s to 1970s-1980s. This turning point of hypoxia might be related with Tropical Storm Agnes and consecutive wet years with relatively small summer wind speed. During 1985-2016 when the riverine nutrient inputs were modestly decreased, the simulated bottom dissolved oxygen exhibited a statistically significant declining trend of ~0.01 mgL-1yr-1 which mostly occurred in winter and spring. Warming was found to be the dominant driver of the long-term oxygen decline whereas sea level rise had a minor effect. Warming has overcome the benefit of nutrient reduction in Chesapeake Bay to diminish hypoxia over the past three decades. By the mid-21st century, the hypoxic and anoxic volumes are projected to increase by 10-30% in Chesapeake Bay if the riverine nutrient inputs are maintained at high level as in 1990s. Sea level rise and larger winter-spring runoff will generate stronger stratification and large reductions in the vertical oxygen supply to the bottom water. The future warming will lead to earlier initiation of hypoxia, accompanied by weaker summer respiration and more rapid termination of hypoxia. The findings of this study can help guide climate adaptation strategies and nutrient load abatement in Chesapeake Bay and other hypoxic estuaries.Item Response of the coastal ocean and estuaries to tropical cyclones(2018) Zhang, Fan; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Landfalling tropical cyclones (TC) pose great threats to public safety. The recent decades have witnessed major advances of knowledge in TC dynamics and improvement in TC forecast models, however, occasionally inaccurate TC intensity and storm surge predictions remain a vital concern. Different representations of subgrid-scale physics by various atmospheric model parameterization schemes lead to uncertainty in predictions of TC’s intensity and associated surges. In a case study for Hurricane Arthur (2014), local closure scheme for planetary boundary layer turbulence produces lower equivalent potential temperature than non-local closure schemes, leading to under-predicted TC intensity and surge heights. On the other hand, higher-class cloud microphysics schemes over-predict TC intensity and surge heights. Without cumulus parameterization for coarse-resolution grids, both TC intensity and surge heights are grossly under-predicted due to large precipitation decreases in the storm center. To avoid widespread predictions, the ensemble mean approach is shown to be effective. Another source of TC forecast error is inaccurate sea surface temperature (SST) prediction, and accurate SST prediction necessitates a better understanding of mixing processes in the coastal ocean. Previously, the importance of TC-induced near-inertial currents (NICs) to mixing in the coastal ocean was overlooked. With high-frequency radar and autonomous glider, long-lasting NICs with amplitudes of ~0.4 m s-1 were observed on the shelf during Arthur. With an atmosphere-ocean model, we find the NICs were dominated by mode-1 vertical structure and were a major contributor to the shear spectrum. Therefore, NICs may be important in producing turbulent mixing and surface cooling during Arthur’s passage. In the future, with warmer SST, sea level rise, and possible hard shorelines in estuaries, increased storm surge hazard is expected. Using Isabel (2003) as a case study, we find storm intensification under 2100 SST raises surge heights in Chesapeake Bay by 0.1-0.4 m given increased energy input. While sea level rise in 2100 reduces surge heights by 0-0.15 m through non-linear processes, it increases total water level by 0.4-1 m. Moreover, hard shoreline further increases surge heights by up to 0.5 m in the middle and upper Chesapeake Bay by prohibiting energy flux towards wetlands.Item MAPPING PREFERENTIAL FLOW PATHWAYS IN A RIPARIAN WETLAND USING GROUND-PENETRATING RADAR(2009) Gormally, Kevin Hill; McIntosh, Marla; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Preferential flow of water through channels in the soil has been implicated as a vehicle for groundwater and surface water contamination in forested riparian wetland buffers. Water conducted through these by-pass channels can circumvent interaction with wetland biota, biomass, and soils, thereby reducing the buffering capacity of the riparian strips for adsorption and uptake of excess nutrient loads from neighboring agricultural fields and urbanized lands. Models of riparian function need to account for preferential flow to accurately estimate nutrient flux to stream channels, but there are currently no methods for determining the form and prevalence of these pathways outside of extensive destructive sampling. This research developed, tested, and validated a new application of non-invasive ground-penetrating radar technology (GPR) for mapping the three-dimensional structure of near-surface (0-1 m) lateral preferential flow channels. Manual and automated detection methodologies were created for analyzing GPR scan data to locate the channels in the subsurface. The accuracy of the methodologies was assessed in two field test plots with buried PVC pipes simulating the riparian channels. The manual methodology had a 0% Type I error rate and 8% Type II error rate; the automated version had a <1% Type I error rate and 29% Type II error rate. An automated mapping algorithm was also created to reconstruct channel geometries from the scan data detections. The algorithm was shown to robustly track the connectivity of PVC pipe segments arranged in a branching structure hypothesized to exist in the riparian soils. These methods and algorithms were then applied at a riparian wetland study site at USDA Beltsville Agricultural Research Center in Beltsville, MD. The predicted structure of preferential flow channels in the wetland was validated by transmission of tracer dye through the study site and ground truth generated from soil core samples (92% accurate). These GPR tools will enable researchers to efficiently and effectively characterize lateral preferential flow without negatively impacting environmentally sensitive wetland areas. Scientists can now directly study these flow mechanisms to investigate the effects of by-pass pathways on nutrient fate in riparian buffers and the interactions of preferential flow with plant and animal systems.