UMD Theses and Dissertations
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Item NONLINEAR INTERNAL WAVES AND SHORT-TERM VARIABILITY OF CARBON SYSTEM DRIVEN BY LATERAL CIRCULATION IN COASTAL PLAIN ESTUARY(2023) Li, Renjian; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent observations in Chesapeake Bay showed that the interaction between lateral circulation and channel-shoal bathymetry generated internal lee waves which subsequently propagated onto shallow shoals and evolved into internal solitary waves, leading to overturning and enhanced turbulent mixing. However, it is unknown under what hydrodynamic conditions the lee waves could be generated and how the nonlinear internal waves evolved. Using an idealized straight channel representative of a coastal plain estuary, we conducted numerical simulations to investigate internal wave generation over a range of river flows and tidal amplitudes. The model results are summarized using the estuarine classification diagram based on the freshwater Froude number Frf and the mixing parameter M. Δh decreases with increasing Frf as stronger stratification suppresses waves, and no internal waves are generated under large Frf. Δh initially increases with increasing M as the lateral flows become stronger with stronger tidal currents, but decreases or saturates to a certain amplitude as M further increases. This regime diagram suggests that internal lee waves can be generated in a wide range of estuarine conditions. To examine the nonlinear evolution of internal waves, a three-dimensional nonhydrostatic model with nested model domains and increasing grid resolution was configured. The lee wave steepens into a shorter elevation wave due to shoaling and soon evolves into a depression with a train of undular waves at its tail as bottom boundary mixing elevates the halocline above the mid-depth. These nonlinear internal waves enhance the turbulent dissipation rate over the deep channel and shallow shoal, suggesting an important energy source for mixing in stratified coastal plain estuaries. In addition, a pH sensor deployed at the middle reach of Chesapeake Bay recorded high-frequency variability in bottom pH driven by along-channel winds. Though wind-driven lateral circulation can advect high pH water downward, the slow air-sea exchange of CO2 limits the lateral ventilation. With DIC and TA budget analysis and comparison with cross-sections at upper- and lower-Bay where strong lateral circulation was confined in the surface layer, we found vertical mixing and replenishment of oceanic water by longitudinal advection could be more important mechanisms to ventilate bottom pH.Item SUBAQUEOUS SOILS OF CHESAPEAKE BAY: DISTRIBUTION, GENESIS, AND THE PEDOLOGICAL IMPACTS OF SEA-LEVEL ALTERNATIONS(2020) Wessel, Barret Morgan; Rabenhorst, Martin C; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soils and sediments make up a substantial portion of the resource base that supports human societies and other life on Earth, yet in the subaqueous environment our understanding of these materials pales in comparison to our understanding and management of upland soils. We must develop an understanding of how subaqueous soils/sediments are distributed, how they form and change over time, and how they will be impacted by rising sea-levels as a result of climate change if we are to wisely manage these resources. The goal of this study is to improve this understanding in Chesapeake Bay subestuaries. The Rhode River subestuary was first surveyed to identify rates of bathymetric change in these settings and to characterize the common material types found in these settings. Bathymetric change was evaluated using hydrographic surveys dating back to 1846, and though the river bottom does change slowly, it has been more or less stable during the years evaluated. Several types of morphologically distinct materials make up the soil profiles in Rhode River. Materials highest in organic matter are easy to identify in the field, and commonly become ultra-acidic if disturbed. Also present were submerged upland soils, colored and structured like soils in the surrounding landscape. To better understand the impacts of submergence on these materials, a sampling campaign was conducted on shallow marine sediments, reclaimed land, and restored aquatic environments under both seawater and freshwater. This demonstrated that shallow marine sediments develop upland soil features and biogeochemical characteristics within 150 years of drainage, and that these characteristics do indeed persist in the subsoil two years after submergence. Topsoil changes more radically, releasing anomalous amounts of Fe while accumulating anomalous amounts of reduced S minerals, a process exacerbated by seawater flooding. Using these results, a soil-landscape conceptual model was developed and used to predict subaqueous soil distribution in the West River subestuary. These predictions were evaluated with a sampling campaign, and found to be significant. This model can now be used in other subestuaries to quickly and efficiently survey subaqueous soils, supporting the development of future land-use interpretations in these environments.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 DISSOLVED OXYGEN AND NUTRIENT CYCLING IN CHESAPEAKE BAY: AN EXAMINATION OF CONTROLS AND BIOGEOCHEMICAL IMPACTS USING RETROSPECTIVE ANALYSIS AND NUMERICAL MODELS(2013) Testa, Jeremy Mark; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hypoxia, or the condition of low dissolved oxygen levels, is a topic of interest throughout aquatic ecology. Hypoxia has both realized and potential impacts on biogeochemical cycles and many invertebrate and vertebrate animal populations; the majority of the impacts being negative. It is apparent that the extent and occurrence of hypoxic conditions has been on the rise globally, despite a handful of reductions due to management success stories. Efforts to curb the development of hypoxia are well underway in many aquatic ecosystems worldwide, where oxygen levels are a key target for water quality management. Long-term increases in the volume of seasonal bottom-water hypoxia have been observed in Chesapeake Bay. Although there is evidence for the occurrence of low oxygen conditions following initial European habitation of the Chesapeake watershed, as well as direct observations of anoxia prior to the mid 20th century large-scale nutrient load increases, it is clear that hypoxic volume has increased over the last 50 years. Surprisingly, the volume of hypoxia observed for a given nutrient load has doubled since the mid-1980s, suggesting the importance of hypoxia controls beyond nutrient loading alone. I conducted a suite of retrospective data analyses and numerical modeling studies to understand the controls on and consequences of hypoxia in Chesapeake Bay over multiple time and space scales. The doubling of hypoxia per unit TN load was associated with an increase in bottom-water inorganic nitrogen and phosphorus concentrations, suggesting the potential for a positive feedback, where hypoxia-induced increases in N and P recycling support higher summer algal production and subsequent O2 consumption. I applied a two-layer sediment flux model at several stations in Chesapeake Bay, which revealed that hypoxic conditions substantially reduce coupled nitrification-denitrification and phosphorus sorption to iron oxyhydroxides, leading to the elevated sediment-water N and P fluxes that drive this feedback. An analysis of O2 dynamics during the winter-spring indicate that the day of hypoxia onset and the rate of March-May water-column O2 depletion are most strongly correlated to chlorophyll-a concentrations in bottom water; this suggests that the spring bloom drives early season O2 depletion. Metrics of winter-spring O2 depletion were un-correlated with summer hypoxic volumes, however, suggesting that other controls (including physical forcing and summer algal production) are important. I used a coupled hydrodynamic-biogeochemical model for Chesapeake Bay to quantify the extent to which summer algal production is necessary to maintain hypoxia throughout the summer, and that nutrient load-induced increases in hypoxia are driven by elevated summer respiration in the water-column of lower-Bay regions.Item The Pursuit of a Sustainable Coastal Fishery: Comparisons of the Oyster Fishery in Chesapeake Bay and Ariake Sea(2012) Ishikawa, Momoko; Kennedy, Victor S; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A coastal fishery brings high productivity and economic profit while supporting cultural norms. However, it also causes environmental degradation and political conflicts, which sometimes collapse a fishery. With the current global shift from capture fisheries to aquaculture, appropriate management is required for social and environmental sustainability. To identify essential factors in coastal fishery management, I compared the oyster industries in Chesapeake Bay (USA) and in Ariake Sea (Japan) from political, environmental, and cultural perspectives, by field observations, interviews, and literature research. Despite their different historical backgrounds, the two regions have lost most of the oyster resource due mainly to 1) failure of environmental management, 2) environmental degradation and 3) resistance of the fishing communities to necessary changes in fishing methods and aquaculture. Based on these lessons, I propose that a coastal fishery management plan should include environmental management, development of government-initiated aquaculture, understanding cultural backgrounds, and cooperation among science, industry and politics as essential factors.Item Sediment Biogeochemistry Across the Patuxent River Estuarine Gradient: Geochronology and Fe-S-P Interactions(2007-12-21) OKeefe, Jennifer; Cornwell, Jeffrey C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although salinity and redox gradients are defining features of estuarine biogeochemistry, compositional changes in sediment characteristics associated with these factors are poorly described in U.S. coastal plain estuaries. Understanding the basics of nutrient sources and sinks, in the context of these defining characteristics, is required to make efficient and effective management decisions regarding estuarine eutrophication. In this study, detailed analysis of long-term nutrient burial has been used as a tool to understand the trajectory of nutrient cycling at 7 stations along an oligohaline to mesohaline transect in the Patuxent River estuary. Sediment mass accumulation rates were determined for 3 of the 7 sites. Cores analyzed for total P, total N, organic C, biogenic silica, δ13C, and δ15N did not provide evidence of historical nutrient reduction actions taken in this watershed. Burial rates of Fe-S mineral phases and inorganic P (IP) indicated pyrite formation limited the availability of Fe-oxides for adsorption and retention of IP.Item The effects of low dissolved oxygen on predation interactions between Mnemiopsis leidyi ctenophores and larval fish in the Chesapeake Bay ecosystem(2006-11-27) Kolesar, Sarah Elizabeth; Breitburg, Denise L; Boynton, Walter R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Differences in predator and prey tolerances to low dissolved oxygen (DO) concentrations are important to planktonic food webs in seasonally hypoxic environments like Chesapeake Bay. Hypoxia alters field distributions, encounter rates, and predator-prey interactions between hypoxia-tolerant ctenophores, Mnemiopsis leidyi, and less tolerant ichthyoplankton and zooplankton prey. To examine the effect of hypoxia on estuarine food web species' interactions, I conducted medium and small-scale experiments, field sampling, and collaborated on individual-based model development, focusing on ctenophore-larval fish dynamics. Laboratory estimates of clearance rates for ctenophores on bay anchovy (Anchoa mitchilli) eggs and yolk sac larvae, and naked goby (Gobiosoma bosc) feeding larvae were the same at low and high DO. Field sampling for M. leidyi, ichthyoplankton, mesozooplankton, and scyphomedusae (Chrysaora quinquecirrha) during day and night at two sites in the Patuxent River indicated increased abundance of most species in the bottom layer with increasing bottom DO. Vertical overlap between predator and prey pairs also increased with higher bottom DO, increasing potential encounters and predation. Larval fish swimming speeds did not differ significantly with DO, but ctenophores swam significantly faster at intermediate DO (2.5 mg L-1) than at either low or high DO. DO did not significantly affect ingestion. Greater ingestion of fish larvae by ctenophores followed multiple encounters (56%) than initial encounters (10%) at all DO concentrations, highlighting the potential importance of repeated predator-prey interactions. DO did not significantly affect encounter model estimates of ingestion rates. Ingestions averaged 0.4 fish larvae d-1 m-3 for first encounters and 2 fish larvae d-1 m-3 for multiple encounters. Results from laboratory and field studies parameterized a spatially-explicit individual based model of a ctenophore-ichthyoplankton-copepod intraguild predation food web. Ctenophore predation had a bigger effect on survival of modeled ichthyoplankton than did competition between ctenophores and fish larvae for shared zooplankton prey, but competition more strongly affected larval fish growth rates. DO did not alter the relative importance of ctenophore predation and competition, but low DO did decrease larval fish survival and increase growth rates. Results suggest that effects of DO on vertical distribution and species overlap are more important to predation than direct DO effects.Item The regulation of bacterioplankton carbon metabolism in a temperate salt-marsh system(2005-09-21) Apple, Jude Kolb; del Giorgio, Paul A.; Kemp, W. Michael; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study describes an investigation of the factors regulating spatial and temporal variability of bacterioplankton carbon metabolism in aquatic ecosystems using the tidal creeks of a temperate salt-marsh estuary as a study site. Differences in land-use and landscape characteristics in the study site (Monie Bay) generate strong predictable gradients in environmental conditions among and within the tidal creeks, including salinity, nutrients, and the quality and quantity of dissolved organic matter (DOM). A 2-yr study of bacterioplankton metabolism in this system revealed a general positive response to system-level nutrient enrichment, although this response varied dramatically when tidal creeks differing in salinity were compared. Of the numerous environmental parameters investigated, temperature and organic matter quality had the greatest influence on carbon metabolism. All measures of carbon consumption (i.e., bacterioplankton production (BP), respiration (BR) and total carbon consumption (BCC)) exhibited significant positive temperature dependence, but the disproportionate effect of temperature on BP and BR resulted in the negative temperature dependence of bacterioplankton growth efficiency (BGE = BP/[BP+BR]). Dissolved organic matter also had an influence on carbon metabolism, with higher BCC and BGE generally associated with DOM of greater lability. Our exploration of factors driving this pattern suggests that the energetic content and lability of DOM may be more important than nutrient content or dissolved nutrients alone in determining the magnitude and variability of BGE. Investigations of single-cell activity revealed that BCC and BGE may be further modulated by the abundance, proportion, and activity of highly-active cells. Differences in single-cell activity among creeks differing in freshwater input also imply that other cellular-level properties (e.g., phylogenetic composition) may be an important factor. Collectively, results from this research indicate that the variability of bacterioplankton carbon metabolism in temperate estuarine systems represents a complex response to a wide range of environmental and biological factors, of which temperature and DOM quality appear to be the most important. Furthermore, this research reveals fundamental differences in both cellular and community-level metabolic processes when freshwater and marine endmembers of estuaries are compared that may contribute to the variability in bacterioplankton carbon metabolism within and among estuarine systems.