UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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Author: search.filters.author.Testa, Jeremy Mark

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    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.
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    Factors Regulating Variability in Water Quality and Net Biogeochemical Fluxes in the Patuxent River Estuary
    (2006-08-10) Testa, Jeremy Mark; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Net biogeochemical production and transport rates for several variables were computed for the Patuxent River estuary from 1985 to 2003 using a box model. Monthly rate estimates were analyzed for temporal patterns and variability in response to climatic factors and nutrient management. The middle estuary was the most productive estuarine region and was characterized by strong pelagic-benthic coupling. Phytoplankton biomass in this region peaked in spring as fueled by seaward nutrient inputs. Nutrients regenerated from decomposition of this spring bloom were required to support summer productivity. Improvements of sewage treatment in the watershed resulted in declining point source nutrient loads to the estuary, but water quality did not improve in the mesohaline estuary. Poor water quality in the middle estuary was maintained by persistent non-point nutrient loads, while degrading water quality in the lower estuary correlated with increasing DIN inputs from Chesapeake Bay, high river flow, and declining herbivorous grazing.