Theses and Dissertations from UMD
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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
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Item DETERMINING FEEDING RATES IN EASTERN OYSTERS (Crassostrea virginica) USING NATURAL SESTON FLOW- THROUGH SYSTEM(2023) Wiltsee, Laura E.; Gray, Matthew W; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bivalves are prized for the ecosystem services they provide. The removal of particles from the water column through filter feeding and resulting water quality benefits, known as the biofiltration services, of bivalves have been studied for over a century. This has created a wealth of knowledge around the mechanistic drivers of bivalve feeding activity. Recently, Chesapeake Bay ecosystem-wide models have begun incorporating Eastern Oyster (Crassostrea virginica) biofiltration. Acute feeding variability is critically important when estimating oyster biofiltration services at ecosystem scale. Typically, natural seston clearance rate studies last a limited timeframe, with a focus on specific environmental events such as an increase in temperature, drop in salinity, or a tidal cycle.To capture the highly variable filter feeding rate of bivalves, such as the Eastern Oyster, studies have used highly controlled laboratory conditions, with single environmental variable modification. These studies often use indirect methods for estimating clearance rates that commonly lack high-resolution capability. Furthermore, these studies are labor intensive and time consuming, and as a result, few studies have monitored bivalve feeding activities over long periods to understand variation in activity or how these rates may change with seasonal shifts in conditions. These limitations have led to a shortage of knowledge around how clearance rates of oysters vary in response to ambient conditions over both short-term (hourly) and long-term (seasonal) time scales. This study leverages advances in semi-autonomous aquatic observing to track high- resolution, long-term feeding responses of bivalves to subtle variations in environmental conditions. Oyster ex situ clearance rates in the Choptank River (Maryland, USA) were estimated under flow-through conditions, and logged in real-time using fluorometers among replicate oysters over 5-day experiments for 9 months. The measured clearance rates from this system were compared to a mechanistic clearance rate model used by the Chesapeake Bay Program, which is used to estimate the role of oysters in controlling water quality in the Bay. Environmental data were evaluated to build a statistical and random forest model to predict how oyster clearance rates respond to prevailing environmental conditions. This monitoring system and resulting models enable a deeper understanding of feeding variability and how natural seston and environmental variability directly influence oyster physiology.Item TIME-SERIES FOCUSED ASSESSMENTS OF CHANGING MARINE BIVALVE COMMUNITIES IN THE BERING AND CHUKCHI SEAS(2021) Goethel, Christina; Grebmeier, Jacqueline; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Pacific Arctic has been experiencing rapid environmental change, including increasing bottom water temperatures, declining sea ice extent, and ecosystem shifts. In the northern Bering Sea (NBS), bottom water temperature was ~1.5°C higher in 2018 than previously recorded. Temperature and sea ice dynamics could alter this benthic-dominated system, potentially shifting the food web to a more pelagic-dominated system. Bivalves, a key component of the benthic macrofaunal community, are important prey items for the spectacled eider in the NBS and for walrus in both the NBS and the southeast Chukchi Sea (SEC). Here, data were collected and analyzed at established time-series stations in the NBS and SEC as part of the Distributed Biological Observatory. The objective was to evaluate changes to bivalve communities, and how those relate to overall benthic community shifts and functioning. By using both time-series and experimental techniques my research: 1) evaluated past trends in detail for a single dominant clam species, Macoma calcarea, 2) tracked the abundance, biomass, and dominant size class of two dominant bivalve species, M. calcarea and Serripes spp., from 2015-2019 when drastic physical changes have been observed, and 3) scaled up connections amongst the individual biological responses to full macrofaunal community population responses to the composite environmental changes using shipboard sediment community oxygen consumption (SCOC) incubation and individual respiration experiments. Results indicate that bottom water temperature and food availability (measured using sediment chlorophyll-a inventories as a proxy) play the largest role in controlling the population dynamics of M. calcarea, and that the population is contracting northward in the NBS. In the SEC, results suggest a shift in the hotspot of M. calcarea from station UTN5 (north) to UTN2 (south). However, size class results showed a larger number of smaller clams in the south and a smaller number of larger clams further north, indicating the biomass hotspot likely remains at UTN5. Serripes spp. were prevalent in 2014-2017, but started to decline in 2018 and 2019. Experimental results demonstrated increased SCOC and individual oxygen consumption in higher temperatures, but that there was little effect to oxygen consumption when food was added.Item PHYSIOLOGICAL, MOLECULAR, AND ECOLOGICAL RESPONSES OF THE EASTERN OYSTER, CRASSOSTREA VIRGINICA, TO HYPOXIA EXPOSURE IN THE CHESAPEAKE BAY(2021) Davis, Anna Manyak; Plough, Louis; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hypoxia is a naturally occurring phenomenon in coastal waters that is increasing in frequency and extent due to human activities. There is a pressing need to understand how organisms will be able to respond and adapt to future oxygen limitation. The eastern oyster, Crassostrea virginica, is an ecologically important bivalve that is threatened by the increasing incidence of low oxygen events. Little is known about the capacity of C. virginica to cope with projected deoxygenation or the potential ecological implications of reduced oxygen availability. The primary objectives of this dissertation research were to 1) characterize the intraspecific variability in physiological and molecular responses to hypoxia for oysters from the Chesapeake Bay and 2) develop a model to predict the implications of hypoxia on oyster population ecology. In Chapter 2 I assessed the survival and heart rate responses under low oxygen stress for oysters sourced from reefs experiencing varying frequencies of hypoxia exposure. Results indicated that prior hypoxia exposure does not confer increased survival under low oxygen stress but may relate to sublethal physiological differences in tolerance, particularly for oysters with a greater frequency of prior hypoxia exposure. In Chapter 3, I used four different analytical approaches, principal components, differential gene expression, co-expression gene network, and transcriptional frontloading analyses, to assess intraspecific differences in oyster transcriptomic response to hypoxia. No statistically significant differences in gene expression response between sites were observed indicating that prior hypoxia exposure may not have affected the regulation of expression under hypoxic stress. However, while not statistically significant, gene expression patterns suggested transcriptional frontloading as a possible mechanism of increased hypoxia tolerance in oysters. Finally, in Chapter 4, I developed a Dynamic Energy Budget model integrating dissolved oxygen concentration as a forcing variable to make predictions about oyster growth and reproduction under varying oxygen conditions. Model outputs indicated that low oxygen exposure reduces oyster growth, fecundity, and spawning frequency. Collectively, this dissertation research affirms that low oxygen availability negatively affects oyster physiology and ecology, and emphasizes the importance of continued research into the capacity of oysters to tolerate future increases in coastal hypoxia.