Biology Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2749

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    SURVIVING THE DEAD ZONE: INTERACTIONS AMONG JELLYFISH, COPEPODS, AND FISH IN THE CHESAPEAKE BAY
    (2020) slater, wencheng katherine; Pierson, James J; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The oxygen-deficient areas (dissolved oxygen < 2 mg L-1) in oceans and estuaries have been increasing worldwide in recent decades and are especially common in populated and developed areas due to eutrophication and warming. The objectives of this dissertation were to understand the effects of hypoxia on zooplankton and the plankton foodweb in the Chesapeake Bay. The study focused on copepod (Acartia tonsa) and its major predators bay anchovy (Anchoa mitchilli), comb jellyfish (Mnemiopsis leidyi), and bay nettle (Chrysaora chesapeakii) with data collected during six cruises in 2010 and 2011 and an individual-based model. Oxygen deficiency was evaluated with both dissolved oxygen concentration (DO < 2 mg L-1) and the oxygen supply and demand of the copepod (pO2 < Pcrit). The effects of hypoxia on zooplankton concentrations were estimated with net tows, and the impact of hypoxia on the plankton foodweb were quantified by comparing copepods’ nonpredatory mortality (estimated with neutral red experiments) and predatory mortality (estimated with gut contents of comb jellyfish and bay anchovy). A copepod behavior model was also built to examine how stress-induced behavior affected copepod vertical distributions and the tradeoffs between avoiding both hypoxia and predation. The results indicated the impact of oxygen deficiency could be underestimated using solely the metric of dissolved oxygen, especially under warm and saline conditions. Both copepod and planktivorous fish concentrations were lower under hypoxic conditions, but gelatinous zooplankton concentrations were higher. Both nonpredatory and predatory mortality of copepods were higher under hypoxic conditions, suggesting a direct linkage between hypoxia and decreasing copepod abundance. Most importantly, the source of copepod mortality changed with both hypoxic severity and season: the relative importance shifted from nonpredatory in spring to a combination of predatory and nonpredatory in summer and autumn, and the dominant predators shifted from juvenile bay anchovies under moderate hypoxia to comb jellyfish under warm and severely hypoxic conditions. The model demonstrated how enhancing stress avoidance would result in aggregating at a shallower depth and thus increasing predation risk, supporting the hypothesis that behavior change under hypoxia may increase predatory mortality. Overall my research has shown that hypoxia directly decreases zooplankton abundance and increases predation impact, and avoiding hypoxia could contribute to higher predation impact.
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    INTEGRATING AUTOMATED IMAGING AND A NOVEL IDENTIFICATION TECHNIQUE TO ESTIMATE MORTALITY AND IDENTIFY FACTORS THAT INFLUENCE THE VERTICAL DISTRIBUTION OF CRASSOSTREA VIRGINICA LARVAE
    (2015) Goodwin, Jacob; North, Elizabeth; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Understanding the population dynamics and complete life cycle of bivalves is important for effectively manage them. Most of the literature and research to date has focused on juvenile and adult bivalves, much less is known about larvae. The larval stage of the bivalve life cycle has been difficult to study due to the lack of a rapid automated approach for identifying species. However, a new technique, called ShellBi, has emerged that utilizes color patterns on the larval shell under polarized light to identify bivalve larvae. The objective of this chapter was to review the scientific basis for ShellBi and to apply it to bivalve larvae in Choptank River with the goal of distinguishing C. virginica from seven other species that spawn at the same time. A digital camera and polarized light microscope were used to capture images of the shells of bivalve larvae under standard and cross-polarized light. Images of C. virginica were distinguishable from other species based on these patterns, especially at later stages of development. These images could serve as a visual guide to identify C. virginica collected from the Choptank River and other tributaries with similar species in Chesapeake Bay.