Biology Theses and Dissertations
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Item EFFECTS OF SHELL DAMAGE ON MORTALITY OF THE EASTERN OYSTER (CRASSOSTREA VIRGINICA) IN NORMOXIC AND ANOXIC CONDITIONS(2022) Schlenoff, Jake I; Miller, Thomas; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Deoxygenation is increasingly problematic in coastal waters globally, with many costal estuaries subject to zones of hypoxia (< 2 mg/L dissolved oxygen) or anoxia (< 0.5 mg/L dissolved oxygen). The presence of hypoxic and anoxic zones can place a unique physiological burden on marine fauna and flora, potentially leading to mass mortality and resulting in dead zones. Anthropogenic stressors, such as increased nutrient input (primarily Nitrogen and Phosphorus), have led to long-term increases of hypoxia in the Chesapeake Bay over the 20th century. Although environmental management policies for the Bay have mitigated hypoxia trends, hypoxia continues to be prevalent through many parts of the Bay. While motile aquatic organisms can change locations to avoid seasonal or long-term bouts of deoxygenation, organisms with sessile adult life stages cannot move to avoid this ecological stressor. The Eastern Oyster (Crassostrea virginica) is a foundational species in the Chesapeake Bay’s ecosystem, performing many ecosystem services such as water filtration, nutrient cycling, and fostering benthic-pelagic connectivity while also serving as an economic resource for commercial fishing. However, long-term trends in hypoxia and anoxia, combined with other anthropogenic stressors, have contributed to a decline in Eastern Oyster in the Bay, leaving populations at a fraction of historical levels, fostering a need for research to better understand the physiological and biomechanical responses of C. virginica to depletion of dissolved oxygen. While the Eastern Oyster has been termed a champion of hypoxic tolerance, and studies have been published exploring the impacts of low DO on oyster mortality and sublethal responses, research is still in search of answers to whether the response of the oyster comes from shell-based behavioral resilience to isolate the animal from environmental conditions, or physiological adaptions from the tissue of the oyster. By drilling holes of three different sizes into one valve of the oyster and exposing it to anoxic external conditions, this study aims to bridge the gap in knowledge of whether anoxic tolerance is a behavioral or physiological response. Oysters with a hole drilled in the shell of any size experienced much faster mortality in anoxic environments than oysters with no hole in the shell (χ2= 8, p = 0.005), while the size of the hole drilled did not impact time to death. These results shed new light on the behavioral response of the Eastern Oyster to depleted dissolved oxygen and the importance of clamping to ostracize internal tissue from environmental deviations.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.Item RESPONSES OF THE COPEPOD ACARTIA TONSA TO HYPOXIA IN CHESAPEAKE BAY(2015) Barba, Allison Patricia; Roman, Michael R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chesapeake Bay experiences seasonal hypoxia each year and while studies have been done investigating how the copepod Acartia tonsa responds to hypoxia, few studies have focused on a comprehensive understanding of how its behavior and fitness are affected by low oxygen. The abundance, distribution, fitness and diel vertical migration patterns of A. tonsa were measured on series of six cruises in 2011 and 2012 in spring, summer and fall. I found that copepod abundance, distribution and vertical migration were significantly affected when hypoxic waters occurred below the pycnocline. I also found that males were less impacted by hypoxia than females, with a greater decrease in female abundance and vertical migration when there were hypoxic bottom waters.Item The effects of oxygen transition on community metabolism and nutrient cycling in a seasonally stratified anoxic estuary(2014) Lee, Dong-Yoon; Cornwell, Jeffrey C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gradients of dissolved oxygen concentrations in seasonally stratified estuarine water columns directly influence microbial composition and metabolic pathways, resulting in annually recurring spatiotemporal chemical gradients of redox-active species. Understanding such microbial responses to variable geochemical conditions and elucidating the diversity of microbial processes are needed to comprehensively identify ecosystem functions. At first this study describes an investigation of the relationships between microbial processes and geochemical conditions. To assess the contribution of different biological redox processes on carbon and nitrogen cycles in the Chesapeake Bay, we used observational and experimental approaches as well as utilization of monitoring datasets to facilitate an assessment of ecosystem-level metabolism. Observations revealed a general positive association of community metabolism with strong gradients of redox-related variables and hydrodynamic characteristics, although geochemical and environmental conditions varied seasonally across oxic transitions and interannually across degrees of stratification. The most distinct evidence supporting the positive association were vertical distributions of community respiration with the highest average rates in the most stratified regions coincident with the depths of the steepest gradient of chemical compounds. Although organic matter availability may be enhanced due to hydrodynamically induced stability, our investigation of factors driving the pattern revealed that differential responses and metabolic strategies of microbial communities result in high respiration near oxyclines. Investigation of vertical profiles of redox-related variables also revealed that the coexistence of oxidants and reduced compounds further provides an optimal condition for other electron accepting processes, including chemoautotrophy and anoxygenic photoautotrophy. The strong interdependence between environmental conditions and variability in microbial metabolism also reflected in patterns of plankton assemblages. An ammonium mass-balance analysis revealed that increases in vertical ammonium dispersion during severe hypoxia cause a shift of plankton assemblages towards heterotrophy, subsequently supporting a deep secondary microbial food web in the vicinity of oxic/anoxic interface. Overall, results from this research indicate that the estimation of more accurate net ecosystem metabolism should take into consideration of the highly variable nature of community metabolism associated with both geochemical gradients and stratification.Item THE EFFECTS OF THE DIEL-CYCLING OF DISSOLVED OXYGEN AND pH ON THE EASTERN OYSTER, CRASSOSTREA VIRGINICA (GMELIN), CLEARANCE RATES AND HEMOLYMPH pH(2014) Clark, Virginia M; Breitburg, Denise L; Harris, Lora A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chronic hypoxia and hypercapnia affect Crassostrea virginica. Less is known about how the co-cycling of these stressors, as occurs in shallow waters worldwide, affects this filter feeder. I used laboratory experiments and age-specific models to test how diel-cycling hypoxia and hypercapnia affect algal clearance rates by C. virginica and C. virginica hemolymph pH. Clearance rates were reduced during periods of low dissolved oxygen, but older oysters compensated by clearing faster when DO returned to normoxia. Models indicated that this compensatory feeding may allow older oysters to avoid decreases in average summertime clearance rates. Low hemolymph pH has been linked to decreased immune function in marine invertebrates and low water pH decreases the hemolymph pH of oysters. My hemolymph experiment also showed that hemolymph pH decreased with decreasing water pH and indicated that oysters may begin to compensate for declining water pH at water pH values between 7.60 and 7.36.Item Polychaetes, Hypoxia, and Nitrogen Cycling in the Mesohaline Chesapeake Bay(2014) Bosch, Jennifer A.; Kemp, W. Michael; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Benthic macrofauna can play an important role in facilitating some of the microbial mediated processes of nitrogen cycling in estuarine sediments. Declines in benthic macrofauna, like polychaete worms, have been attributed to long-term increases in bottom water hypoxia in Chesapeake Bay. Utilizing a large monitoring dataset including benthic macrofaunal abundance, biomass, and concurrent measures of environmental parameters, I examined how environmental conditions regulate the densities of opportunistic polychaetes in a mesohaline estuarine system. This analysis points to a benthic community dominated by euryhaline, opportunistic polychaete worms (M. viridis, S. benedicti, H. filiformis, A. succinea) which have well adapted but varying responses to hypoxia and other stressful conditions. Results of two laboratory experiments with the opportunistic polychaete Alitta (Neanthes) succinea were used to quantify the short-term influence of density and size of surface-feeding polychaetes on sediment-water fluxes of inorganic nitrogen under varying oxygen conditions. Polychaete enhancements of O2 and nitrogen fluxes were strongly correlated with total animal biomass. Solute fluxes were stimulated by presence of both larger and smaller worms, but per capita effects were greater for the deep-burrowing larger polychaetes. Utilizing a unique large-scale monitoring dataset collected in the Chesapeake Bay, I employed Classification and Regression Tree (CART) and multiple linear regression (MLR) analyses to assess the relationship between benthic biomass and NH4+ efflux within different regions of the estuary by season. In addition to labile organic matter, oligohaline and mesohaline tributary temperature and salinity control the rate of nitrogen cycling and benthic macrofaunal biomass. In deeper regions of mesohaline tributaries and the mainstem Bay, dissolved oxygen was found to be the dominating parameter regulating sediment nitrogen pathways as well as the structure of the benthic macrofaunal community. With increased macrofaunal biomass, spring regressions indicated an enhancement of NH4+ efflux. In contrast, fall regressions indicated the enhancement of fixed nitrogen removal from sediments. Summer data lacked a significant relationship, but high NH4+ effluxes under hypoxic/anoxic conditions suggested dissolved oxygen is the primary driver of summer nitrogen cycling. This study, using field and laboratory data, concludes that a complex balance between seasonal and regional dissolved oxygen, temperature and salinity conditions shape not only the benthic community but also the relationship between macrofaunal biomass and sediment nitrogen flux in this eutrophic estuarine system.Item NITROGEN CYCLING AND CONTROLS ON DENITRIFICATION IN MESOHALINE SEDIMENTS OF CHESAPEAKE BAY(2009) Owens, Michael Sean; Cornwell, Jeffrey C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrogen is a key nutrient in the eutrophication of coastal and estuarine systems. In shallow water systems, sediment recycling can be an important source of nutrients for phytoplankton growth. The balance between nitrogen recycling and denitrification regulates the importance of sediments as a nitrogen source. To assess controls on denitrification, we conducted intensive seasonal measurements of sediment water exchange and denitrification using sediment core incubations. Peak rates of denitrification were observed in fall and spring (>100 μmol N-N m-2 h) followed by a decrease to 10 μmol N m-2 h in summer. Although denitrification rates were stimulated by labile organic carbon additions from the water column, the overall efficiency of the process sharply declined as temperature increased and bottom water O2 declined. Macrofauna activity was shown to enhance sediment transport of O2 by >5 fold, increase organic matter decomposition and maintain a high rate of denitrification efficiency.Item A Comparison of the Biochemistry and Physiology of the Eastern Oyster, Crassostrea virginica, and the Asian Oyster, Crassostrea ariakensis(2007-08-13) Harlan, Nicole Porter; Paynter, Kennedy T; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Chesapeake Bay Foundation, US Environmental Protection Agency, and the Chesapeake Bay Commission, requested research on the introduction of the Asian oyster, Crassostrea ariakensis, to help restore the fishery and ecosystem function of the native Eastern oyster, Crassostrea virginica. In order to augment the role of C. virginica in Chesapeake Bay, C. ariakensis will likely require tolerances to low dissolved oxygen similar to that of the native oyster. This research showed that triploid and diploid C. virginica lived significantly longer than C. ariakensis under anoxic conditions, although the oxygen consumption rates of diploid oysters did not differ between species. Free amino acid pools in the gill tissue of oysters exposed to normoxia or hypoxia were analyzed. Alanine increased in both species during hypoxia, indicating the use of alternative metabolic pathways. Aspartate was consumed by C. virginica during hypoxia, confirming the use of a pathway coupling glucose and aspartate fermentation. Differences in the free amino acid pools of these two species suggest an explanation for the disparity in anaerobic metabolism between C. ariakensis and C. virginica.