Date Awarded

Fall 2019

Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Kimberly S. Reece, Ph.D.

Committee Member

Wolfgang K. Vogelbein, Ph.D.

Committee Member

Juliette L. Smith, Ph.D.

Committee Member

Ryan B. Carnegie, Ph.D.

Committee Member

Emily B. Rivest, Ph.D.


Harmful algal blooms (HABs) are expanding globally and are anticipated to continue increasing with climate change. Two dinoflagellate species, Margalefidinium polykrikoides and Alexandrium monilatum, form extensive and dense blooms most summers in the lower Chesapeake Bay. Alexandrium monilatum, which produces the toxin goniodomin A, tends to bloom soon after M. polykrikoides, for which a toxin has not yet been identified. Previous laboratory studies and a more limited number of field studies indicate mortality and pathology in multiple shellfish species associated with exposure to M. polykrikoides and A. monilatum. However, the impacts of sequential exposure to both HAB species on marine organisms in the natural environment are less well understood. Local aquaculturists grow oysters under a variety of conditions that may be differentially impacted by HAB exposure. No extensive and controlled studies have been carried out in lower Chesapeake Bay assessing impacts of sequential exposure to M. polykrikoides and A. monilatum blooms on oysters cultured using different aquaculture strategies. The two main objectives of this study were to: 1) investigate M. polykrikoides and A. monilatum as a potential threat to cultured oysters, and 2) inform mitigation strategies to minimize HAB impacts based on current grow-out methods. To address these objectives, oysters were grown in 2017 and 2018 at sites characterized by differing water energetics and HAB dynamics. At all sites during both summers, oysters were grown intertidally and subtidally, and in 2018 were also grown in floating cages at one site. Water quality parameters were monitored, including cell concentrations of M. polykrikoides, A. monilatum, and two other local HAB species known to negatively impact oysters, Karlodinium veneficum and Prorocentrum minimum, along with oyster health and survival. Blooms of M. polykrikoides and A. monilatum occurred at the study sites, but not the reference site, in summer 2017 with cell concentrations that were lower than those quantified in some previous years. In summer 2018, neither species bloomed, providing the opportunity to assess oysters during both a bloom and a non-bloom year. Overall oyster mortality in both summers was relatively low compared to mortality often seen associated with other oyster stressors such as disease. Results indicated oyster health and survival were more impacted by factors indirectly related to HABs, specifically location factors (i.e. site and placement location), temperature, and DO. In both summers, mortality was significantly higher for intertidal oysters, compared to subtidal oysters, suggesting intertidal placement may incur stress in summer. It is unknown whether this intertidal stress may be further compounded during more HAB-intensive years. The results of this study suggest M. polykrikoides and A. monilatum had little impact on the health and survival of oysters cultured in the lower Chesapeake Bay in the summers of 2017 and 2018. More than one year of bloom data is likely necessary, however, to fully evaluate the impacts of M. polykrikoides and A. monilatum as potential stressors to cultured oysters in the context of inter-annual variability and the expanding distribution of these two HABs in the Chesapeake Bay.




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