Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Nicole C Millette

Committee Member

Kimberly S. Reece

Committee Member

Deborah K. Steinberg

Committee Member

Juliette L. Smith


The influence of bottom-up and top-down controls on the formation and persistence of phytoplankton blooms has been well studied. However, the relative importance of these bottom-up and top-down controls vary spatially and temporally. In the tidal tributaries and mainstem of Chesapeake Bay, the summer dinoflagellate population follows a succession of bloom-producing species. The dinoflagellate species Margalefidinium polykrikoides and Alexandrium monilatum are currently considered the end of this succession. These species form near-annual blooms in the lower half of Chesapeake Bay and are considered harmful algal bloom (HAB) species due to their negative ecological impacts. However, analysis of long-term monitoring data and previous field samples suggest that Ceratium furca, a non-toxic dinoflagellate, might be an overlooked species in this dinoflagellate succession. My objective was to explore the influence of bottom-up and top-down controls on the species succession of the late summer phytoplankton blooms in the lower Chesapeake Bay. In the laboratory I used cultures of A. monilatum and C. furca isolated from the York River to evaluate the abiotic drivers influencing the succession of the late summer bloom from A. monilatum to C. furca. These experiments showed that each species exhibited differing light and temperature preferences but neither had a preferred N:P ratio. Lower light levels and lower temperatures favor non-toxic C. furca over toxin-producing A. monilatum in the York River. This information can help oyster aquaculturists identify regions of Chesapeake Bay that are unlikely to favor A. monilatum and are safer for oyster grow-out practices. Next, I used a combination of weekly field sampling along with in situ experiments during the late summer to assess top-down control of the copepod Acartia tonsa on harmful M. polykrikoides blooms. Sampling occurred during M. polykrikoides blooms in the lower York River in 2021 and 2022 and prey removal experiments were conducted using the water and copepods collected. I found that at M. polykrikoides abundances above 2000 cells mL-1 A. tonsa experienced >50% mortality in the prey removal experiments over 24-hours. Furthermore, A. tonsa abundances within the lower York River declined over the course of the bloom. This suggests that at high concentrations, M. polykrikoides may act as its own grazing deterrent, reducing the influence of top-down control and supporting bloom proliferation and maintenance. The time it takes for the A. tonsa population to recover from the negative impacts of M. polykrikoides blooms may also result in a “window of opportunity”, where a decrease in grazing pressure could support the formation of other blooms, like A. monilatum. This research provides insight into the bottom-up and top-down controls influencing the species succession in late summer phytoplankton blooms in the lower Chesapeake Bay. Evidence supports environmental conditions aiding in the transition from A. monilatum to C. furca. However, decreased grazing potential, as during an M. polykrikoides bloom, may also aid in the succession of species and provide opportunities for blooms to form. These findings can help environmental managers better predict when these blooms will occur and help to mitigate the negative impact of these blooms.



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