Name: Yuren Chen
Date: 06/20/2025
Time (EST/EDT): 01:00 pm
Location: AREL Lecture Hall, Horn Point Laboratory
Remote Access: email: mees@umd.edu

Committee Chair: Dr. Ming Li
Committee Members: Dr. Patricia M. Glibert; Dr. Cynthia Heil; Dr. David Ralston; Dr. Kenneth Rose
Dean’s Representative: Dr. Sujay S. Kaushal

Title: Modeling the Karenia Brevis Blooms in The West Florida Coastal Ocean: Unraveling the Influence of Nutrient Pulses, Hurricane Events, and Mixotrophic Grazing

Abstract:  Karenia brevis, a toxin-producing dinoflagellate, blooms almost annually on the West Florida Shelf (WFS), posing serious ecological, economic, and public health risks in coastal regions. While previous studies have highlighted the importance of nutrient sources, such as offshore nitrogen fixation, the remineralization of organic matter from zooplankton excretion, and terrestrial inputs, the links between K. brevis blooms and episodic nutrient release events, extreme storm disturbances, and mixotrophic processes remain poorly understood. Therefore, this dissertation developed a three-dimensional coupled hydrodynamic-biogeochemical model to investigate the impacts of these factors. Using the 2021 Piney Point release event as a case study, modeled ecological responses revealed a succession from r-strategists, such as diatoms, to K-strategists like K. brevis following the nutrient injection. Nutrient budget analysis suggested that a significant portion of the nutrients initially utilized by the diatom bloom was retained in the system through the deposition of organic matter and dead cells, which could provide additional nutrient sources for the development of the summer K. brevis bloom. In comparison, extreme storm events can deliver not only a huge amount of nutrient loads but also alter coastal circulation. Model results suggested that wind-driven upwelling and expansive, nutrient-rich river plumes caused by Hurricane Ian (2022) created favorable conditions for an earlier and more intense coastal K. brevis bloom. Sensitivity analyses identified storm-induced circulation as a key factor for bloom initiation, while the magnitude of nutrient loading largely determined bloom intensity. In contrast, the effects of suspended sediments and freshwater input were found to be relatively minor and short-lived. In addition, a novel modeling framework was developed in this study, incorporating recent advances in mixotrophic dynamics to investigate the impact of mixotrophy on K. brevis blooms. By conducting a hindcast run and a hypothetical run without mixotrophy, the results demonstrated the importance of mixotrophy in bloom persistence and expansion. Mixotrophic grazing on Synechococcus is shown to be able to not only provide a nutrient and energy source but also a competitive advantage over the picoplankton prey cells with higher nutrient affinity and larger surface-area-to-volume ratio. Overall, this dissertation presents a mechanistic modeling framework that enhances our understanding of K. brevis bloom dynamics and provides important insights for predicting bloom development under future environmental scenarios.