Karenia brevis is responsible for annual harmful algal bloom (HAB) events in the Gulf of Mexico (GoM). Although the factors regulating the initiation of K. brevis blooms have been extensively investigated, the mechanisms leading to bloom decline and termination are not well known. Understanding the triggers of bloom decline could enhance our ability to forecast bloom dynamics and their impact on coastal areas. Microzooplankton grazing and programmed cell death (PCD) have been recognized as important pathways for the loss of phytoplankton biomass. However, neither of these two factors has been comprehensively examined in K. brevis. My dissertation work describes mechanisms of intrinsic cell death in response to environmental stress and develops methods to evaluate microzooplanton grazing pressure on K. brevis in the field. In Chapter 2, I confirmed the existence of PCD in K. brevis using oxidative stress as a model trigger. Acute susceptibility to oxidative stress increased as cultures aged, while the youngest cultures showed the fastest overall rate of cell loss within the first 24 hours. In untreated cells, the prevalence of components in the PCD cascade were not related to cell death, but fluctuated not only between growth phases but also over a diel cycle. Given that K. brevis likely experiences drastic environmental change during bloom season in the GoM, cell death processes in response to high light, hypo-osmotic, cooling and heat stress were described in Chapters 3 and 4. PCD in K. brevis only occurred at the largest light shift (50 to 1000 μmol m⁻² s⁻¹), along with induction of non-photochemical quenching which may enable cells to resist or delay PCD. Intensity-dependent induction of PCD cascade occurred when salinity decreased by 5, 10, or 15 psu, while cell death was only evident after 15 °C temperature change. Lastly, in Chapter 5 I describe the development of a quantitative real-time PCR method that can be used to measure species-specific grazing pressure on K. brevis. Overall, this work adds to our knowledge of cell loss processes in phytoplankton in general as well as insights into the effects of abiotic and biotic factors on the decline and termination of K. brevis blooms specifically