Dinoflagellate Dinophysis acuminata is an obligate mixotroph that can cause diarrhetic shellfish poisoning (DSP) through their production of okadaic acid and dinophysistoxins. Increasing global reports of DSP events underscores the urgent need for real-time monitoring to better understand their population dynamics. Grazing pressure has been recognized as a key factor regulating D. acuminata populations. However, its specific role in bloom development remains poorly understood. Here, an observatory raft with an automated imaging flow cytometer (Imaging FlowCytobot, IFCB) was deployed for continuous in situ monitoring in a bloom hotspot in Massachusetts, USA. During June-July 2024, an intense D. acuminata bloom was recorded, with cell concentrations reaching millions per liter. Seawater samples were collected weekly for 72-h lab incubation to explore vegetative division/mating behaviors, potential grazer and toxin production of D. acuminata across different bloom phases. In each incubation, four treatments were established by mesh sieving and/or dilution to simulate different levels of grazing pressure. Culture samples were collected and analyzed using two IFCBs hourly. Results showed that  D. acuminata concentrations decreased dramatically during daytime, while remained stable or slightly increased at night. lab conditions enhanced mating signals relative to field observations. Vegetative division persisted in both field and laboratory populations but differed in timing and intensity. Microzooplankton, especially ciliates and Gyrodinium spp. were identified as potential key grazers, while mesozooplankton showed limited impacts. Under elevated grazing pressure, the accumulation rates of D. acuminata declined, whereas net toxin production rates increased, suggesting a defensive strategy. These findings contribute to a deeper understanding of D. acuminata population dynamics and its toxin production in response to varing grazing pressure.
 

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