Abstract: Noctiluca scintillans, a species of dinoflagellate, is a globally common species associated with red tides. It can produce a large number of gametes during sexual reproduction, but their role in the formation and termination of blooms remains understudied. We collected samples during a bloom of N. scintillans from offshore of Ningde in 2021, and enumerated the abundance using traditional microscopic counting method. There was an obvious shift from diatoms to dinoflagellates, accompanied by the decrease of inorganic nitrogen and silicate. The maintenance of the population density of N. scintillans relies on the abundance of diatoms, while other dinoflagellates can not be effectively consumed by N. scintillans, which may explain the termination of Noctiluca scintillans bloom. In addition, the temporal and spatial distribution of N. scintillans’ gametes were investigated by quantitative PCR, and it was found that there was a significant positive correlation between gametes and vegetative cells. Under favorable environmental conditions with ample food resources, the gametes released by N. scintillans may sustain blooms. However, when the trophonts density reaches the environmental limits, the the bloom terminated with disintegration of trophonts and the diffusion of gametes. These findings highlight the regulatory role of gametes in bloom dynamics, offering valuable insights into the mechanisms underlying N. scintillans bloom events.