POPULATION DYNAMICS OF GREEN<i>NOCTILUCA SCINTILLANS</i>(DINOPHYCEAE) ASSOCIATED WITH THE MONSOON CYCLE IN THE UPPER GULF OF THAILAND¹

<jats:p>Population dynamics of<jats:italic>Noctiluca scintillans</jats:italic>(Macartney) Kof. et Swezy containing the photosynthetic endosymbiont<jats:italic>Pedinomonas noctilucae</jats:italic>(Subrahman.) Sweeney was investigated in relation to environmental conditions in the upper Gulf of Thailand. A clear association was observed between the abundance of<jats:italic>N. scintillans</jats:italic>and the monsoon cycle, with its blooms occurring during the southwest (SW) monsoon from May to September, and low abundance during the northeast (NE) monsoon from November to February. Nutrient concentrations were higher during the SW monsoon than during the NE monsoon due to the combined effect of increased river discharge into the northern upper gulf and the transport of the riverine inputs by the prevailing clockwise circulation of the water. These nutrient conditions favored the growth of both phytoplankton and the endosymbiont. Correlation analysis revealed that the higher abundance of<jats:italic>N. scintillans</jats:italic>in the SW monsoon was manifested primarily by higher growth through both sexual and asexual reproduction supported by phagotrophy. However, the dependence of<jats:italic>N. scintillans</jats:italic>on the nutrient concentration was not significant, probably because the nutrient supply for the endosymbiont was sufficient due to intracellular accumulation of nutrients within the host cells. Sexual reproduction occurred only during the SW monsoon, and its potential importance in population growth was suggested. These findings showed the bottom‐up control of the population dynamics of<jats:italic>N. scintillans</jats:italic>through growth of phytoplankton as prey. The seasonal shift in the circulation pattern associated with the monsoon cycle played a crucial role in blooming of<jats:italic>N. scintillans</jats:italic>by producing favorable food conditions.</jats:p>