Surface chlorophyll-a dynamics in the upper gulf of Thailand revealed by a coupled hydrodynamic-ecosystem model

Although plankton bloom incidents in the upper Gulf of Thailand (UGoT) have been reported, no dynamic investigation of the phenomenon has been conducted. To address this need, a simple pelagic ecosystem model coupled with the Princeton Ocean Model (POM) was employed to investigate seasonal variations in surface chlorophyll-a (chl-a) distributions to clarify phytoplankton dynamics in this area. The results revealed patterns of seasonal chl-a distribution that correspond to local wind, water movement and river discharge. High chl-a patchiness was found to be concentrated near the western coast following westward circulation near the northern coast developed during the northeast monsoon. During the southwest monsoon high concentrations were observed around the northeastern coast due to eastward flow. The simulated results could explain the seasonal shifting of phytoplankton blooms, which typically arise along the western and eastern coasts during the northeast and the southwest monsoons, respectively. Sensitivity analyses of simulated chl-a distributions demonstrate that water stability, including wind-induced vertical currents and mixing, plays significant roles in controlling phytoplankton growth. Nutrients in the water column will not stimulate strong plankton blooms unless upwelling develops or vertical diffusivity is low. This finding suggests an alternative aspect of the mechanism of phytoplankton bloom in this region.