Abstract

• The spatial and temporal variations of phytoplankton were governed by the physicochemical properties of the water column such as water stratification, vertical mixing, temperature and salinity gradients, and nutrient supply. In this study, the salinity gradient was pronounced in the upper layer, and a strong stratification developed in the water column which in turn influenced large variability in the vertical distributions and cell concentrations of phytoplankton. In terms of cell concentration, nine species such as Lauderia annulata, Skeletonema sp., Thalassiosira spp., Chaetoceros sp., Thalassionema frauenfeldii, T. nitzschioides, Pseudo-nitzschia sp., and Heterosigma akashiwo dominated the phytoplankton community, and the percentage of their abundance comprised 89.5% of the total phytoplankton concentration. Among these species, the cells of Pseudo-nitzschia sp. and H. akashiwo increased at night, approximately 4 to 8 times higher than in the daytime, due to the changes in the water mass. At the most depth range, all species had higher cell concentrations in the upper layers above the pycnocline where Chl-a showed the maximum. However, a significant decrease in cell concentration was noted below the pycnocline where the hypoxic water mass was observed.