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A Study of Salinity Effects on Phytoplankton Community Structure for Monitoring Plankton Blooms along the Coastal Areas of Mueang District, Chonburi Province. The objective of this scientific project was to investigate the relationship between salinity and the stability of phytoplankton community structures, and to compare the ecological dynamics between two study sites: Ang Sila and Wonnapha Beach. Water sam-ples were collected and analyzed from August to November 2025. The results demonstrate the critical role of salinity in maintaining ecological bal-ance. Under conditions where salinity remained at moderate to high levels (23.00 – 30.00 ppt), the community structure exhibited high stability and biodiversity. Various species of diatoms, such as Rhizosolenia spp. and Chaetoceros spp., coexisted harmoniously; this equilibrium effectively inhibited the rapid proliferation of any single species. Conversely, a sharp decline in salinity (below 18.00 ppt) caused by freshwater runoff—particularly observed at Wonnapha Beach in early September led to an ecological imbalance. This triggered monodominance, where the dinoflagellate Ceratium furca rapidly replaced other species, reaching a peak density of 941,750 cells/liter. In conclusion, salinity levels significantly influence phytoplankton community struc-tures and serve as a key indicator of the transition from an ecological equilibrium to a bloom state. These findings are highly beneficial for monitoring and forecasting the risk ofharmful algal blooms (red tides) in coastal environments. Keywords: Salinity , Phytoplankton , Population Structure , Plankton Bloom