Phytoplankton Composition Indicators for the Assessment of Eutrophication at Patong Beach, Phuket

Abstract

Recurrent phytoplankton blooms at Patong Beach, Phuket, Thailand, a major international tourist destination, pose a potential threat to coastal ecosystems, public health, and the local tourism economy. This study monitored phytoplankton community structure, cell abundance, diversity indices, and chlorophyll a (Chl a) at five stations spanning a drainage canal and adjacent beach waters from December 2022 to September 2023. Bloom events were observed at the southern end of the beach in February and April. In February, the estuarine station was dominated by the colonial diatom Chaetoceros socialis (up to 99% of total abundance; 695,750 cells·L⁻¹), while the beach stations showed a strong contribution from the dinoflagellate Peridinium quinquecorne. In April, C. socialis dominated all marine stations, coinciding with sharp reductions in species diversity and Chl a concentrations approaching 100 µg·L⁻¹. Blooms were not observed in the drainage canal during the study period, despite its strong land-based influence, suggesting that physical retention conditions in the bay, together with nutrient inputs, may be important in promoting bloom formation near the beach. Shannon–Wiener diversity (H′) and Margalef richness (d) were the most sensitive indices for detecting bloom-related community change and are recommended for routine monitoring. These findings provide a useful framework for bloom detection and risk assessment at Patong Beach and comparable tropical tourist destinations, while also highlighting the need for future studies integrating nutrient dynamics, sediment cyst surveys, and hydrodynamic observations.