Impact of seasonal variations on microplastic accumulation and characteristics in sandy beaches of Sichang Island, the inner Gulf of Thailand

Microplastics (MPs) are pervasive environmental pollutants whose fate, transport, and ecological impacts require clarification. This study examines the abundance and characteristics of MPs (16–5000 μm) in sandy beach sediments on Sichang Island, the inner Gulf of Thailand, during the dry season and after a four-month wet season. On the western beach, exposed to monsoon winds and currents, MP abundance increased from 2295.38 ± 1227.44 pieces/kg in the dry season to 2386.63 ± 121.45 pieces/kg post-wet season. Conversely, the leeward eastern beach showed a significant decrease, from 686.29 ± 243.90 to 238.63 ± 121.45 pieces/kg. Polypropylene was the dominant polymer, with reduced variation in MP color and shape following the wet season. The proportion of smaller MPs (16–100 μm) decreased from 59 to 62 % in the dry season to 27–42 % post-wet season, while larger debris became more prevalent. These findings highlight seasonal shifts in MP accumulation and dispersal on sandy beaches influenced by hydrodynamic conditions.