Monsoonal and tidal influences on plastic debris accumulation along a tropical coast

Although microplastic contamination has increasingly been documented in Thai coastal environments, comparable information on monsoonal and tidal-zone variation in macroplastic debris remains limited. This study investigated sediment-embedded macroplastic debris (>5 mm) along the Rayong coast, Gulf of Thailand, during the northeast (NE), southwest (SW), and inter-monsoon (IM) periods, and surface macroplastic debris during the SW and IM periods. Embedded debris was collected from triplicate 50 × 50 cm quadrats in each of two operationally defined tidal zones at 12 stations: the high-tide line and a lower-beach position 10 m seaward. Surface litter was surveyed only during SW and IM using one 10 × 10 m quadrat at each station, covering 1200 m² per campaign. Plastic debris was characterized by shape, color, size, and polymer composition using visual examination and ATR-FTIR spectroscopy. Mean embedded macroplastic abundance was 16.2 items m^-2, peaking during NE (30.9) and at the high-tide line (19.3). Negative binomial GLMs confirmed significant monsoonal and tidal-zone effects (p < 0.05). Fibers, white items, mesoplastics (5-25 mm), PE, and PP predominated. PERMANOVA revealed significant monsoon-related variation in shape, color, size, and polymer composition. Surface-litter density was higher during IM than during SW. Correspondingly, item-based Clean Coast Index values increased from 8.27 ("moderate") during SW to 18.12 ("dirty") during IM. Sand dominated and varied spatially, but grain-size fractions were not significant with sediment-embedded macroplastic abundance. These findings provide a regional baseline for season- and location-specific monitoring and targeted plastic-litter management.