This study assessed seawater quality, sediment properties, hydrodynamics, and microplastic contamination across seagrass ecosystems in Chumphon Province, Thailand. Surveys were conducted in Tung Maha Bay (Enhalus acoroides), Tung Kha-Sawi Bay (Halophila beccarii), and Bo-Mao Bay (Halodule pinifolia) during the dry (June 2025) and wet (September 2025) seasons to evaluate spatio-temporal environmental variability.
While most seawater parameters complied with Thailand's coastal water standards, salinity, turbidity, and nutrients showed marked spatial and seasonal variation. Tung Kha-Sawi Bay consistently exhibited higher nutrient levels, while ammonia increased during the rainy season. Wet-season Noctiluca sp. blooms in Tung Maha and Bo-Mao Bays indicated enhanced terrestrial runoff. Sediment characteristics revealed declining nitrogen but increasing orthophosphate concentrations during the wet season, suggesting active biogeochemical cycling.
Microplastic contamination remained relatively low compared to other Thai coastal regions but varied spatially and seasonally. During the dry season, microplastic abundance ranged from 0–20 items kg⁻¹ D.W. in sediments (mean 4.83 ± 4.57) and 0–37.50 items m⁻³ in seawater (mean 7.96 ± 10.45), with the highest averages in Bo-Mao Bay. During the wet season, sediment abundance increased to 0–48.08 items kg⁻¹ D.W. (mean 10.76 ± 11.76), while seawater abundance decreased to 0.14–5.26 items m⁻³ (mean 1.90 ± 1.32), with peak concentrations in Tung Kha-Sawi Bay. Textile fibers, particularly polyester and cotton, dominated. Hydrodynamic simulations indicated circulation in Tung Kha-Sawi Bay is primarily driven by tides and monsoonal winds, with stronger currents during the Northeast monsoon.
Overall, all three seagrass meadows remain in generally good ecological condition despite distinct pressures. Tung Maha Bay exhibited stable conditions, Tung Kha-Sawi Bay experienced higher nutrient enrichment, and Bo-Mao Bay showed the highest microplastic loads. These findings provide critical baseline information to support targeted conservation and sustainable management of regional coastal ecosystems.
 

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