Estuarine Plastic Dynamics: Analyzing Export Patterns from a Typical Semi-Enclosed Bay in Asia

Semi-enclosed estuarine systems are recognized as hotspots for microplastic (MP) pollution, yet their export dynamics remain poorly understood. This study investigates the distribution of microplastics (<5 mm) and macroplastics (>5 mm) in Sanya Bay, a typical semi-enclosed bay in southern China. Based on 45 surface water samples, MP concentrations ranged from 0.298 to 4.010 items/m³ (mean: 1.628 ± 1.055), significantly higher than macroplastics (mean: 0.109 ± 0.338 items/m³). Most particles were fragments smaller than 5 mm, dominated by white, green, and transparent colors. The main polymers identified were polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET). A “dual-center” pattern was observed: low-density plastics (e.g., PP, PE) accumulated near river mouths and offshore islands, while high-density plastics (e.g., PET, PVC) settled in estuarine zones. Principal Coordinate Analysis (PCoA) revealed spatial clustering across estuarine, coastal, and offshore areas, influenced by land-based inputs, human activity, and hydrodynamic transport. This study provides new insights into plastic transport mechanisms in semi-enclosed bays and highlights the importance of integrating plastic properties with environmental forces. The findings support targeted pollution management in similar coastal environments.