A B M Sadique Rayhan ^a, Bowen Cui ^a,Kazi Belayet Hossain^a,  Hongwei Ke ^a, Minggang Cai ^a,* *Corresponding author: Dr. Minggang Cai, Professor
E-mail: mgcai@xmu.edu.cn
Full postal address: College of Ocean and Earth Science, Xiamen University

Abstract
Microplastics (MPs) pollution in coastal sediments poses escalating ecological risks, yet comprehensive mass-informed assessments spanning broad latitudinal gradients remain scarce. This study presents the first integrated evaluation of MPs contamination across 17 major coastal systems from the Bohai Sea to the Beibu Gulf, China. MPs concentrations ranged from 12.40 to 185.0 items/kg (mean: 93.08 ± 51.62 items/kg), with fibers dominating overall shape distribution (43.5%). The total estimated mass inventory reached 171.4 tonnes within the upper 5 cm of sediment, revealing pronounced spatial heterogeneity in mass density (0.12-8.90 kg/km²). Getis-Ord G_i* hotspot analysis identified primary hotspots in the Bohai Sea and Jiangsu Coast (Gi*Z > 3.2, p < 0.001) where semi-enclosed hydrodynamics create effective retention traps, while significant coldspots in Hangzhou, Sanmen, and Quanzhou bays (G_i*Z < -2.45, p < 0.014) demonstrate that high-energy regimes decouple sedimentary burdens from proximate anthropogenic pressure. Convergent validation via K-means clustering confirmed the robustness of spatial classifications. Source apportionment using PCA-MLRA revealed regionally differentiated contributions: fishing and aquaculture dominated southern systems (38-49%), sewage and textile manufacturing prevailed in central regions (52-63%), while industrial and packaging inputs characterized northern hotspots (45% combined). In fishing areas, plastics like PP, PE, and nylon were found, while in places affected by wastewater, materials like PES and semi-synthetics appeared, providing chemical fingerprints substantiating source attributions. These findings reveal that China's coastal regions act as MP sinks rather than conduits to the open ocean, challenging transboundary flux narratives and informing national inventories and management strategies.

Keywords: Microplastics (MPs); Coastal sediments; Mass inventory; Hotspot analysis; Source apportionment.

ACKNOWLEDGEMENT
This work was supported by the National Natural Science Foundation of China (NSFC) ( U25A20790 and 42576044), the Industry-University-Research Joint Innovation Project of Fujian Provincial Department of Education (2025Y4002), and the Guangxi Key Research and Development Program (AB25069113).