Marine plastic pollution is a worldwide problem. Microplastics (MPs) are the predominant form of marine plastic debris, a form small enough to be ingested by and potentially harm marine organisms. It is urgent to develop ecologically relevant metrics for the risk assessment of MPs based on in situ data, especially for coastal areas. For the first time, we performed a comprehensive study of the characteristics of MPs in seawater and zooplankton in the Yellow Sea. For MPs in seawater, the average concentration is 0.13 ± 0.20 pieces/m³, dominated by fragments (42%). The average size is 3.72 ± 4.70 mm, with the most frequent size appearing at 1200 μm. The major polymer types are polypropylene and polyethylene, accounting for 88.13% in total. The distribution of MPs in seawater is patchy, with high MP concentrations close to the coastal cities. The average concentration of MPs in 11 total zooplankton groups is 12.24 ± 25.70 pieces/m³. The average size is 154.62 ± 152.90 μm, with 90% being <500 μm. Fiber is the dominant shape of MPs found in zooplankton, accounting for 46%, but the composition of the polymer type is diverse. The retention of MPs in zooplankton depends on the taxa and their abundance in the Yellow Sea. Siphonophorea, Copepoda, Euphausiacea and Amphipoda are the main repositories compared to other groups, achieving 3.57, 2.44, 1.41 and 1.36 pieces/m³, respectively. The high concentration area of MPs in zooplankton appeared near the adjacent waters of the Yangtze estuary. These results prove that zooplankton act as a repository for MPs in coastal waters. The retention of MPs in zooplankton is recommended as a key index for further ecological risk assessment of MPs.

海洋塑料污染是一个世界性的问题。微塑料（MPs）是海洋塑料碎片的主要形式，其体积小到足以被海洋生物吸收并可能危害海洋生物。迫切需要根据原位数据（特别是沿海地区），开发出与生态相关的指标来进行MP风险评估。我们首次对黄海海水和浮游动物中MP的特征进行了全面研究。对于海水中的MP，平均浓度为0.13±0.20个/ m ³，以片段为主（42％）。平均尺寸为3.72±4.70 mm，最常见的尺寸为1200μm。主要的聚合物类型为聚丙烯和聚乙烯，占总量的88.13％。海水中MP的分布是零散的，MP浓度很高，靠近沿海城市。11个浮游动物各组中MP的平均浓度为12.24±25.70个/ m ³。平均尺寸为154.62±152.90μm，其中90％为<500μm。纤维是浮游动物中MP的主要形状，占46％，但聚合物类型的组成却多种多样。MP在浮游动物中的保留取决于黄海中的生物分类及其丰富度。与其他组相比，虹吸虫，足形纲，紫茎纲和双翅目是主要的储存库，分别达到3.57、2.44、1.41和1.36个/ m ³。浮游动物中MPs的高浓度区域出现在长江口附近水域附近。这些结果证明了浮游动物是沿海水域中MPs的储存库。建议将MPs在浮游动物中的保留作为MPs进一步生态风险评估的关键指标。