[Effects of Flow Concentration on Water Microplastic Pollution in the Luoshijiang Sub-watershed of Erhai Lake Basin].

With the wide application of plastics, the environmental problems and health risks brought by microplastics are gradually becoming prominent and the effects of flow concentration on the microplastic pollution in the water environment of a closed sub-watershed remain unclear. Therefore, a comprehensive survey was carried out at the sub-watershed scale to explore the distribution characteristics of microplastic pollution in the water of tributaries, main streams, and lakes of the Luoshijiang sub-watershed in the Erhai Lake Basin. The effects of flow concentration on water microplastic pollution were also analyzed. The results showed that： ① The abundance of microplastics in the water of the stemflow （14 500 n·m^-3） and lake （16 562 n·m^-3） was significantly higher than that of tributaries （8 714 n·m^-3,P<0.05） in the Luoshijiang sub-watershed. The main polymer types were rayon （47.16%） and polyester （38.41%）. The particle size of microplastics was concentrated in the range of 0.2-1 mm （73.59%）, and more than 99.37% of microplastics were fiber-shaped, and the main color was transparent （65.08%）. ② The microplastic diversity index followed the sequence as main stream>tributary>lake. The stemflow had the highest abundance and polymer and color types of microplastics. The flow concentration in the river showed a net increase in the microplastic abundance and diversity index of the main stream. ③ The pollution characteristics of microplastics including particle size （0.2~0.5, 0.5~1, and 1~2 mm）, color, and shape in water showed a significant positive correlation with that in adjacent soil （P<0.05）. Soil and water PES, 0.5-1 mm microplastic distribution, and microplastic size diversity showed higher similarity. The overland flow caused soil microplastics to migrate into water. In the Luoshijiang sub-watershed with the independent and enclosed features, the flow concentration altered the migration and distribution of microplastics. Plastic pollution control and source reduction at a small watershed scale should be strengthened.