Seasonal pulse effect of microplastics in the river catchment-From tributary catchment to mainstream

Rivers have received extensive attention as a major pathway for microplastics (<5000 & mu;m) from land to ocean. This study investigated the seasonal variation of microplastic contamination in surface water of the Liangfeng River catchment, a tributary of the Li River in China, based on a fluorescence-based protocol, and further explored the migration process of microplastic in the river catchment. The abundance of microplastics (50-5000 & mu;m) was (6.20 & PLUSMN; 0.57)-(41.93 & PLUSMN; 8.13) items/L, of which 57.89-95.12% were small-sized microplastics (<330 & mu;m). The microplastic fluxes in the upper Liangfeng River, lower Liangfeng River, and upper Li River were (14.89 & PLUSMN; 1.24) x 1012, (5.71 & PLUSMN; 1.15) x 1012, and (1.54 & PLUSMN; 0.55) x 1014 items/year, respectively. The 3.70% of microplastic load in the mainstream came from the tributary input. Fluvial processes can effectively retain 61.68% of microplastics in the surface water of river catchments, especially for small-sized microplastics. The rainy season is the main period of microplastic retention (91.87%) in the tributary catchment by fluvial pro-cesses, while exporting 77.42% of one-year microplastic emissions from the tributary catchment into the mainstream. This study is the first to reveal the transport characteristics of small-sized microplastics in river catchments based on flux variation, which not only can partly explain the "missing small-sized microplastic fraction" in the ocean, but also contribute to improving microplastic model.