Rainfall is a significant environmental factor of microplastic pollution in inland waters.

Worldwide, increasing attention is being paid to the issue of microplastic pollution in marine ecosystems, yet there is a relative lack of knowledge concerning the pollution of microplastic in inland water systems, although these microplastics are major sources of ocean pollution. In aquatic environments, previous efforts have mainly been devoted to exploring the impact of human-related activities on microplastic pollution, but little is known about non-anthropogenic effects on microplastic distribution. In this paper, the relationship between rainfall and the microplastic concentration of surface water was studied for the first time. The investigation was conducted in Lake Donghu (the largest urban lake in China) from July 23 to September 1, 2019. The abundance of microplastics in the lake water ranged from 7.4 to 29.6 items/L. The dominant size of the collected microplastics was less than 2 mm, with a dominant fiber shape and a dominant transparent color. Our results demonstrated that microplastic concentration was significantly related to rainfall. Therefore, high frequency sampling and rainfall data are needed to accurately evaluate microplastic pollution as well as its consequences.