Research progresses of microplastic pollution in freshwater systems.

Microplastics (MPs) have received widespread attention as an emerging environmental pollutant. They are ubiquitous in the freshwater system, causing a global environmental issue. The current features and perspectives of MPs in the freshwater systems can provide the concerns of their ecological effects, which has not been addressed widely. Therefore, in this study, we reviewed the characteristics of MPs in freshwater environments and discussed their sources and potential impacts. The abundance of MPs in freshwater system ranged from approximately 3-6 orders of magnitude in different regions. There colors were mainly white and transparent, with polypropylene (PP) and polyethylene (PE) as the major polymers. The main shape of these MPs was fibers with dominant size of less than 1 mm. Analysis indicated MPs in freshwater system mostly originated from human activities such as sewage discharge in highly contaminated areas, while atmospheric long-distance transport and precipitation deposition played an important role in remote areas. Freshwater MPs pollutants also affected drinking water and aquatic organisms. Because the abundance of MPs in organisms was relatively balanced, the pollution level of biological MP pollution cannot accurately characterize the pollution status in the watershed currently. Future research should focus and strengthen on periodic monitoring to characterize the temporal and spatial changes of MPs, and enhance toxicological research to explore MPs pollution impact on biota and humans.