Microplastics (MPs) was first proposed as a new persistent pollutant in marine water and sediments in 2004 and soon attracted widespread attention. MPs are plastic particles and debris with the diameter less than 5 mm (Thompson et al., 2004). According to the global statistics, approximately 300 million tons of plastic materials were produced every year, and it is growing at a rate of 20 million tons annually (Matsuguma et al., 2017; Zhang et al., 2020). MPs main include polyamide fiber (PA), polypropylene (PP), polyethylene (PE), Polyester fiber (PET) etc (Thompson et al., 2004). The shape of MPs is divided into fragments, films, fibrous, granular and foaming. Han (2018) vividly describes MPs as "PM 2.5" in the ocean. MPs from land is proved to be an important source of MPs in the ocean, accounting 80% of plastics in the oceans (Bakir et al., 2012). MPs are widely distributed in environment matrices including rivers (Peng et al., 2020), lakes (Yuan et al., 2019), wetlands (Jian et al., 2018; Yu et al., 2021), soils (Henseler and Gallagher, 2021) and air (MeralYurtsever. et al., 2018). Because of its small size, MPs are easy (Heim and Nemeroff, 2001) to be ingested by aquatic organisms and not easy to be digested by organisms, which may cause damage to growth and development and even death (Yang and Huang, 2019). At the Second Joint Environment Conference in 2016, it was proposed that MPs have been listed as the second largest scientific problem in the field of environmental and ecological science research and become one of the major global problems (Xia et al., 2019).
Lakes are relatively closed bodies of water. Unlike rivers serving as the main transport route, lakes are more like "oceans" in inland waters and act as sinks of pollutants. MPs can accumulate in lakes, and could have a more direct impact on freshwater organisms, terrestrial organisms and humans themselves (Xiong and Wu, 2021). Both lakes and wetlands are places where MPs accumulate and buffer (Qian et al., 2021). Duan et al. (2020) found that MPs were widely detected in Yellow River Delta wetland even in the protection area with little human activities and MPs ranged from 136 to 2060 items /kg in soil samples with component being PET (536 to 660 mg /kg) and PC (83.9 to 196 mg /kg). Zhou et al. (2021a) found the average MPs was (558.4±233.3) items /kg in sediment of Fuhe river in Hebei, China with the proportion of fragmentary MPs accounting for 66.1% of the total. Zhou et al. (2021b) studied MPs in the main upstream river flowing to Baiyangdian lake and found that MPs decreased with the increase of sediment depth with the highest content being 1049 ± 462 items /kg in the topmost sediment layer (0-5 cm). PE and PP were the main plastic polymer types in all sediment samples. Cordova et al. (2021) investigated MPs in mangrove sediment, Indonesia, where the average abundance of MPs was 28.09±10.28 items / kg of dry sediment. Foam form was the most dominant in all the samples and was found more abundant on the outside.
As the largest shallow lake in north China Plain, Baiyangdian lake plays an important role in maintaining the balance of ecosystem in northern China (Li et al., 2021; Zhao et al., 2021). Currently,with numerous treatment and construction measures being taken, great changes are taken place here day by day and the environment of Baiyangdian lake has been greatly improved. While the MPs especially with the µm sizes, which can act as carriers of pollutants, is still a problem restricting the development of ecological environment. In view of this situation, this study selected Baiyangdian lake as our research object and carried out experimental research aiming at clarifying the level and distribution characteristics of MPs in the water and sediment of Baiyangdian lake.