## 4 Discussion  
### Sources of Microplastics in Dishui Lake Sediment  
The Dishui Lake is surrounded by developing towns. Previous studies have shown that urban rivers, hyporheic zone of rivers, the river bed, and inland lakes accumulated large amounts of microplastics, such as the Beijing River (China), River Rhine, and the Main River (Germany), rivers in Northern England, and Lake Winnipeg (Canada) [33; 45; 46; 47; 48]. Our results showed that the distribution of microplastics in the Dishui Lake sediment is affected by human activities. The sampling sites on the north side of the lake (DSL-1, DSL-3, DSL-5), which is relatively densely populated, had higher concentrations of microplastics than the eastern side, which is mostly farmland (DSL-9, DSL-10, DSL-11, DSL-12, D-1). This conclusion is also supported by the elevated concentrations of microplastics detected at site D-2. This sampling site is close to a power supply control center, where various anthropogenic activities take place daily, such as inspection and repair work. Our finding is consistent with observations in previous studies. [PERSON] et al. (2013) found that, among the five Great Lakes, Lake Erie, which is near the most populated cities, had consistently higher amounts of microplastics than Lake Superior and Lake Huron [34]. [PERSON] et al. (2019) showed evidence that human activities affect the abundance of microplastics in the middle and lower reaches of the Yangtze River [49]. Similarly, a recent study by [PERSON] et al. (2018) indicated that population density and quantities of industrial waste were important sources for plastic accumulation in fluvial sediment [44]. The concentration of microplastics in lakes of Wuhan also exhibited a decreasing trend away from urban centers [50].  
Microplastics in the environment come from two main sources--the primary source and the secondary source. The primary source refers to those that directly contribute plastics to the environment, such as urban rubbish, industrial waste, and fishing waste, etc. [51; 52; 53]. Once released into the environment, macroplastic, mesoplastics, and microplastics can be fragmented into smaller pieces under the effects of physical, chemical, and biological processes [38; 54; 55]. These constitute the secondary source of microplastics in the environment. In previous studies, line or fiber microplastics were found to be important plastic components in rivers and coastal waters and were suspected to come from laundering [56; 57]. Our results showed that sheet microplastics were the most abundant microplastics in the Dishui Lake sediment, suggesting that laundry wastewater discharging was not the main source. In addition, we found that PP microplastics were the most abundant type of microplastics. PP is widely used in woven products, food packaging, and pipes. The detection of PSfoams in the Dishui Lake sediment further suggests that food packaging plastic material is likely the primary source of plastic particles in the lake. It is interesting to note that macroplastics, mesoplastics, and microplastics all exhibited similar characteristics in color, shape, and polymer type, suggesting that the different sizes of plastic particles found in the lake sediment were from the same sources.