## 1 Introduction  
Plastic is one of the most widely used materials in modern society. The production of plastics has increased annually from 0.5 million tons in 1950 to 330 million tons in 2016 [1]. However, the legacy has now become an intractable problem. The use and abuse of plastics caused serious white pollution worldwide [2, 3]. According to the ministerial declaration adopted by the United Nations Environment Assembly, plastic waste entering the ocean from land sources was 4.8-12.7 metric tons annually in 2017 [4]. Plastics, as a new type of emerging contaminant in the marine environment, have attracted ever-increasing attention of the scientific community, environmental policy makers, and the society as a whole [5]. Plastics can be divided to micro-, meso- and macroplastics with diameter, respectively, \(<\)5, 5-20 and \(>\)20 mm [6, 7]. Because of their large specific surface area, it is easy for plastics to adsorb chemical contaminants in water, such as persistent organic pollutants (POPs), metals, etc., and play an important role in contaminant transport across ecosystems [8, 9]. Thus, the combined toxicity of plastic particles and the absorbed pollutants is currently a hot research topic [10, 11, 12, 13, 14]. Meanwhile, the environmentally harmful additives like plasticizers, UV stabilizers, flame retardant, etc., which are added to improve plastic properties, would release to the environment [15; 16; 17; 18]. Furthermore, plastic particles could migrate in the food chain [19]. Thus, plastics can act as a powerful carrier for the transport of chemical contaminants in the environment through the food chain and eventually reach the human body [20; 21; 22; 23; 24]. Plastic particles, especially micoplastics, are now found almost everywhere worldwide, from the polar regions to the deepest part of the ocean--the Challenger Deep of the Mariana Trench [25; 26; 27; 28; 29; 30], and even in the air that we breathe [31]. At present, most of the research has focused on marine microplastic pollution. In fact, anthropogenic stress, including plastic pollution, has also caused huge irreversible changes in rivers [32]. Research on plastics in continental environments is increasing in recent years [33]. However, most of those studies are directed on large water bodies. [PERSON] et al. (2013) studied plastics in the Great Lakes of the United States and found that lake water had a very high concentration of microplastics (average concentration of 43,000 particles per km\({}^{-2}\)), and lakes in densely populated areas contained higher concentrations of microplastics than in remote areas [34]. Urban rivers, such as the Danube River [35] and the Rhine River [36], had plastics pollution in varying degrees. Furthermore, inland rivers with sparse population density, such as the Qinghai-Tibet Plateau (China), cannot avoid plastic intrusion [37]. The distribution of plastics in inland rivers and lakes is inseparable from the development of urbanization, from sewage discharge, surface runoff, littering, and atmospheric deposition [38; 39]. Although results of these past studies have increased our understanding of microplastics in the marine and continental environments, knowledge gaps remain as to the role of aquatic systems in the transitional zone between the two environments, in the accumulation, reworking, redistribution, and transport of plastics to the marine environment. Currently, there is limited understanding on fate and behavior of plastic particles across the freshwater-saltwater bodies and their potential contribution of plastic debris to the marine environment. We hypothesize that coastal lakes, located between and connected with the continental and marine environments, can act as a buffer zone in reprocessing and redistribution of plastic particles at catchment scale across ecosystems. In this study, we sampled a coastal lake in southeast Shanghai--the Dishui Lake.