## 2 Materials and Methods  
### Study Area and Litter Sampling  
Sampling was carried out in Como Bay, which is located in the southern end of the western basin of Lake Como (Figure 1a,b). In this basin, Lake Como has no outlet [45]: the only outlet for Como Lake is the Adda river, that flows instead in the eastern basin (near Lecco city).  
Figure 1: (**a**) Satellite view of the Lake Como area. The location of Lombardy region is highlighted in orange in the map in the upper left corner. The main cities are indicated in orange, and Como (the study area) is highlighted in red. (**b**) Detail of the sampling area of Como, located in the southern west Como branch: sampling points are highlighted in orange, main towns are indicated by yellow dots, and the yellow dashed line indicates the main route of the local ferries. Main tributaries of lake Como in the area are also highlighted.  
Como city is instead drained by two streams, Valduce creek and Cosia stream (Figure 1b), which are presently artificially buried, and have an underground flow path through the urban area until it reaches the lake. These hydrological features can be important sources of plastic litter [46]. Valduce creek provides an ephemeral flow, and is mainly recharged by a small karst basin, whereas Cosia stream drains a larger watershed area (about 33.5 km\({}^{2}\)) with measured flows of up to 33 m\({}^{3}\)/s during its maximum flow regime. Moreover, in the area of Cernobbio town, the Breggia stream outflows into the lake after a 12 km path. This stream collects water from the western border of Lombardy and part of Switzerland (with a watershed area of about 47.4 km\({}^{2}\)).  
The demographic data of the study area can give insights into the littering sources: human pressure, in fact, has been identified as being the main cause of water pollution worldwide [47]. In this area, the principal municipalities are represented by Como and Cernobbio. According to the data for 2021, as reported by the National Institute of Statistics [48], the populations of these cities were 84,250 and 6498, with a density of 2300 and 536 inhabitants/km\({}^{2}\), respectively [48]. Moreover, this area is a known tourist destination: in the case of the Como Bay, the highest number of visitors occurs in spring and fall, with a weekly increase during the weekends [47].  
According to the most widely applied dimensional categorization of plastic fragments [49], this research work exclusively focused on plastic litter with dimensions >5 mm [50] and that were recognizable to the naked eye. Plastic beach litter was collected in 5 different locations on the lake shores (Figure 1b and Table 1). All beaches were surveyed during May 2021 and, for each one, the extension (in square meter) was registered. Beach litter was sampled along parallel transects, until the whole beach area was inspected. To determine the predominant factors in the deposition of plastic beach litter, for each location, the following features were considered: the distance from the urban centers, public accessibility, the presence of tributary streams, and the exposure of the beach. The characteristics of the sampling sites are recorded in Table 1.  
Since the data of the main flows are not available and the main currents are mostly constant during the season, the likely movement of the plastic litter was assessed, considering other natural and anthropic factors. Firstly, the wind speed and the velocity data in Como city (from Como station, located about 400 m away from the TP sampling point) were collected for the whole year of 2021 from the Lombardy environmental protection  
agency [51]. Another disturbing factor by waves, i.e., the passage of ferries, was also analyzed in this study. Indeed, this area of the lake is supported by an extensive navigation network that connects the city of Como with other towns on the lake shore (i.e., Cernobbio, Tavernola, and Blevio, Figure 1b), increasing wave rates toward the coast, especially in E-W direction.