## 3 Results
### Comparison of the Microplastic Content of Samples Collected in 2019 and 2020
It is also reflected by the fact that the location of the most polluted sediment in 2019 (No. 11: 7533 item/dry kg) shifted downstream to the end of the section (No. 18: 4477 item/dry kg), and the originally polluted No. 11 site became the least polluted in 2020 (440 item/dry kg). In 2019, the four studied tributaries of section S2 were the most polluted of all tributaries, but in 2020, the average concentration of the MPs in their sediments decreased by 84% (Table 2). In 2019, the Nagyag was the most polluted tributary (D1: 7115 item/dry kg), but in 2020, the Talabor had the maximum MP contamination (C1: 1080 item/dry kg). Section S3 is the only section where the mean MP content of the samples increased (by 7%) between 2019 and 2020 (Table 2); thus, the least polluted section in 2019 became the most polluted by 2020. On the other hand, the minimum and maximum MP contamination values decreased by 75 and 17%, respectively, and their sites were relocated (Figure 9). The most polluted sample (No. 25: 8067 item/dry kg) became slightly polluted one year later (986 item/dry kg), and highly polluted sediment appeared at the No. 24 site (4187 item/dry kg). Contrary to the Tisza samples, the MP content of the tributaries of this section decreased by 6%. In 2019, the most polluted tributary was the Bodrog (G1: 5017 item/dry kg), but one year later, the Sajo River became the most contaminated (I2: 6383 item/dry kg). In section S4, the pollution of the 2020 samples was 17% lower than in 2019. While in 2019, this section was more polluted than section S2 and S3, in 2020, this section became the most polluted section along the entire Tisza (Table 2). The most polluted site (No. 39: 7809 item/dry kg) in 2019 became the least polluted (299 item/dry kg) in 2020 (Figure 9), as the amount of the MP decreased by 97%. Interestingly, the purest sample of this section during the 2019 measurement (No. 38: 528 item/dry kg) became the most polluted by 2020 (6523 item/dry kg). The amount of MP in the sediments of the tributaries joining this Tisza section decreased by 46% (Table 2). In both years, the least polluted tributary samples were the upstream samples of the Zagyva River (at J1 in 2019: 900 item/dry kg; 2020: 787 item/dry kg). However, in 2020, the most polluted tributary's sediments were collected at the impounded, near-to-the-confluence section of the Zagyva (J2: 6533 item/dry kg), though in the previous year, the Koros River was the most contaminated (K1: 2780 item/dry kg). The amount of MP on each geomorphological form was also compared to see whether they changed in a similar way between 2019 and 2020 (Figure 10). While the MP content of the sediments decreased in general, this was not typical for the point-bars, which have  
Figure 9: Differences in microplastic content (item/dry kg) of sediments collected at the same locations from coarse sediments in 2019 and 2020. (The capital letters refers to the sample ID of the tributaries, see Figure 1). become more polluted by 10% in a year. At the same time, the amount of the MP decreased by 29% in the side-bars and by 51% in the sedimentary sheets.