## 3 Results
### Comparison of the Microplastic Content of Samples Collected in 2019 and 2020  
Some of the sampling points were different during the 2019 and 2020 sampling campaigns, as we were able to collect samples in some places in 2019 but not in 2020 (e.g., along the Ukrainian-Romanian country border), and some new sampling points were selected in 2020 (e.g., Lower Tisza in Serbia). In addition, in 2019, only coarse sediments were collected [29]. Therefore, only coarse samples from the same location were selected for the comparison, and those sampling sites were excluded, which were sampled just once. The samples collected in 2019 were much more polluted (Tisza: 3149 \(\pm\) 2010 item/dry kg; tributaries: 3827 \(\pm\) 1584 item/dry kg) than those collected in 2020 (Tisza: 2220 \(\pm\) 1697 item/dry kg, tributaries: 2007 \(\pm\) 1821 item/dry kg). Thus, the average microplastic pollution of the Tisza sediments decreased by 30% and that of the tributaries by 48% in 2020 (Table 2). The greatest decrease was detected in the sediments of the tributaries, which was observed in 90% of the samples. However, the extent of the decrease varied from section to section. In section S1, the average MP content of the sediments decreased significantly by 79% (Table 2). While in 2019, this section was the most contaminated, in 2020, it became the least polluted one. The location of the minimum MP contamination remained at the same site (No. 3 on Figure 9); however, its amount decreased by 90% (2019: 2657 item/dry kg; 2020: 260 item/dry kg). The most polluted sampling point in 2019 was one of the source branches of the Tisza (No. 1: 6738 item/dry kg), but in 2020, the most polluted sampling point was identified further downstream (No. 4: 2620 item/dry kg), though here the amount of MP in the sediment still decreased below the minimum value of the section in 2019. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline  & & **Mean in 2019** & **Mean in 2020** & **Changes (\%) between** & & \\  & **(item/dry kg)** & **(item/dry kg)** & **2019 and 2020** & & **Range in 2019** & **Range in 2019** \\ \hline \multirow{4}{*}{**S1:** Upper Tisza, upstream & 4102 \(\pm\) 1568 & 880 \(\pm\) 979 & –79 & 2657–6738 & 260–2620 \\  & S2: Upper Tisza, downstream & 3073 \(\pm\) 2066 & 1856 \(\pm\) 1505 & –40 & 729–7533 & 440–4477 \\  & S3: Middle Tisza, upstream & 2334 \(\pm\) 2042 & 2501 \(\pm\) 1948 & +7 & 906–8067 & 237–6707 \\  & S4: Middle Tisza, downstream & 3549 \(\pm\) 2049 & 2928 \(\pm\) 1572 & –17 & 528–7809 & 299–6523 \\  & Tisza average & 3149 \(\pm\) 2010 & 2220 \(\pm\) 1697 & –30 & & \\ \hline \multirow{4}{*}{**S2:** Upper Tisza, upstream & - & - & - & - & - \\  & S2: Upper Tisza, downstream & 4362 \(\pm\) 1422 & 706 \(\pm\) 269 & –84 & 2997–7115 & 420–1080 \\ \cline{1-1}  & S3: Middle Tisza, upstream & 3687 \(\pm\) 1155 & 3479 \(\pm\) 2103 & –6 & 2211–5017 & 537–6383 \\ \cline{1-1}  & S4: Middle Tisza, downstream & 3138 \(\pm\) 2482 & 1706 \(\pm\) 820 & –46 & 900–6533 & 787–2780 \\ \cline{1-1}  & Tributaries average & 3827 \(\pm\) 1584 & 2007 \(\pm\) 1821 & –48 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Microplastic content (item/dry kg) of coarse sediments collected along the Tisza and its tributaries in 2019 [29] and 2020. Figure 8: (**A**) Relationship between the grain size (\(d_{50}\)) of the samples and their microplastic content (item/dry kg) collected on various in-channel forms from the reaches of the Tisza. (**B**) Spatial changes in mean microplastic content (item/dry kg) of the various forms collected from the three reaches of the Tisza. Upper Tisza: S1–S2; Middle Tisza: S3–S4; Lower Tisza: S5. In section S2, the difference between the two surveys decreased, as the average MP content of the samples collected in 2020 was just 40% lower (Table 2). In 2019, the average MP content of this section decreased by 25% compared to the upstream section S1; however, in 2020, the trend had changed, and the MP content doubled. This reflects an emptying MP pollution from the upstream section S1 (Figure 9).