## 3 Results
### Microplastic Content in Various Geomorphological Situations  
Sediment samples were collected from the different in-channel depositional forms of the Tisza (Figure 6). The point-bar sediments contained a slightly higher amount of MP (mean: 2049 \(\pm\) 1218 item/dry kg) than the side-bars (mean: 1692 \(\pm\) 1335 item/dry kg) or the sediment sheets (mean: 1584 \(\pm\) 1464 item/dry kg). However, remarkably high MP content was not measured on a point-bar but a side-bar (No. 38: 6523 item/dry kg) and sediment sheet (No. 29: 6707 item/dry kg). The difference between the geomorphological forms of the tributaries was smaller, and here, slightly fewer MPs (\(1617\pm 2015\) item/dry kg) accumulated on the point-bars, while the side-bars and sediment sheets were slightly more polluted (1965 \(\pm\) 1496 and 1871 \(\pm\) 1120 item/dry kg, respectively). Point-bars developed along the meandering sections (S2-S4), but they are missing from the upstream section of the mountainous Upper Tisza (S1), and they disappeared from the Lower Tisza (S5) as the result of the incision initiated by channel regulation works. According to the results, the amount of the MP particles in the point-bar sediments showed a similar trend (Figure 6) as the river sections themselves; thus, the point-bars in the S3 section had the highest MP content (mean: 2985 \(\pm\) 672 item/dry kg). In each section, more MPs were deposited in the point-bars than the section average; the largest difference (E: +43%) from the section average was measured in section S3, while in the other sections (S2 and S4), the difference was only 7-8%. It is also interesting to see how the pollution developed in the point-bars in successive sections. The point-bars show the strongest enrichment in section S3 containing 70% more MP than section S2 and 28% more than the section below. _Side-bars_ formed along the slightly sinuous sections, and they can be found along the entire length of the Tisza (Figure 6). The amount of the MP particles on the side-bars of the individual sections showed a similar trend as the sections and the point-bars; thus, the amount of MP was the highest in section S3 of the Middle Tisza (average: 2272 \(\pm\) 1302 item/dry kg). However, in contrast to the point-bars, only slightly more MPs were deposited in the side-bars (E: +0-3%) than the mean of the actual section. The only exception was section S3, where the side-bars accumulated 9% more MP than the section average. As these forms occur along the entire Tisza, the spatial difference between the individual sections could be well-assessed. The enrichment rate was the highest between section S1 and S2, where the amount of the MP increased by 66% on the side-bars, but it decreased (E\({}_{\text{S3/S2}}\): 39%). However, from this section, the side-bars contain less and less MP toward the downstream sections, as the mean values of the sections decreased by 14% (E\({}_{\text{S4/S3}}\)) and by a further 72% (E\({}_{\text{S5/S4}}\)). It must be noted that these changes were similar to the changes of the section averages. The _sediment sheets_ were deposited on the steep sides of the almost straight channel. Except for the uppermost section, such forms were sampled all along the Tisza. The spatial changes of the mean MP content of the sediment sheets did not follow the same trend as the sections, point-bars or side-bars (Figure 6), as they had the highest MP concentration (mean: 1983 \(\pm\) 943 item/dry kg) in section S4. Considering the downstream changes of the MP content of the sediment sheets, it was interesting that from section S2 to S3 it increased by 63%, similarly to the point-bars. However, the MP content further increased toward section S4 by 16%, which showed a completely different spatial pattern than the other forms. The amount of the MP particles in the sediment sheets was lower than the average of a given section by 2-36%, which is in sharp contrast to the enrichment in the point-bars.