# Spatial Variations in Microfiber Transport in a Transnational River Basin
## 3 Results
### Comparison of the Microplastic Transport of the Tisza and Its Tributaries in 2021 and 2022  
The average microplastic pollution of the Tisza in 2022 was 22.4 \(\pm\) 14.8 items/m\({}^{3}\); thus, compared to the 2021 data, the contamination increased by almost 20% (Table 1, Figure 5). While in 2021 at the most polluted point (Site \"i\"), the transported MP was 42 items/m\({}^{3}\), in 2022 more sites had even higher values: Site \"f\": 45 items/m\({}^{3}\); Site \"u\": 63 items/m\({}^{3}\); and Site \"x\": 46 items/m\({}^{3}\).  
In 2022 the tributaries transported 27 \(\pm\) 19 MP items/m\({}^{3}\) on average, which was 20% greater than the mean value of the Tisza. The most contaminated tributaries were the Zagyva (J: 63 items/m\({}^{3}\)) and the Szamos (E: 48 items/m\({}^{3}\)). The Hernad (H), Sajo (I) and Maros (L) were moderately polluted (23-25 items/m\({}^{3}\)), while the least polluted (11-11 items/m\({}^{3}\)) tributaries were the Kraszna (F), Bodro  
Figure 5: The amount of transported microplastics in the fluvial system of the Tisza in 2022, and the difference between the data of the surveys made in 2021 and 2022.  
Although in 2022 no measurements were made along the Ukraine section, the two Hungarian samples from the Upper Tisza S2 section reflected also considerable MP pollution increase (Site \"#\" from 7 to 45 items/m3; and at Site \"#\" from 0 to 16 items/m3). The increased and remobilized contamination is reflected by Site \"g\", where in 2021 the water did not contain suspended MP, but in 2022 it increased to 45 items/m3.  
Then, similarly to the previous year, the amount of transported MP decreased heading downstream; by reaching the upstream section of the Middle Tisza (S3) it was almost halved (mean: 16.5 \(\pm\) 6.6 items/m3), and here the subsequent sites reflect gradual drop in MP pollution. In the S3, section the MP pollution was similar in the two years, and the same sampling site (\"i\") remained the most polluted, although here the MP transport decreased by 40% (from 42 items/m3 to 25 items/m3). The high MP pollution at this point clearly can be linked to the joining tributaries, as upstream of this point the highly polluted Szamos River (E: 48 items/m3) joins the Tisza, causing additional pollution.  
In contrast to the previous year, in 2022 between the S3 and S4 sections the mean MP pollution increased by ca. one third, though in 2021 it decreased further on; thus, the average MP pollution of the S4 section increased by 45%. This section had the greatest MP transport variability, as in 2022 the highest value of the entire Tisza (site \"u\": 63 items/m3) and the lowest value (site \"s\": 4 items/m3) were measured here. However, this variability appeared temporally as well, as at some points (e.g., sites \"p\" and \"u\") the MP transport increased by 3-6 times between the two surveys. Interestingly, in the impounded parts of the S3 and S4 sections, very similar transported MP values were measured in both years, referring to similar MP input and flow conditions.  
The mean MP transport increased further in the Lower Tisza (S5), showing similar a spatial trend in both years, though in 2022 the mean value (27.6 \(\pm\) 14.2 items/m3) was higher by 22% than the 2021 average. Along this section, in both years, the amount of transported MPs increased steadily downstream, probably as the result of impoundment by the Novi Becej Dam and the Danube.  
The 2022 survey reflected that the transported dominant MP type remained fiber (Figure 4); however, its proportion increased from 84.2% to 97.8%. Most of the fibers were colored, and similarly to the previous year, the increase in colorless fibers was typical in the samples with higher MP pollution. In 2022, only 0.5% of the particles were fragments and they were found only at two sampling sites (\"f\" and \"q\"); 1.7% were spheres, which were found especially in the water of the Middle Tisza and in the tributaries originating in Slovakia and Hungary (I: Sajo, H: Hernad and J: Zagyva).