# Spatial Variations in Microfiber Transport in a Transnational River Basin
## 3 Results
### Microplastic Transport of the Tisza in 2021  
In 2021, the MP transport (including microfibers; Figure 4) of the Tisza River was 19 \(\pm\) 13.6 items/m\({}^{3}\) on average (Table 1). The most polluted section (39 \(\pm\) 31.1 items/m\({}^{3}\)) of the river was the upstream (S1) section of the Upper Tisza in Ukraine; and the most polluted site was found here too, at the village of Gyilove (Site \"b\": 61 items/m\({}^{3}\)), which is built right along the banks of the river in the deep valley, where no sewerage cleaning facilities exist. Towards the downstream sections, the MP content of the water in gradually decreased, as in S4 section the mean pollution was just 14.5 \(\pm\) 7.9 items/m\({}^{3}\). The decreasing trend was also obvious along the Upper Tisza when the sites are compared, as in the upstream Ukrainian settlements (27-32 items/m\({}^{3}\)) the pollution was higher than in the Hungarian section (0-7 items/m\({}^{3}\)). It has to be noted that in the larger cities of the Ukrainian section (e.g., Rahiv, Bustino, Szolotvino, Tyacsuy) there are WWPS, but that due to the poor condition of the WWTPs [54], these continuously and significantly pollute the Tisza. The situation is not better along the Hungarian section of the Upper Tisza, as here half of the villages (e.g., Tivadar) have no WWPS, while in the other half only 61-78% of the households are connected to the sewage network [67]. Figure 3: Longitudinal changes in suspended sediment concentrations (SSC) along the Tisza and its tributaries in 2022. The S2-S5 indicate the hydro-morphologically uniform sections of the Tisza. The Middle Tisza was the least polluted reach of the Tisza, especially its downstream section (S3: 14.5 \(\pm\) 7.9 items/m\({}^{3}\)). The MP pollution of the upstream S4 section was also low, despite the fact that 10 out of the 15 municipalities along the section are without WWPSs. At one site, the water was heavily polluted (\"i\": 42 items/m\({}^{3}\); however, it is located downstream of the Kraszna River (F), which carries large amounts of suspended sediment and MPs. In the downstream section of the Middle Tisza (S4), the MP pollution was more uniform, though a slight increase could be observed downstream. In the Middle Tisza, the sampling sites between \"1\" and \"0\" are represent the reservoirs behind dams. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & & **Upper Tisza** & \multicolumn{2}{c}{**Middle Tisza**} & **Lower Tisza** & **Tributaries** \\ \hline
**Year** & **Average** & **S1** & **S2** & **S3** & **S4** & **S5** & \\ \hline
2021 & 19 \(\pm\) 13.6 & 39 \(\pm\) 31.1 & 18.6 \(\pm\) 14.2 & 15.8 \(\pm\) 13.8 & 14.5 \(\pm\) 7.9 & 22.6 \(\pm\) 10.1 & no data \\ \hline
2022 & 22.4 \(\pm\) 14.8 & no data & 30.5 \(\pm\) 20.5 & 16.5 \(\pm\) 6.6 & 21.1 \(\pm\) 17.8 & 27.6 \(\pm\) 14.2 & 27 \(\pm\) 19 \\ \hline \end{tabular}
\end{table} 
Table 1: Mean microplastic content (items/m\({}^{3}\)) of the Tisza and its tributaries in 2021 and 2022. Figure 4: Amount transported fibres, microplastic spheres and fragments in 2021 and 2022. Along the impounded section of the river, the MP content of the water gradually decreased (from 19 to 11 items/m\({}^{3}\)). However, downstream of the Kiskore Dam, the MP content of the samples was relatively high with an increasing trend: between sites \"p\" and \"q\" it increased from 12 to 21 items/m\({}^{3}\), as the clear water erosion probably mobilized the sediments at the channel bottom. In the Lower Tisza (S5), the average MP content (22.6 \(\pm\) 10.1 items/m\({}^{3}\)) increased by ca. 25%. This can be partly explained by impoundment and trapping by the Novi Becej Dam, the influence of the tributaries (K: K\(\acute{\rm o}\)rs and L: Maros Rivers), and the low degree of sewage collection and treatment in Serbia. The morphological types of the transported MPs were also identified (Figure 4). Plastic fibers dominated (mean: 84.2%) in all samples along the entire length of the Tisza in 2021.