## 4 Discussion
### Parameters Affecting the Rearrangement of Deposited Microplastic Contamination
In addition, the MPs were transported in different ways onto these forms (as bed load or suspended load) and deposited in a different flow environment (flowing or standing water). This is supported by the results of previous researchers [7; 13], who found that the MP is transported in suspension under a continuous flow and its sedimentation rate is very low. The sedimentation of suspended MP particles is supported by the low-energy environment created by a slope drop [15; 19; 30]. The accelerated deposition on the point-bars could also be connected to the fact that these forms are located at the greatest distance from the thalweg, where the MP transport is the most intensive [62]. Above thepoint-bars, due to the reduced flow rate, the MP particles easily form aggregates with natural colloids, promoting the appearance of contaminating communities on their surface, which enhances sedimentation [52]. The MP content of the point-bars and the side-bars changed with a similar spatial pattern as that of the uniform hydrological sections. Thus, they can be considered as representative indicators of the downstream changes in the MP concentration. However, in the most polluted section (S3), the MPs were more enriched in the point-bars compared to the section average (by 43%) or compared to the point-bars of the previous section (by 70%) than in the other form. Thus, the point-bars may show significant MP overload in some situations. Furthermore, the spatial distribution of the MP content of the sediment sheets showed a different spatial pattern than the sections or other forms, as their MP pollution reached its peak further downstream, in section S4. The amount of 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 MP enrichment in the point-bars or side-bars in the S1-3 sections. It is also interesting to compare whether the MP content of the different geomorphological forms changed similarly or not between the two surveys. While the MP content of the sediments decreased in a year, this was not typical for the point-bars, which have become more polluted by 10%. At the same time, the amount of the MP decreased by 29% in the side-bars and by 51% in the sedimentary sheets. Therefore, it does matter what forms are sampled, as they influence the changes and spatial patterns we conclude. The novel result of our research is that if the sediment samplings are performed on different channel forms, their results may show completely different longitudinal trends, and some forms over represent the MP pollution of the section (e.g., point-bar), others (e.g., sediment sheets) underrepresent it. We recommend selecting side-bars for sampling, as on the one hand, in alluvial rivers, these forms are more common than the point-bars. In addition, their MP values were the closest (+0-9%) to section averages, and they changed with a similar spatial and temporal trend. Finally, we also evaluated the connection between the _particle size_ of the sediments and their MP content, but no clear general relationship was found. However, other researchers [26; 50; 63] found a positive correlation between the clay fraction of the sediments and their microplastic content. On the contrary, in the lower sections of the Tisza (S2-4), the coarse-grained samples contained more MP, and as the slope of the sections decreased, the difference between the MP content of the two sample types increased: the coarse-grained samples contained more and more MP. The difference between the two sample types was the most evident in the sediments of point-bars, as on these forms, the MP content of the coarse material was higher by 34-55% than at the same site in the fine-grained samples. However, in the case of the side-bars or sediment sheets, no characteristic differences were found between the MP content of the coarse- or fine-grained samples. This may be related to the fact that the MP particles form aggregates with other sediment particles [14; 64], or biofilm is formed on their surface [15; 65].