# Spatial Variations in Microfiber Transport in a Transnational River Basin
## 1 Introduction
The longitudinal variability of MP in rivers is governed by various factors. Several studies tried to connect the MP contamination in fluvial sediments or in water to land use types. However, usually no unambiguous correlation was found [14; 29; 30; 31]. However, some studies have revealed higher MP concentrations near to urbanized or industrialized areas than rural territories, or in the vicinity to WWTPs [18; 26; 32]. The lack of correlation was explained by the importance of the hydrodynamic conditions of the river for redistributing MPs [29]. The most important is the spatial distribution of point sources, such as effluents of WWTP [33], industrial and agricultural drains [34], and tributaries [35], whereas non-point sources include surface run-off from roads, urban and agricultural areas [36], aquaculture activities [37], wind transport and direct deposition by humans [34]; ultimately, the contribution of point-source and non-point-source polluters to MP transport has not yet been cleared. Conversely, the spatial changes in flow velocity, discharge, slope, roughness, and channel morphology also influence the MP pollution along a river [38]. As these hydrological parameters could be altered by man-made hydraulic structures, such as dams, barrages, and weirs, they can accelerate the deposition process of MPs upstream of these constructions [39]. Finally, the geomorphological setting of the depositionary environment, such as sedimentary bodies (e.g., point and side bars), and impoundment at confluences can also influence the MP transport and deposition processes [35; 38]. Although many studies have investigated the spatial variability of MP along rivers, there is no agreement on the longitudinal downstream trend [14; 35; 40]. Studying transported MPs in water samples, [PERSON] et al. [41] found a downstream decrease in MP concentrations (River Estarreja, Portugal); however, [PERSON] et al. [40] found no downstream trend (Gallatin River, USA). At the same time, [PERSON] et al. [42] and [PERSON] et al. [43] found an increasing upstream trend on the St. Lawrence (Canada) and Brantas Rivers (Indonesia). These different results may be caused by the fact that the transport of MPs in rivers does not only depend on the slope or the discharge, but may also be governed by the location, time, and magnitude of MP input [44]. Similar conclusions were reached by [PERSON] et al. [42] and [PERSON] et al. [43], who explained the increasing downstream trend with an increase in incoming waste sources. [PERSON] et al. [45] revealed that the concentration and composition of MPs did not differ significantly between source area and the areas further downstream at the Gold Coast Broadwater, Australia. Considering the actual number of transported MPs, there is a great range at calculated values. In Hungarian rivers, the MP content varies between 3.52 and 32.5 items/m\({}^{3}\)[37]. An MP pollution (120 \(\pm\) 10 and 160 \(\pm\) 20 items/m\({}^{3}\)) orders of magnitude higher was measured in the St. Lawrence River (Canada) by [PERSON] et al. [42], and 50-725 items/m\({}^{3}\) were reported from the Zhangjiang River (China) by [PERSON] et al. [46]. Even higher amounts (4390 items/m\({}^{3}\)) were measured in the Langat River (Malaysia; [PERSON] et al. [47]), whereas the Brantas River (Indonesia) could also be classified as highly polluted (133-5467 items/m\({}^{3}\)[43]). Single measurements give a snapshot on the MP transport in rivers; however, based on repeated measurements over time, the relationship between MP transport and hydrological conditions could be revealed. For example, [PERSON] et al. [41] performed measurements in different hydrological situations on the Antua River (Portugal), and they concluded that 58-193 items/m\({}^{3}\) MP particles were transported during high flows, while this increased to 71-1265 items/m\({}^{3}\) during low flows. [PERSON] et al.