## 4 Results
### Pathways of the Microplastics Released from the Siberian Rivers  
The results of the second experiment, where microplastic particles were released from Siberian Rivers Ob and Yenisey, are shown in Figure 6. The figures illustrate the seasonal distribution patterns of MPs of various sizes.  
Figure 5: (**a**) Time series of immersion depth (right scale, lower plots) and biofilm thickness BT (left scale, upper plots) for particles D001 (red) and D05 (blue) during the simulation period. Positive depth values indicate that the particle is frozen in the ice. (**b**) Time series of immersion depths for particles D05 and D001 in 2017 in order to refine details and identify differences.  
The majority (66%) of the particles with a diameter of 0.5 mm were frozen into ice during the winter season (Figure 6a). In addition to the particles located near the river mouths, the area of active freezing of MPs into the ice was located between the islands in the northern part of the Barents Sea. The preceding experiment demonstrated that particles located in the Kara Sea, in the area of the continental slope and the northern part of the Barents Sea, rapidly reached the surface after the disappearance of the biofilm in the fall and  
Figure 6: Simulated microplastic particle distribution after 5 years of continuous riverine influx. Results of the experiments: for a 0.5 mm particle in (**a**) March 2020 and (**b**) September 2020; for a 0.01 mm particle in (**c**) March 2020 and (**d**) September 2020. The particle color is determined by its immersion depth, shown in the panel below. Circle diagrams show the percentage of the particles in each layer.  
then were frozen into the ice. Subsequently, the distribution of these MPs was dependent on the ice drift. In the Barents Sea, Fram Strait, and Greenland Sea, the majority of the particles were localized in the surface waters, as these regions are devoid of ice cover, or its amount does not exceed 50%.  
The vertical distribution indicates that during the summer, the prevailing concentration of particles can be observed in the water layer, extending from the surface to a depth of 75 m (Figure 6b). Additionally, some of the particles that entered the Barents Sea in the winter through its northern boundary may have been transported further to the central part of the sea by the summer currents. This type of trajectory was illustrated in Figure 3 for particle Mr05 released in March.  
For the 0.01 mm particles, only 15% were embedded in the ice during the winter season (Figure 6c). These included particles that were released during the winter and then immediately frozen into the ice. The largest portion of the particles was represented by those located in the layer between 10 and 30 m. This feature is consistent with the results of the first experiment, which demonstrated a tendency for small particles to remain underwater most of the time, gradually rising to the surface. During the summer months, due to ice melting and subsequent biofouling, the particles became submerged in the water (Figure 6d). A mere 22% of the particles were distributed within the 30 to 75 m layer. The majority of these particles were located in the northern region of the Kara Sea. In the future, they will enter the deep part of the Eurasian basin and spread along the continental slope. Almost half of the particles descended to a depth of only 30 m. As illustrated in Figure 6d, this percentage primarily included particles in close vicinity to the river mouths in the shallowest part of the Kara Sea.  
There was a discernible difference in the trajectories of the particles of various sizes. The primary direction of 0.01 mm MP transport beyond the Kara Sea shelf was along the continental slope of the Eurasian basin, in accordance with prevailing water circulation. The predominant route for the larger particles, which tended to become frozen into ice during the winter months, was towards the Barents and Greenland Seas, in accordance with the prevailing ice drift direction.