## 5 Discussion  
This study extends our previous investigation [45], which analyzed the trajectories of microplastic particles released from the Siberian Rivers into the Kara Sea and their deposition areas in the Arctic Ocean. The results of this study demonstrated that microplastic biofouling, which alters a particle's buoyancy, represents a pivotal process that affects the depth of a floating particle's submergence, its trajectory, and the rate of settling on the seafloor. In contrast to the present study, our previous research used a simplified approach to modeling seasonal biofouling of microplastics in marine environments, which did not account for processes affecting biofilm degradation. Since a more advanced model of microplastic particle biofouling has been used in the present study, we note the main points that distinguish the results obtained from those published previously. The previous study [45] concluded that smaller microplastic particles covered in biofilm sink at a faster rate than larger particles. This was based on a comparison of particles with a diameter of 0.5 and 5 mm. In the present study, this conclusion is also valid when comparing particles of 0.01 and 0.5 mm in size in the beginning of the biofouling process. Figure 7 illustrates the behavior of the sinking velocity (\(w_{s}\)) as a function of biofilm thickness (\(BT\)) for particles of various sizes. The sign of the velocity value determines the particle movement direction (positive is downward). The presented graphs demonstrate that smaller particles lose their buoyancy at a lower biofilm thickness than is necessary for larger particles. A particle with a diameter of 5 mm requires a biofilm thickness of 0.35 mm to start sinking. For a 0.5 mm diameter particle, a biofilm thickness of 0.035 mm represents the boundary value at which the vertical velocity changes sign. For a 0.01 mm particle, a biofilm thickness of 0.00065 mm is sufficient for immersion. However, when some biofilm thickness is reached, the sinking velocity of a larger particle becomes greater than that of a smaller particle. This explains the greater immersion depth and oscillatory behavior of the 0.5 mm particles, whose vertical velocity changed more strongly with biofilm thickness changes compared to the 0.01 mm particles. A comparison of our previous [45] and current results indicates that the primary area of distribution for microplastics remains unchanged. Especially, it was previously shown in [45] that, being transported by sea ice or ocean currents, microplastic particles after 5 years of modeling appeared not only on the Arctic shelf but also outside it, in the deep Arctic Ocean and off the coast of Greenland. In addition, it was determined that there is the possibility of microplastic pollution extending from the Kara Sea to the Barents Sea due to ice drift. The present experimental setup yielded comparable results with regard to the distribution of particles. The notable discrepancy between the images of microplastic particles of 0.5 and 0.01 mm, as observed in Figures 3 and 4, can be attributed to our findings that particles of 0.5 mm are driven by sea currents during the summer but by sea ice drift during the winter, while particles of 0.01 mm are predominantly transported by the sea currents. As indicated in [PERSON] et al., 2024 [45] and illustrated in the Supplementary Materials (Figures S1 and S2), the velocity of sea ice drift and the upper and intermediate layers of the Arctic Ocean exhibit distinct differences. Consequently, the trajectory of a particle may vary depending on its location within the ice, at the ocean surface or within a deeper layer. The geographical position of a particle within ocean waters can also exert an influence on its subsequent trajectory. This is due to the fact that the vertical velocity of the particle is determined by the difference between the density of the particle and the density of the surrounding water, which in turn depends on the temperature and salinity of the water.