## 4 Results  
In the results discussed below, we demonstrate how accounting for biofilm formation and degradation processes on plastic particles affects their trajectory in the Arctic Ocean. The main features of the Arctic Ocean circulation and sea ice drift have been discussed in our previous papers. In particular, the Arctic Ocean model results used in the present study were described in [45]. We have included the illustrations of ocean circulation and ice drift in the Supplementary Materials, Figures S1 and S2.  
The simulation results obtained in [45] demonstrated that the transport of microplastics released from the Siberian Rivers into the Kara Sea on and off the Arctic shelf is the result of the combined influence of ocean currents and sea ice drift. It was highlighted that the velocity of sea ice drift and that of the upper and intermediate layers of the Arctic Ocean are not identical. Consequently, the trajectory of a particle may vary depending on its location within the ice, at the ocean surface, or within a deeper layer. However, the vertical displacement of the MPs in the water, primarily determined by the difference between the particle and seawater densities, affects the vertical immersion and deposition of the particle. Negatively buoyant microplastics have a higher density than water. Unlike buoyant particles, they begin to sink immediately after entering the sea and accumulate near the river mouth. Buoyant plastics can remain in the surface layer or in the sea ice for a long time. The results of this study demonstrate 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 its rate of settling on the seafloor.  
The results of the present study show that the utilization of the more advanced biofouling model described in the preceding section resulted in oscillatory behavior of the particles in a vertical direction, which subsequently affected their trajectory. The oscillations occurred as a result of repeated processes involving the sinking and rising of the microplastic particles in ocean waters. Oscillatory particle behavior, accounting for biofilm growth and degradation, has been demonstrated by [PERSON] et al., 2017 [42]. Due to biofouling, the increased density of a particle coated with a biofilm exceeds the density of the surrounding marine water, causing the particle to submerge. The thickness of the biofilm is determined by chlorophyll concentration, shortwave radiation penetration, and ambient temperature (in accordance with Equations (9)-(11)), and the immersion depth due to the biofilm formation is dependent on water density (in accordance with (1), (2)). A particle with a biofilm will sink until it reaches a depth at which favorable conditions for biofilm growth are absent (i.e., where there are low levels of available chlorophyll and the water is cold). At this depth, the particle rapidly loses its biomass as a result of mortality and respiration, leading to a decrease in density and an increase in buoyancy. The particle then begins to rise towards the sea surface. Once the particle has ascended to a level at which chlorophyll is present, it accumulates biofilm once more and initiates a downward movement. The rate of ascent or submersion of a particle is dependent upon both the density of the particle itself and the density of the surrounding water, taking into account the current velocity. In the numerical experiments, buoyant microplastic particles of the same type (HDPE) but different sizes were considered. As will be demonstrated, the rate of vertical movement is also dependent upon particle size.