**4. Distribution and fate of microplastic**

The distribution and fate of the degraded plastic and microplastic in the marine system are attributed to anthropogenic activities (e.g., tourism, wastewater treatment effluent) in the form of primary microplastics [87]. Environmental factors lead to the introduction of secondary microplastics into the marine habitat, as discussed in Section 3. For example, the wastewater treatment plant effluent releases ~7 million microplastic particles every day [87, 88]. Hence, the marine environment serves as the primary sink for microplastics. Once into the marine system, their accumulation and distribution depend on a number of parameters pertaining to microplastics (e.g., density, size, shape, and chemical composition) and environment (e.g., wind and ocean current speed) [89, 90]. The fate of microplastics is related to their immediate source of disposal, and they can be translocated to remote areas such as artic seas and ice-capped regions [91]. Depending on the density of the microplastics, they can either remain suspended in the surface water or sink into the deep sediments. The density and other chemical properties of the most common types of microplastics are given in **Table 1**. If the density of the microplastic is lesser than that of the seawater (usually ~1.025 g/cm3 ) [61], the microplastic may remain suspended in surface water and would be transported to distant locations through horizontal distribution driven by ocean circulations (Section 4.1). If the density of the microplastic is greater than that of the seawater, the microplastic may sink to the sea floor through a pathway of vertical distribution (Section 4.2) [61, 92, 93]. Data show that around 15% of microplastics remain in the suspended form, whereas 70% of microplastics accumulate in sea sediments [94]. In the United States, ~260 tons of PET are released from the used containers of personal care products alone, and this contributes to 25% of microplastics in the North Atlantic Ocean gyre [95]. Due to the variation in degradation mechanisms of different plastics, the continuous generation of plastic

waste, and the dynamic nature of the environmental conditions (since the velocities of wind and ocean circulation vary along with the changing weather conditions), the fate of microplastic is not constantly steady and difficult to predict. This necessitates a proper understanding of the distribution of the microplastics once it enters the marine system.
