**4.1 Horizontal distribution**

Coastal current, rainfall, and wind are responsible for the movement of the plastics from the coastal shorelines/beaches into the marine system [96–98]. Once the macroplastics enter the marine environment, they can undergo further degradation as a result of ocean abrasion or biotic degradation, as discussed in Section 3. The fate of the microplastics, those carried from the terrestrial shorelines and/or formed as the result of degradation in the marine system, depends on their intrinsic properties and ambient conditions. Depending on the velocity and direction of flow of the regional wind and water current, these microplastics can either be transported to remote regions or return to the coastal shorelines/beaches [32, 87, 99], resulting in the accumulation of microplastics in the oceanic/regional water gyres in the marine environment. Meanwhile, 5–13 million tons of plastic debris enter the ocean (data for 2010) [3], and approximately 7–35 thousand tons of suspended microplastics remained in the ocean surface water [100]. This implies that the remaining plastic debris was translocated (either by horizontal or vertical distribution pathways). **Figure 2** shows the distribution pathways for plastic and microplastics in the marine environment.

## **4.2 Vertical distribution**

As stated previously, microplastics with density greater than that of the marine/ region water may sink to the seabed. This process is mediated by vertical turbulent mixing, biota transfer (via fishes or other marine organisms), biological fouling (also known as biofouling), and aggregate formation [61, 101]. Biofouling is the accumulation of existing marine microorganisms, planktons, algae, microalgae, and small marine organisms on the plastic debris/microplastics [102]. This process depends on the polymer type, surface area, and size of the microplastic, as well as the microorganisms present in the marine environment, temperature, salinity, pH, nutrient/ metals, and oxygen concentration of the water [66, 103–106]. For example, the presence of a plethora of bacterial species (*Alteromonas, Zoogloea, Ruegeria, Roseobacter, Nautella, and Pseudomonas*) in the benthic (6 m in depth) and the planktonic (2 m in depth) zones of the Arabian Gulf resulted in the biofouling of PET and PE [107]. Another study showed that the water conditions, primarily oxygen concentration and the presence of iron in the water resulted in biofouling of PET, PE, and PS by cyanobacteria, bacteria, and algae [108]. Biofouling starts with the attachment of the organisms, nutrients, flocculants, and dissolved organic compounds on the microplastic surface [109]. Subsequently, extracellular polymeric substances are released by the microorganisms to form a biofilm, which further attracts other marine invertebrates and worms [110]. As a result, the aggregate forms, the overall density of the microplastic increases, and it eventually sinks.

The density of marine water varies at different depths. Therefore, depending on the density of the aggregate formed, different layers can serve as a sink to accumulate microplastics [102]. Heavier aggregates can sink into the deep oceanic layers. The fate of the microplastics accumulated in the marine sediments is affected by the

**Figure 2.** *Distribution pathways and fate of microplastics in the marine environment.*

disturbance in the sediment zone, resulting in releasing the accumulated microplastics back into the water zone [111]. Also, similar to surface water currents, bottom water currents can also lead to the transportation of the microplastic to remote regions (**Figure 2**) [101].
