WHY and WHERE PLASTICS ACCUMULATE?
Litter shape and size
Proximity to source and transport mechanisms
The amount of anthropogenic litter, especially plastic, has been increasing in oceans over the last decades and can be nowadays found on six distinct compartments: sea surface, water column, shorelines, seafloor, sea ice and biota. Marine litter is usually a mixture of locally-derived items and litter that has been originated somewhere.
The vertical position of plastic in the water column is dependent mainly on the item’s density, even though also other factors, such as size and shape can have an effect too. Depending on their composition, plastics can be positively, neutrally or negatively buoyant in the water. The density of seawater is approximately 1.03 g cm-3 and for example polypropylene (PP) tends to float according to its specific gravity (0.83–0.85 g cm-3), whereas polyvinyl chloride (PVC) is denser than seawater (1.38 g cm-3) and hence usually sinks towards the seafloor. Only few plastics are naturally neutrally buoyant in seawater.
The temperature and salinity of the seawater affects its density. Therefore the behaviour of plastic litter in the environment may also vary depending on these factors. In the world´s oceans salinity is high, but in estuaries and brackish water basins the salinity is much lower. When salinity declines, the water density decreases: compared to the salinity and density of the seawater (35 ‰, 1.02 g cm-3), the brackish water with a salinity of 5 has approximately a density of 1.004 g cm-3.
Another factor affecting the water density is the temperature. The density of water increases when the temperature decreases. Seasonal variation in the surface water temperatures of the Baltic Sea is high, and therefore it is suggested that sinking velocities of plastic litter might be slightly lower during cold winter months compared to summer. Additionally, temperature stratification in the water column has been suggested to further affect the sinking velocity of plastics . Despite of these suggestions related to differences in seawater density, it is however not yet known if the density gradients present in the environment truly have an influence on marine litter distribution.
The plastic density is not, however, always stable – many factors including weathering, leaching of additives, fouling and ingestion may alter the specific gravity of plastic items. It has been suggested that photodegradation or biological degradation could result in weight-loss of the plastic particles and hence decrease their sinking velocities, but more research is needed to confirm this. Link to Marine litter is a persistent and cumulative threat.
During the time spent in the sea, marine litter is being covered with a community of micro-organisms, commonly known as biofilm. Biofilm includes organisms from bacteria to unicellular organisms, invertebrates and algae. Biofilm formation can change the buoyancy of litter by increasing its density, which results in higher sinking velocity. Experimental studies in the waters of Plymouth, UK, have shown that a biofilm can form on the polyethylene (PE) plastic bag in a week and increase over time. When starting the experiment plastic was positively buoyant and floating in the seawater-air interface, but after two weeks it started to sink and showed signs of neutral buoyancy. It was also noticed that after removing the biofilm the positive buoyancy of the plastic bag was restored. Field experiments have showed that plastic can undergo defouling when submerged and return back to the sea surface. Defouling is suggested to happen when the fouling organisms experience adverse conditions, such as decreasing irradiance, or are foraged by marine consumers. A long-term study conducted in the Bay of Bengal observed that the biofilm formation is also affected by the seasons and differs between plastic types.
Many animals have been observed to ingest plastics in the nature. If these plastics can then pass the digestive tract, they are excreted in faecal pellets as observed in the case of e.g. zooplankton in laboratory conditions. It has been suggested that plastics may alter the density of faecal pellets thus having an impact on their sinking rates, but at the same time microplastics would be transferred towards the bottom in faecal pellets. Similar transport can also happen when an animal containing plastics dies and sinks to the bottom.
Litter shape and size
Sometimes the density alone does not determine the position of a plastic item in the marine environment. Some items, such as bottles and foamy plastics, have a shape that trap air and therefore can float on the sea surface despite high density or heavy fouling. Furthermore, the geometry of the item affects how it can be transported on the sea surface. If the item is floating well above the water level, it has a high windage and can therefore be more easily transported by winds. In a situation where plastics are entirely submerged under the sea surface, they are not exposed to direct wind stress and hence their distribution in the ocean depends mainly on the currents.
Plastics can travel for long distances, even across oceans. The larger the buoyant litter item is, the more subjected it is to wind-induced transport on the water surface. Since small microplastics cannot have a high windage, they travel more slowly than light-weighted macroplastics.
When winds mix surface waters, plastic litter can get submerged below the surface. It has been observed that smaller plastic particles are more susceptible to vertical transport in the water column by wind-induced mixing than larger plastic items even under low wind conditions (1 kn).
Transport of plastic litter in the water column is a combination of their size, shape and density, which generally determine the minimum velocity of water that is required for their transport. An experimental study investigating the sinking rates of diverse types of microplastics in the water column stated that especially the shape affects strongly to the sinking behaviour. The more spherical the particle is the smaller is the surface area related to the volume. Irregularly shaped particles may sink more slowly, because the sinking motion is unstable and may lead to rotating, oscillating or tumbling of the particle.
Proximity to source and transport mechanisms
The distribution of plastic marine litter is patchy, and it is often very difficult to trace litter back to its source. Correlation between human population density and the amount of beached macrolitter is usually high, and likewise in active fishing areas fishing related litter accounts for a large fraction of litter found the seafloor. Similarly, also microplastic fibres have been found to be more abundant on the shoreline sediments in densely populated areas. This is not, however, always the case since the litter distribution and accumulation can also be affected by prevailing winds, hydrodynamics and geomorphology. Enclosed seas, such as the Mediterranean, have high densities of marine litter partly due to densely populated coastline, but also because of limited water exchange.
Buoyant plastic litter can be transported considerable distances via winds and sea currents. The circulation patterns on the surface waters transport marine litter even to the remotest places of the globe including polar areas and isolated islands. The converging surface currents also trap and retain buoyant marine litter in the five subtropical gyres, which are located in the northern and southern Atlantic Ocean, northern and southern Pacific Ocean and in the Indian Ocean. Plastics can also be transported between these oceanic gyres; a travel time between southern and northern Atlantic gyres is only couple of years.
In addition to surface currents, the oceans exhibit also deep water circulation patterns that redistribute cooled water towards the seafloor. Deep water circulation works in connection with surface circulation: when surface waters enter polar regions, they start to cool down and sink. It has been suggested, that sinking water could transport marine litter to the underlying water column and ocean floors, but more research is needed to confirm this.
Even though large-scale trends in marine litter accumulation have been detected, the role of ocean dynamics and wind patterns on litter distribution on a smaller scale are not yet fully understood due to their complexity. Tides, winds, local currents, storm events and river outflows all transport and redistribute litter on a local scale. For example rivers with high flow rate may transport litter far from the shore, whereas in small, slowly running rivers and estuaries litter may more easily accumulate on the riverbanks or next to salinity fronts.
From the surface waters winds can mix floating items into the underlying water column, where they can continue sinking. When entering seafloor, bottom currents and topography may further influence where litter is distributed. In the Lagoon of Venice it has been observed that microplastics tend to accumulate where the currents are weakest. The geometry of the site may have an effect on accumulation: high concentrations of microplastics have been found from harbour areas that are partially enclosed and have a low flushing rate.
Despite of the properties of plastic litter and physical forces affecting their distribution in the marine environment, also biological factors can have their effect on the transport of plastics between different compartments. For example seabirds that catch litter from the sea surface may transport these items to the shore. An experimental study has revealed that sinking phytoplankton aggregates are able to trap microplastics and increase their sinking rate. Sinking phytoplankton might thus affect the vertical distribution of microplastics in the water column transferring them quicker towards the benthos. When on the seafloor, the distribution of microplastics can further be affected by benthic animals. A study made in a laboratory has demonstrated that benthic invertebrates are able to transfer microplastics from the sediment surface deeper into the sediment when they move around on the seafloor. This mixing of sediments, termed bioturbation, is an important process affecting the vertical distribution of particles and solutes in soft bottom environments and can thus have an influence also on the distribution of microplastics.