Sometimes plastics can account for 95 % of all the waste in the marine environment, but there is a considerable amount of spatial variation in the abundance and distribution of plastics as well as other types of marine litter. The fate of litter in the structurally three-dimensional marine environment depends on several physical factors including the density, size and shape of the item, but also on biological interactions with micro-organisms and various animals. Furthermore, also the proximity to litter sources and different transport mechanisms, such as winds and sea currents, as well as sedimentation processes at the sea influence the distribution of litter in our seas.
Litter is widespread on the sea surface and consists of both of large floating objects and tiny particles. Highest densities of floating litter are usually found near densely populated coasts and areas with high human activity. However, floating litter can also be easily transported by sea currents and may hence be found far from the original source. It has been for example observed that floating litter tend to accumulate in five subtropical gyres that are present in the oceans on the both sides of the Equator.
Not all litter is afloat on the sea surface; some of it is submerged and floating below the surface or sinking slowly towards the seafloor. The knowledge about litter on the water column is still scarce and most of what we know now comes from studies that have explored the distribution of small particles in sub-surface waters or deeper in the water column. Up to date it has occurred that small particles can be distributed below the surface for example by winds that mix the surface waters, or when covered by fouling organisms.
Litter seen on the shorelines is either about to become marine litter or recently washed ashore from the sea. The abundance, composition and distribution of litter on coastal areas is constantly shaped by the tides, waves and winds that remove, replace and redistribute litter in the interface of marine and terrestrial ecosystems. The efficiency of these processes is suggested to depend for example on the different properties of the litter and characteristics of the beach. Surveys conducted around the world have revealed that a majority of beach litter is usually plastic, and it is not only found laying on the beach but also buried in the sand.
Due to the extensive distribution of litter on the seafloor, it is considered to be one of the main accumulation sites for marine litter. Despite the preponderance of plastics among benthic litter, also metal and glass are frequently encountered from the seafloor. It has been observed that marine litter tends to accumulate in areas with low circulation, such as in bays and estuaries. In addition to larger litter items, also microplastics are discovered from the seafloor sediments, even from the deep ocean floors thousands of meters below the sea surface.
Plastic pollution has spread all around the globe even to the remote Arctic waters – recently microplastics have been found incorporated also in the Arctic Sea ice. Plastic materials such as polyester, nylon, polyethylene, acrylic and polystyrene, have been identified from the ice in two separate studies, but the origin of these particles is still unclear for the time being. Besides trapping particles, sea ice can have a potential to also transport these microplastics and release them when melting.