To recall, sea-birds confuse small bits of plastic with food. They swallow the bits of plastic and it kills them. [a] To recall, abandoned fishing gear entangles fish, turtles and marine mammals, and its kills them. Floating plastic bags do the same. And so on, but that is not the end of the story .
|The NOAA Marine Debris Program funds projects |
to remove derelict fishing nets and other debris
from marine waters, where they can entangle marine life.
Each year some 6.4 million tonnes of litter are entering the oceans, or about one kilogram for each human being. Most litter stems from cities. “The world's cities currently generate around 1.3 billion tonnes of municipal solid waste a year, or 1.2 kilogram per city-dweller per day.” [b Thus, the litter discharged into the sea corresponds to about 4-5% of the municipal solid waste produced in the cities of the world. In that sense its a small part of waste produced by the nine billion humans, but as any dumping it causes problems.
More generally, marine litter is defined as ‘‘any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment." Persistent littering the sea likely has started with disposing “clinker” from steamships and currently has found its peak with “plastic”.
Clinker, the residue of burnt coal, was commonly dumped from steamships well into the 20th century. In the Mediterranean Sea, its occurrence on the deep sea-floor has been shown to coincide with such shipping routes.
Currently, the most abundant marine litter is plastic. Only part of the plastic items float at the sea surface or close to it. Two third of the plastic sinks to the bottom of the sea when converted by fouling organisms. Therefore plastic accumulation on the seabed is more abundant than in the open sea. Thus, below the floating garbage is an even bigger garbage dump at the sea bottom.
Litter is present in all marine habitats, from beaches to the most remote points in the oceans, such as the Midway Islands in the Pacific [a. The particular distribution and accumulation of litter in the ocean is influenced by water movements, bottom morphology and economic activities.
On the global scale, the ocean currents sweep the litter to the centre of the ocean-gyres where it accumulates, called the big garbage patch. On the local scale, litter is washed upon the beach. On the hidden scale, litters is channelled to the deep sea. Marine litters accumulates in particular high densities in submarine canyons.
(from coast down to > 4000m depth)
Submarine canyons act as passages for litter transport from continental shelves into deeper waters. Submarine canyons are areas where organic debris accumulates. The debris is food for bottom-dwelling fauna and suspension-feeding invertebrates. They are abundant in submarine canyons. The accumulation of plastics in submarine canyons likely has a detrimental effects on theses deep-sea organisms.
A survey  of European seas published in April 2014, confirmed again that marine litter is found everywhere, from the beach down to the deep sea. The survey uses standardized methods to categorize the litter and to quantify its abundance. Most common litter items included, non-surprisingly, plastic bags, glass bottles and abandoned fishing lines and nets:
- Plastic represented 41% of the litter items in European seas, whilst abandoned fishing gear accounted for 34% of the total. Clinker, glass and metal are least common. Density and composition of litter in European seas is comparable to what has found in other parts of the world ocean.
- Litter density in submarine canyons reached an average of 12.2 – 6,4 items per ten-thousand square meters, or the double of the litter density found elsewhere.
A litter density of 12.2 – 6,4 items per ten-thousand square meters means to find about 10 items of litter on a surface wide as a football field!
A list of locations with highest litter densities in European waters includes for example the Lisbon Canyon in continental shelf of Portugal or the Blanes Canyon in the continental shelf of the Mediterranean sea of Spain. Theses canyons were formed when the sea-level was much lower than today.
Litter is a serious risk for the marine environment. Entanglement in abandoned fishing gear is a serious threat for birds, turtles and marine mammals, it is also causing high mortality of fish through ‘‘ghost fishing''. Beyond other threats, plastic is a source of toxic chemicals that is lethal to marine fauna.
The degradation of plastics generates micro-plastics that are ingested by organisms, leading to contaminants across trophic levels up to the fish  that we may eat. So plastic debris may return to their source, finally.
 José G.B. Derraik, 2002, The pollution of the marine environment by plastic debris, Marine Pollution Bulletin 44(9), pp. 842-852.
 Christopher K. Pham et. Al, 2014. Marine Litter Distribution and Density in European Seas, from the Shelves to Deep Basins. PLOS ONE, 9(4), pp. 1-13.
 Chelsea M. Rochman et al. 2013, Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress, Nature, doi:10.1038/srep03263;
[b] Economist, 7th June 2012, http://www.economist.com/blogs/graphicdetail/2012/06/daily-chart-3