Plastic Garbage in the Sea
Estimates suggest that 80% of the waste from land reaches the sea through rivers. Therefore the problem of plastic waste in the sea does not affect only countries with a marine coastline but essentially all regions where plastic is used. Plastic also becomes a problem in lakes, rivers, streams and wetlands.

Plastic is everywhere…
Light pieces of plastic float around on the surface and are carried by the currents for long periods across great distances. Numbers of them are washed onto the coasts.  Plastics with a density greater than that of water sink to the seafloor.So far little research has been conducted into the situation on the sea or ocean floor. It seems likely that especially large collections of heavy sorts of plastic, such as PET and HDPE, have formed there, mostly off densely populated areas of coastline and near river deltas. It is feared that the concentration of plastic restricts the exchange of gas between the seafloor and the water. 

Ocean Report – marine litter, Surfrider Foundation Europe

…also in the Stomachs of Animals
Research work carried out by Kara Lavender Law, an oceanographer at the Sea Education Association (SEA), has shown that, over the last 20 years the amount of plastic measured on the surface of the Atlantic has stagnated, even though the amount entering the sea is growing continuously. She evaluated more than 7500 samples collected on research expeditions made by SEA over a period of 25 years. What happens to the surplus?
One of Law’s theories suggests that a considerable amount of the debris is consumed by plankton-eating animals. Another theory says that sea organisms colonize the pieces of plastic which thus become heavier and sink to the bottom. Plastic is consumed on the seafloor, too: Erin Graham and Joseph Thompson proved in 2009 that sea cucumbers ingest large amounts of plastic with their food.