Plastic contamination in the food of ocean organisms is very common nowadays, and the number of species known to be affected by the ingestion of plastic is increasing (Kühn et al. 2015).
Ocean organisms ingest plastic, then they spread it over a wide range. This is because animals that have ingested plastic often move from their original location to a different place. This is called a biological pump.
For example, Balaenoptera borealis in the Mediterranean is thought to ingest several thousand pieces of microplastic per day using its big mouth (Fossi et al. 2014). Subsequently, they defecate at another place, which means they carry plastic to other areas of the sea or the deep sea.
Sea birds are known to be a biological transportation of plastic. For instance, at the South Pole, the parent birds of Wilson’s storm petrel (Oceanites oceanicus) gather plastic from the sea and carry it back to the breeding grounds, where the hungry chicks are waiting (Van Franeker & Bell 1988).
Chicks die by ingesting the plastic that has been mixed in with the food given to them by their parents. Thus, plastic carried by the parent birds remains in Antarctica permanently (Van Franeker & Bell 1988).
A similar situation occurs at the Midway Atoll. Healthy Diomedea immutabilis individuals living there ingest 10g of plastic on average. However, dead chicks have been shown to ingest an average of 20g of plastic. Thus, parent Diomedea immutabilis (about 600 thousand individuals) are estimated to carry approximately 6 tons of plastic from the sea to the land every year (Auman et al. 1997).
The issue is not only their carrying but also their changing the size of the plastic by ingesting it. Cape petrels grind what they eat in their gizzards for a month before regurgitating 75% of it (Van Franeker & Bell 1988).
In the case of ingesting plastics, the size of the plastic being regurgitated becomes smaller than it was prior to being eaten. This makes it easy for tiny marine organisms to ingest the pieces of plastic (Van Franeker & Bell 1988).
Plastics remain floating in the sea for a long time, however, they can become fouled by periphyton, such as barnacles. As a result, they become heavy and eventually sink (Ye & Andrady 1991).
Small microplastics become entangled in the mucus of phytoplankton, condensing to a more dense mass and sinking to the bottom of the sea (Woodall et al. 2014).
Megafauna, such as whales, that have died due to entanglement in plastic fishing nets sink to the bottom of the sea. At the same time, these plastic fishing nets sink with the whale’s body.
It is hard for microplastic and nanoplastic to sink because of their tiny size. However, these tiny plastics are ingested by animals, and then they are carried to the deep layers at the bottom of the sea (Wright et al. 2013, Setälä et al. 2014, Cole et al. 2016).
For example, many zooplankton species as well as some types of fish such as anchovies forage for their food on the surface of the water during the night. In the daytime, these organisms dive deeper to hide in darker waters. Such animals ingest microplastic on the surface and transport it down vertically. Consequently, plastic is carried from the surface to the bottom of the sea (Choy & Drazen 2013).
In some cases, microplastic that has been mixed with excrement sinks quickly. For example, microplastic ingested by zooplankton is excreted, however it is packed in with the excrement, therefore turning into marine snow and sinking to the deep sea (Cole et al. 2013).
In addition, when fish feed on plastic and die, their bodies settle in the deep sea together with the plastic (Van Cauwenberghe et al. 2013)．Such vertical transport by organisms is called a biological pump, because it appears to send plastic from the surface of the sea to the bottom as if using a pump.
In the sea, there is a certain little type of zooplankton known as larvaceans (appendicularians). This small creature is believed to help transport plastic to the deeper layers (Katija et al. 2017).
Larvaceans live in a house made of mucus. They generate a water flow by wagging their tails to create a flow of suspended solids floating on the sea into the house and then filter feed on them. However, they have a too high ability to filter and, as a result, their house quickly fills (Alldredge 1976, Hopcroft & Roff 1998).
Therefore, larvaceans discard their houses and make new ones. They sometimes change their house six times a day, and sometimes even more than 20 times, especially on hot days (Sato et al. 2003). Their discarded houses sink to the deeper layers as marine snow. Microplastic could flow into the larvaceans’ houses and become carried to the deeper layers together with the house.