The first and most direct effect of this pollution is the imprisonment of animals in driftnets or large debris. It is a major cause of mortality for marine mammals, turtles and birds.
A second direct effect is ingestion. We now admit that this concerns the entire food chain of the marine ecosystem. There is a continuum of plastic debris sizes, from several centimeters to a micron (thousandth of a millimeter) or even a nanometer (millionth of a millimeter).
Each size of marine organism in the food chain corresponds to a size of debris that risks being ingested. After ingestion, plastic accumulates in the digestive system of animals, which then eat less and eventually die.
A large number of organisms, some of which can be invasive, agglutinate on plastics and are transported with them by the flow of currents, over thousands of kilometers and this for several decades. It is a real danger for the balance of ecosystems.
The organisms associated with plastic are as diverse as fish, algae, shells, etc. They may be visible to the naked eye or microscopic in size. In addition, it has been shown that the bacteria that grow on plastics in gyres are different from natural bacteria found in the marine environment. Some could be potentially pathogenic.
Plastic debris represents chemical pollution for several reasons. They contain compounds that can be chemically transferred to marine organisms during ingestion (they are said to be bio-available). Some of these molecules are potentially toxic and can accumulate in the body (they are bioaccumulative). In addition, during the aging of plastic in the environment, chemical compounds incorporated during its manufacture (mainly additives) can be released into the environment or when they are ingested by organisms.
Plastics are also vectors of persistent organic pollutants. Certain plastics thus have the capacity to concentrate pollutants present in the environment during their long stays in rivers and oceans. Plastics can thus multiply the initial concentration of these molecules by a factor of up to 100,000. These molecules are also likely to bio-accumulate in living organisms, and to concentrate along the food chain.