The accumulation of plastic waste fragments in sea sediment has sharply increased around the world in the 21st century, according to a recent study on microplastic litter.
The study, led by Hideshige Takada, a professor at Tokyo University of Agriculture and Technology, found the concentration of microplastics, defined as plastic particles up to 5 mm in size, is higher in sediment than in sea water, raising concerns that it could affect organisms living in and on the bottom of the ocean.
“Sediment is becoming one of the places where microplastics build up. There is an urgent need to deal with the situation by reducing the use of plastic,” Takada said.
Microplastic particles are believed to come from waste such as plastic bags and containers broken down by waves and ultraviolet rays, and also from microbeads widely used in cosmetics and toothpaste. They absorb hazardous chemicals and could become concentrated in birds and fish that mistakenly eat them.
In the study, the professor examined samples of sediment collected from the sea floor and elsewhere in areas including Tokyo, Southeast Asia and Africa.
In the Imperial Palace moat in Tokyo, nearly 80 microplastic particles were found per 10 grams of mud that was likely to have accumulated around 2000. But mud dating back to the 1950s had only about 10 particles and mud before 1900 did not contain any such particles.
The microplastics may have been carried from the ground into the moat by rainwater, or originated as larger plastic fragments that broke up as they floated in the water, according to Takada.
Takada’s group also found that more microplastic particles tend to exist in more shallow parts of sediment on the sea floor in the coastal areas of South Africa, Thailand, Malaysia and Vietnam, although details of the age of the sediment have yet to be determined.
Off Durban in South Africa, for example, the amount of microplastics was four times larger in mud 2.5 cm to 5 cm in depth compared with the mud 20 cm to 22.5 cm in depth.
The study signals that the marine environment has been further contaminated around the world this century.
Microplastics are often floating in the water, but could sink as microorganisms stick to them and accumulate in the mud. The density of microplastic particles in a sample of sediment of Tokyo Bay, collected in 2012, was much higher than that in the seawater, according to the study.