Around four billion minute fibres could be littering each square kilometre of some of the world's deep seas, says an international study led by Plymouth University which looked for unaccounted plastic debris in the seas.
Rayon, a man-made non-plastic polymer used in personal hygiene products and clothing contributed most at 56.9% of recorded fibres, with polyester, polyamides, acetate and acrylic among the others recorded.
The study looked at depths of up to 3500 metres in parts of the Indian and Atlantic Oceans, and the Mediterranean Sea.
In areas of the Indian Ocean there could be around 4 billion plastic fibres per square km, suggest the scientists.
Many of the fibres are used in a wide range of domestic and industrial applications and can be introduced to the marine environment via sewage. They have been discovered in ice cores and digestive systems of marine wildlife.
While presence of plastics has been noticed in water along coastlines, this was seen to be not in lines of predicted concentrations given the growing use of plastics. The present study shows that this discrepancy is because most of the plastic has sunk to the depths as microplastic.
Fibres were recorded as being up to four times more abundant in the deep seas than in shallow and coastal waters.
"The puzzle for marine scientists has been to establish where plastic debris is going. Part of the answer is that much of this waste is breaking down into fibres invisible to the naked eye and sinking to the sea floor. It is alarming to find such high levels of contamination, especially when the full effect of these plastics on the delicate balance of deep sea ecosystems is unknown," says Dr Lucy Woodall, zoologist at the Natural History Museum.
Deep-sea sediment and coral samples were collected from 16 sites worldwide and analysed at Plymouth.
Microplastics were abundant in all the samples and commonly around 2-3mm in length.
With deep seas extending to 300 million square kilometres globally, the potential for harm from the fibres can be imagined.
The study was published in the Royal Society Open Science journal. The University of Barcelona, the University of Oxford and the Scottish Association for Marine Science were involved in the work.