Microplastics on the ocean floor have 'tripled in 20 years' say scientists

Microplastics deposited on seabeds have tripled in just 20 years, reveals new research.

The study also shows that microplastics are retained unaltered in marine sediments for several decades.

Scientists found that the total amount of microplastics deposited on the bottom of oceans has tripled in the past two decades – with a progression that corresponds to the type and volume of consumption of plastic products by society.

The findings, published in the journal Environmental Science and Technology (ES&T), provide the first high-resolution reconstruction of microplastic pollution from sediments obtained in the north western Mediterranean Sea.

The research team explained that, despite the ocean floor being considered the final sink for microplastics floating on the sea surface, the historical evolution of that pollution source in the sediment compartment, and particularly the sequestration and burial rate of smaller microplastics on the ocean floor, was unknown.

The new study shows that microplastics are retained unaltered in marine sediments, and that the microplastic mass sequestered in the seafloor mimics the global plastic production from 1965 to 2016.

Researcher Dr Laura Simon-Sánchez said: ‘Specifically, the results show that, since 2000, the amount of plastic particles deposited on the seafloor has tripled and that, far from decreasing, the accumulation has not stopped growing mimicking the production and global use of these materials.’

The researchers say that the sediments analysed have remained unaltered on the seafloor since they were deposited decades ago.

Scientist Dr Michael Grelaud said: ‘This has allowed us to see how, since the 1980s, but especially in the past two decades, the accumulation of polyethylene and polypropylene particles from packaging, bottles and food films has increased, as well as polyester from synthetic fibres in clothing fabrics.’

He said the amount of these three types of particles reaches 1.5mg per kilo of sediment collected, with polypropylene being the most abundant, followed by polyethylene and polyester.

Despite awareness campaigns on the need to reduce single-use plastic, Dr Grelaud said that data from annual marine sediment records show that we are still far from achieving that.

He said policies at the global level could contribute to improving the ‘serious’ problem.

Dr Grelaud said: ‘Although smaller microplastics are very abundant in the environment, constraints in analytical methods have limited robust evidence on the levels of small microplastics in previous studies targeting marine sediment.

‘In this study they were characterised by applying state-of-the-art imaging to quantify particles down to 11 µm in size.’

The degradation status of the buried particles was investigated, and it was found that, once trapped in the seafloor, they no longer degrade, either due to lack of erosion, oxygen, or light.

Professor Patrizia Ziveri, of ICTA-UAB, added: ‘The process of fragmentation takes place mostly in the beach sediments, on the sea surface or in the water column.

‘Once deposited, degradation is minimal, so plastics from the 1960s remain on the seabed, leaving the signature of human pollution there.’

The investigated sediment core was collected in November 2019, on board the oceanographic vessel Sarmiento de Gamboa, in an expedition that went from Barcelona to the coast of the Ebro Delta, in Tarragona, Spain.

The research team selected the western Mediterranean Sea as a study area, in particular the Ebro Delta, because rivers are recognised as ‘hotspots’ for several pollutants, including microplastics.

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