Plastic bag floating in the ocean with seaweed

Microplastic quantities on the seafloor triple in 20 years

Image credit: Dreamstime

The total amount of microplastics deposited on the seafloor has tripled over the last 20 years, researchers have said.

The team from the Autonomous University of Barcelona (AUB) and Aalborg University said the increase corresponds with the type and volume of consumption of plastic products by society over that time.

The study is the first high-resolution reconstruction of microplastic pollution from sediments obtained in the north-western Mediterranean Sea.

Despite the seafloor generally being the final sink for microplastics floating on the sea surface, the level of build-up and the sequestration and burial rate of smaller microplastics on the ocean floor is unknown.

It was found that microplastics are retained unaltered in marine sediments, and that the microplastic mass in the seafloor mimics the global plastic production from 1965 to 2016.

“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,” said AUB researcher Laura Simon-Sánchez.

The sediments analysed were found to have remained unaltered on the seafloor since they were deposited decades ago.

“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,” explained AUB researcher Michael Grelaud.

The team found roughly 1.5mg of these three types of plastic per kilogram of sediment collected, with polypropylene being the most abundant, followed by polyethylene and polyester.

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.

It was also found that, once trapped in the seafloor, the particles no longer degrade, either due to lack of erosion, oxygen, or light.

“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,” said AUB professor Patrizia Ziveri.

The researchers selected the western Mediterranean Sea as a study area, in particular the Ebro Delta, because rivers are recognized as hotspots for several pollutants, including microplastics. In addition, the influx of sediment from the Ebro River provides higher sedimentation rates than in the open ocean.

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