Underestimated sources of marine pollution

Plastic waste pollutes oceans across all regions of the world. Marine animals may become entangled in larger plastic debris such as nets and bags or mistake smaller pieces for food. Ingested plastic can block or injure the gastrointestinal tract. The smallest plastic particles in the micro and nano range are mostly excreted, but a small proportion can pass through the intestinal wall and enter the bloodstream.

So how much nanoplastic is actually present in the oceans? Most scientific attention has so far been focussed on macro- and microplastic because their larger size makes them easier to study.  Quantitative data on the pollution of the oceans by nanoplastic particles smaller than 1 µm have been scarce until now because the particles are very small, prone to change, and often difficult to distinguish from other environmental particles using standard methods.

During a 2020 expedition aboard the RV Pelagia, the largest Dutch research vessel and flagship of the NIOZ, researchers from the UFZ and Utrecht University recorded the occurrence of nanoplastic along a transect from the European continental shelf to the subtropical North Atlantic Gyre. Samples were taken at 12 measuring points: in the uppermost water layer at around 10 m, in the intermediate layer at around 1,000 m, and 30 m above the seabed. “With the data from these measuring points, we can make statements about the vertical and horizontal distribution of nanoplastic in the North Atlantic”, says Dr Dušan Materić, chemist at the UFZ and lead author of the study.

Led by Materić, the scientists used a high-resolution proton transfer reaction mass spectrometer (PTR-MS) coupled with thermal desorption (TD) to measure the concentrations of organic trace gases. With this TD-PTR-MS, the tiny plastic particles in the samples can be combusted. By heating them, gases are released; these can then be quantified in the mass spectrometer. According to Materić, who developed the method in 2020 while working at Utrecht University, because each polymer produces a distinct chemical fingerprint, its identity and concentration can be reliably determined.

The researchers detected nanoplastic at all depths analysed across the 12 measurement sites. “They are present everywhere in such large quantities that we can no longer neglect them ecologically”, says Materić. The research team most frequently found nanoparticles of polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC), which are commonly used in disposable and reusable plastic bottles, films, drinking cups, and cutlery. At nearly all measuring points, the researchers detected these types of plastic in the uppermost water layer. “This is because, on the one hand, the redistribution from the atmosphere occurs via the sea surface and, on the other hand, a lot of plastic is introduced via the estuaries of rivers”, says Materić. The intermediate layer (i.e. the layer between the oxygen-rich surface water and the oxygen-depleted deep water) is dominated by PET nanoparticles. According to Materić, a higher concentration of nanoplastic was found in the North Atlantic subtropical gyre, an area where surface microplastics are known to accumulate because of ocean currents.

The researchers found the lowest concentrations of nanoplastic in the water layer near the sea floor. They detected PET nanoplastic at all measuring points there – even at depths of more than 4,500 m. This nanoplastic most likely originated from the fragmentation of synthetic clothing fibres but possibly also from previously unknown processes. “Nanoplastic and nanoparticles are so small that the physical laws governing larger particles often no longer apply”, says Materić.

The research team were surprised to find no polyethylene (PE) or polypropylene (PP) at any of the measuring points. Both PE and PP are commonly used in bags and packaging, which often end up as marine plastic waste. “There is a lot of PE/PP microplastic on the sea surface, but we did not find any PE/PP nanoparticles that could have been formed as a result of solar radiation or abrasion by the waves”, says Materić. The PE and PP nanoplastic may be mineralised or molecularly altered to such an extent that they are no longer detected as plastic by the PTR-MS, or there might be some other dynamic sedimentation and removal processes we are not yet aware of.

The scientists extrapolated the mass of nanoplastic in the North Atlantic from the concentration measurements. Based on these results, around 27 million tonnes of nanoplastic – 12.0 million tonnes of PET, 6.5 million tonnes of PS, and 8.5 million tonnes of PVC – are stored in the uppermost water layer of the North Atlantic, up to 200 m deep, from the temperate to the subtropical zone. “This is in the same order of magnitude as the estimated mass of macro- and microplastics for the entire Atlantic”, says Materić. This means that nanoplastic accounts for a large proportion of plastic pollution in the oceans and has not yet been factored into current assessments of the marine plastic balance. “Only a couple of years ago, there was still debate over whether nanoplastic even exists. Many scholars continue to believe that nanoplastics are thermodynamically unlikely to persist in nature, as their formation requires high energy. Our findings show that, by mass, the amount of nanoplastic is comparable to what was previously found for macro- and microplastic – at least in this ocean system”, says Materić.

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