The amount of microplastics in the Southern Ocean has been significantly underestimated | Polarjournal
In February and March 2021, the research team took 17 water samples from the southern Weddell Sea to investigate the concentration of microplastics. Photo: Clara Leistenschneider

The pollution of the Southern Ocean with microplastics has been significantly underestimated, as researchers from the University of Basel and the Alfred Wegener Institute have shown.

All 17 water samples taken by the research team in the Southern Weddell Sea during an expedition on the German research icebreaker Polarstern showed a significantly higher concentration of microplastics compared to previous studies. The new study was published in Science of the Total Environment.

“The reason for this is the type of sampling we conducted,” explains Clara Leistenschneider, PhD student at the Department of Environmental Sciences at the University of Basel and lead author of the study, in a press release issued by the university.

Clara Leistenschneider (left) and research group leader Prof. Dr. Patricia Holm at the sample tank after taking samples and before the water is filtered. Photo: Patricia Holm, University of Basel

While previous studies mostly used nets to collect samples, which only capture particles larger than 300 micrometres, Clara Leistenschneider and her team used a pump to take water samples, which they filtered on board. This method also allowed them to capture smaller particles and fibers. For their study, they focused on microplastics between 11 and 500 micrometers in size.

Their analyses showed that 98.3 % of the particles and fibers are smaller than 300 micrometers. This means that earlier studies missed most of the microplastics during sampling.

These results are very worrying, because the smaller plastic particles are, the more toxic they could be for marine organisms, according to several studies. For example, nanoplastics, i.e. even smaller particles, can change the development and behavior of krill, as the particles penetrate tissue and cross the blood-brain barrier.

Left: The study area is located in the Southern Weddell Sea (black rectangle), which is influenced by the Weddell Gyre (yellow). The Antarctic Slope Current (red) flows along the Antarctic continental shelf. Right: The study area in detail. Samples 1 and 2 were taken north of the continental shelf, samples 3 – 17 on the continental shelf. Figure: Leistenschneider et al. 2024

The extent of pollution was greatest in the two offshore samples taken north of the continental slope, with around 260 microplastic particles per cubic meter. All other samples came from the continental shelf and contained between 0.5 and 56 microplastic particles per cubic meter. Even the sample that the researchers took in a crack between the Brunt Ice Shelf and the recently calved iceberg A74 contained around 19 particles per cubic meter.
The most commonly found polymer types are polypropylene (PP) and polyamide (PA).

The polymer composition of the microplastics found and the concentration of particles (left) and fibers (right). Figure: Leistenschneider et al. 2024

The researchers cannot explain these large differences in the concentrations between the offshore samples and the other 15 with certainty. They suspect that the plastic particles are bound by the ice near the coast and are only released back into the water when the ice melts.

Ocean currents – including the Antarctic Slope Current, which flows along the continental shelf in a westerly direction – could also play a role. “They might work like a barrier, reducing water exchange between the north and south,” says Gunnar Gerdts, a scientist at the Alfred Wegener Institute on Helgoland, in the press release.

In the current study, the water samples could only be taken near the water surface. However, in order to investigate the role of ocean currents, it is crucial to also sample deeper water layers, “since the deep currents differ greatly from the surface currents and thermohaline circulation leads to exchange with water masses from northern regions,” says Clara Leistenschneider.

The researchers can also only speculate about the origin of the microplastics. They named ships from tourism, fishing and research as possible sources. Whether microplastics can ever leave the region again is questionable, as the Antarctic Circumpolar Current is a powerful barrier.

Despite the alarming results, Clara Leistenschneider says: “Research on the topic has dramatically increased awareness in recent years of the problems that microplastics cause for the environment and all living organisms.” Although there is no all-encompassing solution, she notes that a variety of stakeholders all over the world are working intensively to better understand the problem and develop innovative ideas to reduce plastic pollution. And, of course, “every individual who engages in environmentally-conscious behavior can bring about positive change.”

Julia Hager, PolarJournal

Link to the study Clara Leistenschneider, Fangzhu Wu, Sebastian Primpke, Gunnar Gerdts, Patricia Burkhardt-Holm: Unveiling High Concentrations of Small Microplastics (11-500 μm) in Surface Water Samples from the Southern Weddell Sea off Antarctica, Science of the Total Environment (2024). DOI: 10.1016/j.scitotenv.2024.172124

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