Clumps of the algae known as Melosira arctica contained an average of 31,000 microplastic particles per cubic meter, about 10 times the concentration of the surrounding water, researchers said.
They said the average came with a range of ±19,000, meaning that some clumps may have had as many as 50,000 microplastic particles per cubic meter.
The study was conducted by the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, based on samples collected during an expedition with the research vessel Polarstern in 2021.
The samples were analyzed by the Ocean Frontier Institute, Dalhousie University and the University of Canterbury and the findings were published Friday in the journal Environmental Science & Technology.
"The filamentous algae have a slimy, sticky texture, so it potentially collects microplastic from the atmospheric deposition on the sea, the seawater itself, from the surrounding ice and any other source that it passes," Deonie Allen, of the University of Canterbury and Birmingham University, who is part of the research team, said in a news release.
Fish, such as cod, dine on the algae and are in turn eaten by other animals including humans, passing along a "variety of plastics" including polyethylene, polyester, polypropylene, nylon and acrylic that have been detected throughout human bodies.
"People in the Arctic are particularly dependent on the marine food web for their protein supply, for example through hunting or fishing," Melanie Bergmann, a biologist who led the study, said in a statement.
"This means that they are also exposed to the microplastics and chemicals contained in it. Microplastics have already been detected in human intestines, blood, veins, lungs, placenta and breast milk and can cause inflammatory reactions, but the overall consequences have hardly been researched so far."
The study said that clumps of dead algae also transport the microplastics "particularly quickly" into the deep sea, which explains "high microplastic concentrations in the sediment," another key finding of the new study.
The algae grow quickly under the sea ice during the spring and summer months and form meter-long cell chains there that become clumps when the cells die. The clumps can sink thousands of feet to the bottom of the deep sea within a single day.
"We have finally found a plausible explanation for why we always measure the largest amounts of microplastics in the area of the ice edge, even in deep-sea sediment," Bergmann said.
The researchers said in the release that, until now, scientists only knew that microplastics, which concentrate during the formation of sea ice, were released into the surrounding water when the sea ice melted.
"The speed at which the alga descends means that it falls almost in a straight line below the edge of the ice. Marine snow, on the other hand, is slower and gets pushed sideways by currents so sinks further away," Bergmann said.
"With the Melosira taking microplastics directly to the bottom, it helps explain why we measure higher microplastic numbers under the ice edge."
Bergmann added that research has shown that reducing the production of more plastic is the most effective way to reduce plastic pollution.
"This should therefore definitely be prioritized in the global plastics agreement that is currently being negotiated," Bergmann said.
She noted in the release that she will attend the next round of negotiations to develop a United Nations treaty for reducing plastic pollution. The talks are to begin in Paris at the end of May.
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