Plastic pollution, particularly microplastics (particles <5mm), poses a significant threat to marine ecosystems globally. While the Arctic was once considered pristine, recent research reveals increasing microplastic contamination in its waters, ice, sediments, and biota. However, the Eurasian Arctic, encompassing the Kara, Laptev, and East-Siberian Seas, remains understudied regarding microplastic distribution. The primary pathways for microplastic transport to the Arctic include long-range transport via the global Thermohaline circulation and input from local and coastal sources. Another significant pathway is the transport of microplastics within Arctic sea ice, with subsequent release during melting – a process predicted to intensify with ongoing sea ice retreat. Rivers are also major vectors for plastic runoff to the oceans. The Arctic receives substantial freshwater discharge, primarily from the Ob, Yenisei, and Lena rivers, forming extensive freshened surface layers on the continental shelf. In contrast, the Barents Sea receives less riverine input and is characterized by warm, saline Atlantic water. This study aimed to quantify microplastic pollution across different water masses in the Eurasian Arctic, assess spatial distribution, abundance, weight, size, morphology, and polymer types, and identify potential pollution sources.

Existing literature highlights the growing concern over microplastic pollution in the Arctic Ocean, but knowledge gaps remain concerning sources, pathways, and interactions with biota. Several studies have reported microplastics in Arctic waters, sea ice, sediments, and biota, highlighting the widespread nature of this pollution. However, some areas, such as the Eurasian Arctic, lack sufficient data to fully understand the sources and transport mechanisms. The role of long-range transport via the Thermohaline circulation and local sea-based and coastal sources has been recognized, but the contribution of Siberian rivers to microplastic pollution in the Arctic has not been fully quantified. Prior studies have suggested that the North Atlantic drift transports large quantities of plastic to the Barents Sea, but the fate of these microplastics after entering the Arctic remains unclear. This study directly addresses the gap in knowledge regarding microplastic input from Siberian rivers and their overall distribution pattern across different water masses in the Eurasian Arctic.

This study employed a combination of surface and subsurface sampling methods in the Eurasian Arctic (Barents, Kara, Laptev, and East-Siberian Seas) during September-October 2019. Surface water samples were collected using a neuston net (mesh size 200 µm) towed for 30 minutes at 2 knots. Subsurface water samples were collected using a shipboard underway pump-through system with an intake at 3m depth. The subsurface system filtered water through stainless steel meshes (1.5 mm and 100 µm pore size) and collected samples onto 80 µm mesh filters. To minimise contamination, rigorous procedural steps were implemented, including pre-rinsed equipment, clean-airflow cabinets, and field and procedural blanks. Microplastic identification involved visual inspection under microscopes and chemical characterisation using Fourier Transform Infrared spectroscopy (FTIR). Particle size, surface area, morphology (fibres or fragments), and polymer type were determined. Statistical analyses, including Pearson correlation, Student's t-test, Fisher exact test, and Friedman and Conover tests, were conducted to assess differences in microplastic characteristics across various water masses and sampling locations. The quality control procedures included analyzing field and procedural blanks to account for potential contamination during sampling and processing. No microplastics were found in most of the blanks, which reinforced the validity of the collected samples.

The study found relatively low but consistent microplastic presence across all four seas. Surface samples averaged 0.004 items/m³ (800 items/km²), while subsurface samples averaged 0.8 items/m³. The highest weight concentration of microplastics (12.5 µg/m³) was observed in the Barents Sea, significantly higher than other seas (0.4-1.1 µg/m³). Microplastic characteristics differed significantly between Atlantic surface water, Polar surface water, and Siberian river plumes, supporting the identification of two main sources: Atlantic transport and Siberian river discharge (p<0.05 for surface area, morphology, and polymer types). Surface water microplastics were primarily polyethylene (PE), while subsurface microplastics were predominantly polyester fibres. Siberian river plumes contained smaller microplastics in their inner parts (near estuaries) and larger ones in the outer parts. The Polar surface water showed an absence of floating microplastics but similar subsurface microplastic abundance and composition to the Atlantic water, suggesting that surface microplastics sink after ice-melt. Subsurface microplastic fibers were found ubiquitously, while fragments were primarily found in saline waters, absent in the inner river plumes. Statistical analysis revealed significant differences in abundance, weight concentration, morphology, and polymer type between the Atlantic water and river plumes in both surface and subsurface samples (p<0.05).

The findings highlight the significant roles of both Atlantic water inflow and Siberian river discharge in shaping microplastic distribution in the Eurasian Arctic. The distinct differences in microplastic characteristics between these two sources allowed for their statistical differentiation. The spatial distribution patterns are directly linked to the transport and mixing of these water masses. The absence of surface microplastics in the Polar surface water mass, combined with the presence of subsurface microplastics with similar characteristics to Atlantic water, supports the hypothesis that surface microplastics are trapped in sea ice and later sink after melting. The lower abundance of microplastics compared to some previous studies may be attributed to a higher detection limit and rigorous quality control measures. The study emphasizes the importance of considering both river discharge and global ocean circulation patterns when modeling microplastic transport in the Arctic. The observed variations in microplastic properties across different water masses could be valuable for future water mass identification and tracking.

This study demonstrates that Atlantic waters and Siberian rivers are major sources of microplastics in the Eurasian Arctic, with distinct microplastic characteristics allowing for source identification. Microplastic distribution is strongly influenced by the advection and mixing of these water masses. The observed sinking of microplastics after ice melt requires further investigation. Future research should focus on improving sampling methods to capture smaller microplastics and expand temporal and spatial coverage to better understand seasonal variations and long-term trends. Harmonized methodological approaches are crucial for comparing results across different studies.

The study's relatively limited spatial and temporal sampling may not fully capture the highly variable nature of microplastic distribution in the Eurasian Arctic. While quality control measures minimized contamination, the possibility of some undetected contamination remains. The study focused on particles >100 µm, excluding potentially significant smaller microplastics. The weight estimation for subsurface microplastics involved assumptions about particle shape, potentially introducing uncertainty in weight concentration calculations. The study did not account for the influence of atmospheric transport of microplastics.