From Surface Hotspots to Abyssal Sediments: Vertical Distribution of (Micro)plastics in the North Pacific Ocean

While plastics are ubiquitous contaminants in the marine environment and, as part of "novel entities", recognized as a planetary boundary threat, there are still critical knowledge gaps regarding their vertical distribution, particularly for small microplastics.

A recent survey of floating plastics across the North Pacific Oceans documented the widespread distribution and identified areas with increased concentrations of plastics. These include the North Pacific Subtropical Gyre, which is well predicted by models based on ocean currents, and a previously unpredicted hotspot of surface-floating plastics in the World Heritage Site Papaha̅naumokua̅kea Marine National Monument. Based on these surface observations, we here present measurements of (micro)plastics across the water column and in abyssal sediments at three stations, exceeding depths of 5 km.

Particle samples across the water column at multiple depths and from deep-sea sediments were collected during research cruise SO268/3 aboard the German RV SONNE (May – July 2019), covering both hotspots and a less contaminated intermediate open ocean site. Plastic items were isolated from the particulate matrix using a combination of enzymatic and chemical digestion methods and density separation. Identification and characterization of microplastics present in the samples was carried out using FT-IR imaging down to an analytical size detection limit of 11 µm.

Microplastics were detected across the whole depth range down to the sediments at all three stations. Concentrations in the water column ranged from 8 to 2600 items m^-3 and 1100 to 3200 items kg^-1 in sediments. Distribution patterns among stations differed, indicating site-specific transport and deposition mechanisms due to different environmental factors and conditions. Across stations and depths, the polymer composition was broadly consistent and dominated by polyethylene and polypropylene. A substantial share of detected plastic items was close to the lower size detection limit, emphasizing the importance of this small particle fraction and the need to include it in future surveys.

Taken together, our results contribute to narrowing critical knowledge gaps regarding the distribution of marine microplastics, demonstrate their widespread vertical dispersion. The similar polymer composition and uniform size distribution across all samples and depths suggest that plastic items partly originate from surface fallout. Furthermore, our results underscore the role of the water column and the deep seafloor as substantial but comparatively understudied reservoirs of microplastics, ultimately highlighting the need for expanded research efforts and effective strategies to mitigate marine plastic pollution.