2:15 PM - 2:30 PM
[MIS09-03] Vertical distribution of small microplastics in the North Pacific: observations and possible causes
Keywords:Microplastics, Water mass, North Pacific Ocean, Marine debris
Marine plastic debris is attracting a global attention: approximately 70% of plastics entering from land to oceans remain unobserved to date (Isobe & Iwasaki, 2022, Sci. Total Environ.). Much of this "missing plastics” is believed to exist as microplastics (MPs: < 5 mm) spreading widely from the sea surface to the deep-sea floor. However, 80.5% of aquatic environmental surveys have not considered microplastics below 300 µm (hereafter small microplastics; S-MPs) (Conkle et al., 2018, Environ Manage). Due to their small size, sampling and subsequent analyses of S-MPs have not yet well established, resulting in an inaccurate assessment regarding their quantities in oceans. To address the marine debris problem, it is crucial to focus on less buoyant S-MPs that potentially penetrate into the subsurface layers to elucidate their presence and behavior in the oceans. Liu et al. (2020) proposed that MPs sinking to deeper layers may be influenced by water mass structures (Liu et al., 2020, Water Res.), although their speculation remains inconclusive due to the absence of field surveys for their vertical distributions. Therefore, understanding how s-MPs’ distribution is related to water mass structures remains unclear as a current demand for elucidating their behavior and impacts. In this study, we established onboard S-MPs sampling method and investigated the vertical distribution of S-MPs in the North Pacific.
At seven observation stations in the North Pacific, we conducted seawater sampling using Niskin bottles installed with a conductivity temperature depth profiler to measure salinity and water temperature. The seawater samples were collected at 12 depths from the surface to 1000 m. To eliminate the contamination during sampling, the seawater samples were filtered in a clean booth installed on the survey vessel using a metal filter (mesh size: 10 µm) to collect S-MPs. The collected S-MPs were then analyzed to quantify quantity (particles counts), sizes, and polymer types using micro Fourier-transform infrared spectroscopy. Additionally, we calculated concentrations (pieces/m³) for each sampling layer by dividing the number of S-MPs by the volume of seawater sampled.
The results exhibit that the sizes of collected MPs at each station peaked at 20-60 μm, indicating the presence of abundant S-MPs beneath the sea surface, implying that currently unaccounted S-MPs may exist in the ocean. Furthermore, at three stations between 20° and 30°N, there was an increasing trend in S-MPs concentration from the sea surface to the 24 σθ isopycnal layer (at a depth of 200 m), followed by a decreasing trend from the 24 σθ to the 26 σθ isopycnal layers. However, from the 26 σθ (at a depth of 600 m) to the 27.5 σθ (at a depth of 1000 m) isopycnal layers, an increasing trend in S-MPs concentration was again observed.Given the established presence of North Pacific Tropical Water (NPTW) within the potential density range of 24-25 σθ, and North Pacific Intermediate Water (NPIW) within the range of 26.7-26.9 σθ, it was suggested that S-MPs concentrations may increase not only within the layer associated with NPTW but also in the subsurface layer below NPIW.The outcropped area of subsurface water with potential density of 24 σθ in the North Pacific coincided with the high concentration zone of surface MPs (Isobe et al., 2021, Microplastics and Nanoplastics). In addition, the surface MP concentration decreases as we move to the north where surface seawater becomes heavier to approximately 26σθ. This implies that S-MPs sink from the surface to subsurface layers and are subducted southward along the isopycnal layers. In addition, increasing trend below the NPIW layer, which never outcrops to the sea surface, suggests that the S-MPs in the deep layers were derived from settling processes from the upper ocean.
At seven observation stations in the North Pacific, we conducted seawater sampling using Niskin bottles installed with a conductivity temperature depth profiler to measure salinity and water temperature. The seawater samples were collected at 12 depths from the surface to 1000 m. To eliminate the contamination during sampling, the seawater samples were filtered in a clean booth installed on the survey vessel using a metal filter (mesh size: 10 µm) to collect S-MPs. The collected S-MPs were then analyzed to quantify quantity (particles counts), sizes, and polymer types using micro Fourier-transform infrared spectroscopy. Additionally, we calculated concentrations (pieces/m³) for each sampling layer by dividing the number of S-MPs by the volume of seawater sampled.
The results exhibit that the sizes of collected MPs at each station peaked at 20-60 μm, indicating the presence of abundant S-MPs beneath the sea surface, implying that currently unaccounted S-MPs may exist in the ocean. Furthermore, at three stations between 20° and 30°N, there was an increasing trend in S-MPs concentration from the sea surface to the 24 σθ isopycnal layer (at a depth of 200 m), followed by a decreasing trend from the 24 σθ to the 26 σθ isopycnal layers. However, from the 26 σθ (at a depth of 600 m) to the 27.5 σθ (at a depth of 1000 m) isopycnal layers, an increasing trend in S-MPs concentration was again observed.Given the established presence of North Pacific Tropical Water (NPTW) within the potential density range of 24-25 σθ, and North Pacific Intermediate Water (NPIW) within the range of 26.7-26.9 σθ, it was suggested that S-MPs concentrations may increase not only within the layer associated with NPTW but also in the subsurface layer below NPIW.The outcropped area of subsurface water with potential density of 24 σθ in the North Pacific coincided with the high concentration zone of surface MPs (Isobe et al., 2021, Microplastics and Nanoplastics). In addition, the surface MP concentration decreases as we move to the north where surface seawater becomes heavier to approximately 26σθ. This implies that S-MPs sink from the surface to subsurface layers and are subducted southward along the isopycnal layers. In addition, increasing trend below the NPIW layer, which never outcrops to the sea surface, suggests that the S-MPs in the deep layers were derived from settling processes from the upper ocean.