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Settling and along-isopycnal subduction of small microplastics into intermediate layers over the North Pacific Ocean
Summary
Researchers investigated the vertical distribution of small microplastics (10-300 micrometers) from the sea surface to 1,000 m depth in the North Pacific Ocean using seawater sampling and hydrographic surveys, finding average concentrations of 6,910 particles per cubic meter and identifying along-isopycnal subduction as a key mechanism transporting small microplastics into intermediate water layers.
A candidate of the fate of ocean plastics is fragmentation to small microplastics unmonitorable in the current observation framework. In the present study, the vertical distribution of small microplastics (S-MPs; 10 μm < sizes < 300 μm) and its relation to water masses were investigated by seawater samplings and hydrographic surveys from the sea surface to 1000-m depth in the North Pacific Ocean. The average ± standard deviation of S-MP concentrations in 12 layers of four stations were 6910 ± 2620 particles m–3. The S-MP concentrations become high in isopycnal layers between potential densities of 23 σθ and 25σθ and again rapidly increased beneath the North Pacific Intermediate Water (NPIW) with the salinity minimum around 26.6–27.0σθ isopycnal layers. A simple model approach to reproduce the observed S-MP distribution suggested two pathways for S-MPs originally floating in surface convergence zones. One is the fast settling of S-MPs via biological processes from the surface euphotic layer to deep layers which are never outcropped at the sea surface like NPIW. Meanwhile, the weak settling of S-MPs of which density become close to neutral causes the other pathway, that is, the along-isopycnal subduction from isopycnal layers outcropped at the sea surface to intermediate layers. In fact, microplastic concentrations at the sea surface between 23σθ and 25σθ contour curves (archived in the AOMI database) become larger than those in the surrounding isopycnal layers in a similar fashion to the S-MP vertical profile. S-MPs are also abundant in the same surface area along with MPs, as is likely due to the surface convergence, so that the surface S-MPs are suggested to migrate below the sea surface by the along-isopycnal subduction. The global inventory of near-neutral S-MPs with weak settling is expected to be greater in intermediate layers, requiring carefully designed field surveys for both sampling and subsequent processing.
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