High concentrations of plastic hidden beneath the surface of the Atlantic Ocean

Marine microplastics (10–1000 µm) are pervasive contaminants entering the ocean via diverse pathways including rivers, atmosphere, dumping, and at-sea activities. Although substantial data exist for surface waters and seafloor, most observations target particles >250 µm, leaving smaller microplastics and the ocean interior largely unquantified. This gap prevents closure of the marine plastic mass balance: estimates of plastic inputs from land and marine sources cannot be reconciled with observed surface stocks, implying a missing sink. Polymer-specific behavior also varies due to differing physical and chemical properties, affecting fragmentation, degradation, aggregation, biofouling, and ingestion. The study aims to quantify polymer-specific (polyethylene, polypropylene, polystyrene) microplastics down to 25–32 µm across a basin-scale Atlantic transect and with depth below the mixed layer, to assess their contribution to the Atlantic plastic inventory and implications for the “missing plastics” problem.

Previous ocean plastic assessments have sparse geographic coverage and largely focus on particles >250 µm at the sea surface or seafloor. Only one full-depth survey in the Arctic Central Basin examined sub-surface waters, and even there, most particles were fibres >250 µm. Observations indicate deep-sea sediments contain microplastics >11 µm, implying downward transport from the surface. Global input estimates from mismanaged waste and rivers are highly uncertain and often omit atmospheric and maritime sources and under-represent microplastics <300 µm. Meta-analyses identify PE, PP, and PS as the most abundant polymers in marine environments, but open-ocean interior distributions are poorly constrained. Models and net-based surveys suggest accumulation in subtropical gyres for larger floating debris, yet small-size classes and depth profiles remain under-sampled, hindering robust inventories and fate assessments.

Study area and design: Twelve stations along a ~10,000 km North–South Atlantic transect were sampled (AMT26 JR16001 cruise, Sep–Nov 2016). At each station, in situ stand-alone pumps (SAPs) simultaneously filtered large seawater volumes (507–1534 L over ~50 min) at three depths: 10 m (near-surface), an intermediate depth ~10–30 m below the local mixed layer depth (MLD), and a mesopelagic depth 100 m deeper than the intermediate horizon (typically >200 m). MLD was determined from CTD profiles using a fixed temperature-based criterion (ΔT = 0.8 °C; ΔT = 0.3 °C at highly mixed stations). Contamination control: All sampling and processing occurred in air-controlled environments (laminar flow hoods; ISO-5 lab), using pre-combusted filters and primarily non-plastic labware. SAP housings were thoroughly cleaned; chemicals were pre-filtered. Procedural blanks (n replicates) processed identically showed no detectable PE, PP, or PS, indicating contamination-free procedures. Sample processing: Particles were collected on 55 µm stainless-steel and 1 µm nylon meshes at sea and stored at −20 °C. In the lab, particles were rinsed from meshes with filtered artificial seawater, split into four equal subsamples using a Folsom splitter, and digested with KOH (47%, 60 °C, 72 h) to remove organics. The digests were filtered onto 25 mm stainless-steel discs (25 µm aperture), rinsed, ethanol-wetted to reduce surface tension, and dried. FTIR imaging and polymer ID: A Spotlight 400 FTIR Imaging System with Frontier IR Spectrometer (PerkinElmer) acquired hyperspectral images in transmission (4000–750 cm−1, 8 cm−1 resolution, 25 µm pixel). An area of 6 × 6 mm (18% of the 201 mm² filter) centered on the disc was scanned (57,600 spectra; 88 min per image). PCA was used to extract representative spectra, which were matched against a 18,711-polymer library; hits with quality >0.7 were accepted. High-quality spectra for PE, PP, and PS served as references to create correlation maps; polymer-specific binary images were analyzed in FIJI/ImageJ to count and size particles (Feret diameter, area). Quantification: For each sample, four 3 × 3 mm markers within the imaged area served as replicates to derive mean counts and SD, scaled to particles m−3 using known imaged/filtered areas, split fraction, and SAP-filtered volume. Error propagation included SAP flow rate uncertainty (±2%) and splitting error (~5%). Particle sizes ranged from 32 to 651 µm (set by instrument resolution and detection). Mass estimation: Individual particle mass was computed as volume (from 2D dimensions under assumed minimal-thickness “flake” geometry) times polymer density, following established methods; the most conservative conversion (Method IV/Flake) was reported. Sample mass concentrations (µg m−3) equaled mean particle mass times number concentration. Statistics used nonparametric Mann–Whitney U tests at α = 0.01. Basin-scale extrapolation: Average mass concentrations of PE, PP, and PS (32–651 µm) across stations and sampled depths were extrapolated over the Atlantic Ocean area to 200 m depth to estimate total mass in the upper ocean interior. A literature-based mass budget compiled inputs (1950–2015) and surface/seafloor stocks for comparison.

• Polymer presence and concentrations: PE, PP, and PS microplastics were detected at all stations (except PP and PS at the southernmost intermediate depth, where other polymers were present). Mean ± SD number and mass concentrations across all samples: PE 1602 ± 1551 particles m−3 and 389 ± 377 µg m−3; PP 490 ± 822 particles m−3 and 262 ± 568 µg m−3; PS 180 ± 439 particles m−3 and 58 ± 241 µg m−3 (PE > PP > PS by Mann–Whitney U, p < 0.001). - Size distributions: Particle sizes spanned 32–651 µm (mean 81 µm; n = 1444). The majority were <100 µm (PE 68%, PP 49%, PS 67%), with peaks at 50–75 µm. Particles >300 µm were rare (PE 1.1%, PP 2.5%, PS 0.7%). Mean sizes: PP 117 ± 76 µm (n = 302) > PE 96 ± 61 µm (n = 1017; p = 1.42×10−9) and > PS 87 ± 64 µm (n = 125; p = 8.94×10−8). - Vertical patterns: Near-surface mean ± SD: PE 1732 ± 1793 particles m−3 and 591 ± 460 µg m−3; PP 822 ± 1250 and 258 ± 354 µg m−3; PS 228 ± 350 and 148 ± 424 µg m−3. Below MLD, substantial loads persisted; mesopelagic counts decreased to PE 1052 ± 452 particles m−3 and PS 62 ± 90 particles m−3. Mass concentrations for PE and PS declined with depth, with PS decreasing faster. Mean particle sizes for PE and PS were smaller below the MLD (PE 105 ± 74 to 89 ± 57 µm, p = 0.000582; PS 102 ± 95 to 78 ± 38 µm, p = 0.01077). PP showed lowest counts and mass at intermediate depth; size differences across depths were not significant. - Depth-integrated mass loads: Combined mass concentrations below the MLD and in the mesopelagic averaged 511 ± 440 µg m−3 and 642 ± 916 µg m−3, respectively, similar in magnitude to larger floating debris loads in prior Atlantic assessments. - Spatial patterns: No pronounced enhancement in subtropical gyres was observed for these small size classes; elevated PE and PP near South Georgia (up to 2553 and 726 particles m−3, respectively) suggest advection by fronts/currents and/or local fisheries inputs. PS showed a significant southward decrease in surface mass abundance (Mann–Whitney W = 233, p = 0.0053). - Atlantic-scale inventory (upper 200 m): Extrapolated total mass of PE+PP+PS microplastics (32–651 µm) is 11.6–21.1 million tonnes (PE 6–14 MT, PP 4–5 MT, PS 0.95–1.6 MT). Near-surface (0–10 m) PE+PP+PS contributes ~0.8–1.6 MT, compared to ~0.1 MT of larger (>300 µm) floating debris from literature. Seabed stocks of >5 mm debris are ~5.6–13.5 MT (scaled from global estimates). - Mass balance implication: Including small polymer-specific microplastics in the upper 200 m reconciles and can exceed cumulative plastic inputs to the Atlantic since 1950 (estimated 17–47 MT to waters and sediments), indicating prior underestimation of both inputs and interior stocks.

The study provides the first basin-scale, depth-resolved, polymer-specific quantification of small microplastics (32–651 µm) in the Atlantic. High concentrations at and below the mixed layer, with dominance of <100 µm particles, demonstrate that the ocean interior is a major sink for small microplastics and that downward transport from the surface is substantial. Size-selective processes—enhanced vertical mixing, rapid biofouling reducing buoyancy, aggregation with marine snow, and preferential ingestion by zooplankton leading to fecal pellet export—likely drive the observed interior accumulation. Differences among polymers (PE, PP, PS) in abundance, size, and vertical profiles reflect distinct material properties and behaviors (e.g., density, degradation rates, products like expanded PS), emphasizing the need for polymer-resolved assessments. Basin-scale extrapolations show that the mass of small PE, PP, and PS in the upper 200 m (11.6–21.1 MT) alone can balance or exceed cumulative inputs since 1950, addressing the long-standing “missing plastics” paradox. The findings imply that previous budgets underestimated both inputs (omitting atmospheric and maritime sources and small manufactured microplastics) and stocks (omitting small size classes and subsurface waters). Improved, repeated, and harmonized sampling across sizes, depths, and regions is essential to constrain sources, sinks, and transport processes and to inform risk assessments and policy.

Small, polymer-specific microplastics are abundant from the near-surface to the ocean interior across the Atlantic and constitute a major, previously hidden component of the marine plastic inventory. By quantifying PE, PP, and PS (32–651 µm) with FTIR imaging and robust contamination controls, the study demonstrates that interior loads can reconcile or exceed estimated inputs since 1950, negating the need for a “missing sink” beyond the surveyed compartments and size classes. Main contributions include: (i) a depth-resolved Atlantic dataset for key commodity polymers, (ii) evidence for size-selective downward transport and pervasive subsurface contamination, and (iii) a revised Atlantic plastic mass balance highlighting the dominance of small microplastics. Future research should: expand polymer coverage beyond PE, PP, and PS; extend size ranges to smaller microplastics and nanoplastics; measure deeper waters and sediments; integrate atmospheric and maritime inputs; and implement repeated, harmonized, polymer-specific monitoring with improved mass-conversion methods and autonomous platforms.

• Size and polymer scope: Only three polymer types (PE, PP, PS) and a particle size range of 32–651 µm were quantified; particles <32 µm, including nanoplastics, and other polymers were not included. - Depth and spatial coverage: Sampling extended to ~200–270 m at 12 stations along a single transect; abyssal waters and comprehensive seabed inventories were not measured. - Mass conversion uncertainties: Particle mass was inferred from 2D imaging using assumed shapes and minimal thickness (conservative Method IV/Flake), introducing uncertainty, particularly for irregular fragments. - Extrapolation assumptions: Basin-scale estimates assume measured concentrations represent the broader Atlantic and to 200 m; true distributions likely vary with regional oceanography and sources. - Variability and low counts: High spatial heterogeneity leads to large standard deviations, especially for PS due to low counts, increasing uncertainty. - Input comparisons: Input estimates from literature may omit atmospheric and maritime sources and underrepresent small manufactured microplastics, complicating direct comparisons. - Potential biases: Despite rigorous contamination controls and clean blanks for target polymers, general limitations of FTIR imaging (resolution, spectral matching thresholds) and sampling logistics remain.