From surface to deep sea: A depth-resolved vertical profile of microplastics in the Canary Basin.

Microplastic (MP) contamination of the open ocean is well documented, yet the vertical structuring of MPs and the role of water masses in shaping deep-water inventories remain poorly resolved along the eastern boundary of the North Atlantic Subtropical Gyre. Here we present a high-resolution vertical profile of MPs in the Canary Basin, based on discrete water samples collected during the MSM/127 cruise (R/V Maria S. Merian, March 2024) at three offshore stations south of Gran Canaria, comprising one full-depth profile (seven depths, 5 to 2720 m) and two stations sampled at shallower horizons; consequently, the deep-water structure below 1500 m reflects a single-station profile and should be interpreted as indicative rather than basin-wide representative. Particles were characterised by stereomicroscopy, ATR-FTIR, micro-Raman spectroscopy and SEM-EDX. MPs were detected in 100% of samples (n = 33), with a mean abundance of 19.5 ± 14.9 MP L-1, the large dispersion reflecting a markedly non-monotonic vertical distribution: a subsurface maximum of 39.3 ± 4.0 MP L-1 at 500 m, coinciding with the upper boundary of the North Atlantic Central Water and the top of the regional Oxygen Minimum Zone; a vertical minimum of 2.0 ± 1.0 MP L-1 at 2000 m; and a secondary deep-water signal between 2500 and 2720 m. MP abundance was strongly correlated with temperature (Pearson r = +0.77), salinity (r = +0.66) and potential density (r = -0.76; all p < 0.001), whereas its association with dissolved oxygen was weak and non-monotonic (Spearman ρ = -0.06, ns), consistent with water-mass structure, rather than dissolved‑oxygen levels, as the principal physical correlate of MP vertical distribution. With depth, the assemblage converged toward fine (<500 μm), dark, fibrous polyethylene terephthalate (PET) particles, reaching 100% PET between 1500 and 2720 m. SEM-EDX revealed weathered, biofouled surfaces bearing Saharan mineral and biogenic carbonate residues, consistent with a Saharan dust and biogenic origin, suggesting that the deep Canary Basin acts as a potential accumulation zone delivering an aged PET-fiber signature to abyssal layers.