Vertical distribution and composition of microplastics and marine litter in the open ocean surrounding the Canary Islands (0-1200 m depth)

Abstract

In the marine environment most abundant polymer are polyethylene (PE) and polypropylene (PP), exposed to oxidation, fragmentation and degradation processes. This study assesses the presence, abundance, distribution and weathering processes of small microplastics (SMPs) and other marine plastic litter in the Canary Islands region. Macroplastic sampling were collected by the Marine Litter Observatory of Fuerteventura (OBAM) in 2023. SMPs were collected from two distinct mesoscale eddies during oceanographic cruises in 2021 and 2022, using two different sampling systems: Niskin bottles and bottle-nets. Fourier transform infrared spectroscopy (FTIR) was used to determine the polymer composition of macroplastics, while micro-FTIR (mu FTIR) was applied to SMPs ranging from 100 mu m to 1 mm. The results highlight the wide distribution of SMPs throughout the Canary Islands water column, extending even beyond 1000 m depth. Synthetic fibers were predominantly composed of polyester, while the presence of colored cellulosic fibers signalled notable human-derived input. Some plastic fragments showed advanced degradation that made their identification difficult, although their spectral characteristics indicated similarity to oxidated PE and PP, so they were classified as oxidized polymers (OxPol). These findings emphasize the critical role of combining advanced polymer analysis with detailed vertical sampling to more accurately trace the transport pathways, breakdown, and ultimate fate of SMPs in deep-sea environments. They also reveal the deep ocean around the Canary Islands as a significant reservoir of degraded microplastics, challenging the assumption that buoyant polymers remain near the surface.