Nowadays it is widely known that pollution by microplastics (MP) at the open ocean covers immense areas. Buoyant plastics tend to accumulate in areas of convergence at the sea surface such as subtropical gyres, while non-buoyant plastics accumulate at the seafloor. However, previous studies have revealed that the total amount of plastic in the different oceans is not well correlated with the concentrations measured at the sea surface and the sea floor, evidencing a significant amount of missing plastic in the oceans. This deviation could be related to an underestimation of the role played by small fragments of plastic and fibers in the oceans. Furthermore, microplastic fragments with a density lower than the density of seawater have been gathered hundreds of meters below the sea surface in the Pacific Ocean due to their size and shape. The main objective of this study is to carry out, for the first time, an equivalent analysis along the water column for the Atlantic Ocean. In that sense, a total number of 51 samples were collected during four different oceanographic cruises between February and December 2019, from the sea surface down to 1150 m depth at the open ocean waters of the Canary Islands region (Spain). For each sample, 72 l of seawater were filtered on board with a mesh size of 100 μm, where the presence of microplastics has been clearly observed. Our results reveal the presence of microplastics at least up to 1150 m depth, at the Northeastern Atlantic Subtropical Gyre with noticeable seasonal differences. The spatial distribution of these small fragments and fibers at the water column is mainly related to the oceanic dynamics and mesoscale convective flows, overcoming the MP motion induced by their own buoyancy. Moreover, these microplastics have being transported by the ocean dynamics as passive drifters.
Keywords: Depth samples; Marine microplastics; Oceanic dynamic; Passive drifters; Physical variables; Water column.
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