Coherent eddies redistribute microplastics across the water column.

The ocean interior is emerging as a hidden reservoir of microplastics, yet the mechanisms governing their accumulation below the surface remain poorly understood. Despite growing interest in microplastic distribution throughout the ocean, in-situ observations within smaller-scale oceanographic features such as mesoscale eddies and fronts remain sparse. Here we present in-situ observations of microplastic distributions from 50 to 1200 m within two coherent mesoscale eddies south of the Canary Islands, an anticyclone and a cyclone. Combining physical oceanographic measurements and microplastic sampling with Lagrangian Coherent Structure (LCS) analyses, we show that both eddies enhanced microplastic concentrations in the upper 200 m by up to threefold relative to regional background waters. Small fragments (< 200 μm) dominated microplastic abundances, while fibers were significantly more abundant in the cyclone and larger fragments prevailed in the anticyclonic eddy. These findings reinforce that coherent eddies act as dynamic drivers of microplastic redistribution in the Atlantic Ocean, emphasizing the role of particle-specific inertial dynamics in shaping the fate of plastics throughout the water column.