Submesoscale ocean structures are ubiquitous across the global ocean and play a disproportionate role in vertical transport, mixing, and biogeochemical exchanges. Yet, their influence on the distribution and quantification of anthropogenic contaminants remains poorly constrained. Here, we investigate microfiber (MF) distributions within a submesoscale cyclonic eddy in the Western Mediterranean Sea using high-resolution hydrographic and contaminant measurements collected during the CALYPSO 2022 campaign. We observed pronounced subsurface enrichment of textile microfibers within the eddy interior, with concentrations reaching 0.34 MF L¹ compared to 0.09 MF L¹ in surrounding waters, indicating a nearly fourfold increase associated with coherent submesoscale circulation. Elevated concentrations persisted after cyclone fragmentation, suggesting that transient submesoscale features can continue to function as effective retention structures. The observed patterns point to interactions between physical retention mechanisms, including vertical recirculation, frontal convergence, turbulent mixing, and upwelling, and the settling behaviour of negatively buoyant fibers. Enhanced chlorophyll-a concentrations within the upper water column further indicate coupling between physical dynamics, biological productivity, and contaminant accumulation. These findings suggest that submesoscale processes not only shape microfiber transport and residence times but may also strongly influence exposure pathways within marine food webs. Importantly, the strong spatial heterogeneity generated by these widespread ocean features highlights a potential source of bias in estimates of microfiber and microplastic concentrations and in marine contamination assessments, as localised sampling may overestimate or underestimate regional contaminant loads depending on whether submesoscale retention structures are intercepted. Accounting for these fine-scale dynamics is therefore essential for improving sampling strategies, interpreting field observations, and refining contaminant transport models. More broadly, this work underscores the need to integrate submesoscale ocean physics into global assessments of marine pollution and ecosystem vulnerability.

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