The Gulf of Mexico (GoM) is a highly dynamic marginal sea where the Loop Current and its associated anticyclonic Loop Current eddies (LCEs) exert a dominant influence on the transport and spatial distribution of biogeochemical tracers. It is well known that mesoscale dynamics regulate the distribution of nutrients and primary productivity, but its influence on bioessential trace metals remains poorly constrained. Here we examine existing observations of dissolved nickel (dNi) and dissolved copper (dCu) obtained in the deep-water region of the GoM to analyze how mesoscale dynamics shape trace-metal distributions. Although these elements display distinct vertical distributions, they exhibited pronounced spatial variability associated with the presence and activity of LCEs. Dissolved Ni presented a nutrient-type distribution characterized by low concentrations in surface waters and increasing concentrations with depth. Anticyclonic eddies depressed isopycnals and displaced dNi-poor surface waters downward, deepening the dNi-rich core of Tropical Atlantic Central Water and reducing dNi inventories in the upper ocean. In contrast, cyclonic circulation promoted the upward displacement of subsurface waters enriched in dNi and macronutrients, enhancing their potential availability to primary producers. These patterns indicate that through the vertical displacement of water masses, mesoscale dynamics control dNi distribution and macronutrient supply, thereby modulating their biological availability to the resident phytoplankton community. In contrast, dCu responded differently because its distribution was strongly influenced by external sources. Elevated dCu concentrations in surface waters were associated with freshwater inputs derived from the Mississippi River plume, which were transported hundreds of kilometers offshore by an eddy recently detached from the LC. The highest dCu concentrations were observed in the western GoM within a decaying anticyclonic eddy colliding with the continental margin. This enrichment was attributed to enhanced cross-shelf exchange and off-shelf transport of dCu-enriched waters receiving benthic inputs from the continental shelf and slope. Overall, these observations reveal that mesoscale eddies act as a relevant process affecting the vertical and horizontal distributions of trace metals across the GoM. By trapping, transporting, and vertically displacing water masses, eddies contribute to regulate the exchange of trace metals between the coast and the open ocean and modify their inventories within the upper layer. The contrasting responses of dNi and dCu highlight how mesoscale dynamics interact with element-specific biogeochemical processes to control trace-metal spatial and temporal variability. These findings underscore the need to incorporate mesoscale dynamics into conceptual and quantitative models of trace-metal cycling in marginal seas.

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