Distribution and export of dissolved organic carbon along a loop current eddy in the Gulf of Mexico

The ocean plays a major role in regulating the global climate by sequestering carbon, and dissolved organic carbon (DOC) represents one of the largest marine carbon reservoirs. Mesoscale eddies can enhance DOC export to the mesopelagic zone through eddy-wind interactions and eddy-induced strain, with this export often increasing during winter convective mixing. This study investigated how these processes affect DOC distribution and vertical fluxes within the anticyclonic Loop Current Eddy Yazoo in the oligotrophic Gulf of Mexico during the onset of Nortes winds in November 2022. Using high-spatial-resolution sampling along a transect, we mapped the spatial variability of DOC concentrations and hydrographic properties across LCE Yazoo and surrounding waters. We found pronounced horizontal and vertical variability in DOC concentrations and did not observe the hypothesized systematic enrichment within the eddy expected in a nutrient depleted environment. Total dissolved carbohydrates, which are important components of labile and semi-labile DOC fractions, accounted for 7.4% of total DOC in the epipelagic zone, decreasing to ~4% in the mesopelagic zone. Significant correlations were observed between DOC and Prochlorococcus , Synechococcus , picoeukaryotes, and heterotrophic bacterial biomass, indicating close spatial coupling between DOC and microbial community structure. Meanwhile, the relationship between DOC and apparent oxygen utilization in the oxycline indicated a greater apparent contribution of DOC remineralization to oxygen consumption within LCE Yazoo (~40%) than in adjacent waters (~29%). Upward and downward fluxes driven by eddy-Ekman pumping redistributed semi-labile DOC within the euphotic zone but did not result in net export. In contrast, upward and downward fluxes forced by eddy-induced strain led to the net subduction of semi-labile DOC, with particularly strong fluxes near the eddy edges. Overall, the findings support the idea that anticyclonic eddies may be important hotspots for semi-labile DOC export, potentially facilitating the microbial carbon pump in oligotrophic oceans.