Mesoscale eddies are important regulators of nutrient cycling and biological nitrogen fixation in oligotrophic oceans, yet their influence on diazotroph communities remains incompletely understood. Here, we investigated hydrographic structure, nutrient distributions, nitrogen fixation rates (NFRs), diazotroph assemblages, and nutrient-addition responses across a cyclonic eddy in the western tropical North Pacific.
The eddy core was characterized by lower sea level anomaly (SLA), depleted dissolved inorganic phosphorus (DIP), and a markedly elevated upper-water-column DIN ratio relative to the eddy edge, while integrated dissolved inorganic nitrogen (DIN) varied little across the transect. Nitrogen fixation exhibited pronounced spatial heterogeneity, with integrated NFRs exceeding 200 μmol N m⁻² d⁻¹ at the eddy edge but remaining substantially lower in the core.
Diazotrophs exhibited clear vertical and spatial partitioning. Surface-dominant UCYN-B declined markedly in the eddy core, whereas UCYN-A lineages increased in the subsurface layer of the core, indicating a shift in diazotroph community structure associated with eddy-driven environmental gradients. Nutrient-enrichment experiments further revealed contrasting nutrient limitation regimes: phosphorus additions stimulated NFRs in the eddy core, consistent with P limitation of diazotroph activity, whereas nitrogen additions enhanced chlorophyll-a concentrations at the eddy edge, suggesting N limitation of phytoplankton growth.
SLA was negatively correlated with the integrated DIN:DIP ratio (R = −0.87, P = 0.005) and positively correlated with NFRs (R = 0.77, P = 0.026). Together, these results demonstrate that cyclonic eddies alter nutrient stoichiometry, reshape diazotroph community composition, and regulate the spatial distribution of nitrogen fixation in the western tropical North Pacific.
Keywords: mesoscale eddy; biological nitrogen fixation; diazotroph community; nutrient stoichiometry; western tropical North Pacific.

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