Flux‐driven evolution: Nitrogen fixation underlies contrasting stoichiometric strategies and genetic lineages in an open‐ocean copepod

Variability in nutrient supply is a defining feature of oligotrophic oceans, yet how nutrient fluxes shape consumer physiology and population structure remains poorly resolved. Here, we examine how diazotrophic nitrogen fixation relates to stoichiometric metabolism and genetic structure in the widespread pelagic copepod Pleuromamma xiphias across the subtropical North Pacific. Using in situ excretion assays, body elemental composition, and genome‐wide single‐nucleotide polymorphism analyses, we show that individuals inhabiting regions of high nitrogen fixation excrete nitrogen and phosphorus in near‐Redfield proportions (N : P ≈ 18) and maintain higher body nitrogen and carbon contents. In contrast, copepods from regions with low nitrogen fixation exhibit strongly nitrogen‐sparing excretion (N : P ≈ 7.5) and reduced body elemental contents. These contrasting stoichiometric phenotypes align closely with two genetically distinct lineages, indicating that spatial variation in nitrogen flux is associated with coordinated physiological and population‐level divergence. Our results demonstrate that diazotrophic nitrogen supply can structure zooplankton‐mediated nutrient regeneration and population differentiation, with implications for nutrient cycling and ecosystem functioning in oligotrophic oceans.