East‐West Variability in Nitrogen Sources Sustaining Pelagic Food Webs in the Subtropical North Pacific Ocean Revealed by Stable Isotopic Analyses

Abstract

Biological N ₂ fixation sustains pelagic ecosystems in subtropical oligotrophic waters. However, its impact on food webs as a nitrogen source remains poorly understood. Here, we quantified the contribution of diazotroph‐derived nitrogen to grazing food chains in the subtropical North Pacific in summer, where N ₂ fixation activity varies markedly along a zonal gradient in the supply of the essential micronutrient, iron. To resolve this spatial variability, we conducted an ecosystem‐scale analysis based on δ ¹⁵ N budgets along a longitudinal transect from 120°W to 137°E at 23°N and near the California Current in August‐September 2017. The contribution of N ₂ fixation to export (new) production varied from approximately 20% in the western part of the subtropical gyre (137–150°E) to approximately 50%–60% in the eastern and marginal areas of the gyre (140–170°W), whereas it decreased to almost 0% at the subarctic‐subtropical boundary. The δ ¹³ C‐δ ¹⁵ N biplots revealed consistent trophic enrichment from suspended particulate organic matter to mesozooplankton and micronektonic fish throughout the sampling areas. Despite the east‐west difference in the diazotrophic nitrogen contribution across the subtropical gyre, δ ¹⁵ N values of biotic components were at similar magnitudes. This apparent discrepancy can be attributed to lower nitrate δ ¹⁵ N in the western gyre (∼4‰) than in the eastern (5–6‰), which depresses the δ ¹⁵ N signatures across food‐web components to levels comparable to those in the eastern gyre, where diazotroph‐derived nitrogen inputs are substantial. This nitrate δ ¹⁵ N pattern suggests that regenerated‐nitrified nitrogen coupled with lateral transport plays an important role in sustaining biological production in the subtropical western North Pacific.