Ammonium recycling limits nitrate use in the oceanic subarctic Pacific.

Seasonal and diel changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH 4 + on NO 3 − assimilation. Surface‐water NO 3 − concentrations ranged from 6 to 17 µ M while NH 4 + concentrations ranged from 0 to 0.4 µ M. Total N assimilation ranged from 84 to 732 nM d −1 but showed no seasonal trend. NH 4 + and urea concentrations were <1% of total dissolved inorganic N, but use of this “regenerated” N still accounted for 44–89% of total N assimilation. Rates of NO 3 − assimilation were negatively correlated with ambient NH 4 + concentrations, and concentrations of NH 4 + between 0.1 and 0.3 µ M caused complete inhibition of NO 3 − assimilation. NO 3 − was more important as a source of N in spring than in summer. We attribute this pattern to a summer increase in turnover rates for NH 4 + . Turnover times for the dissolved NH 4 + pool were half as long in August as in May. Grazing and recycling in the euphotic zone apparently both play significant roles in preventing depletion of NO 3 − in the oceanic subarctic Pacific.