Ammonium recycling limits nitrate use in the oceanic subarctic Pacific.

Abstract

Seasonal and diel changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH4+ on NO3− assimilation. Surface‐water NO3− concentrations ranged from 6 to 17 µM while NH4+ concentrations ranged from 0 to 0.4 µM. Total N assimilation ranged from 84 to 732 nM d−1 but showed no seasonal trend. NH4+ 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 NO3− assimilation were negatively correlated with ambient NH4+ concentrations, and concentrations of NH4+ between 0.1 and 0.3 µM caused complete inhibition of NO3− assimilation. NO3− 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 NH4+. Turnover times for the dissolved NH4+ 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 NO3− in the oceanic subarctic Pacific.