Ammonium recycling limits nitrate use in the oceanic subarctic Pacific

• Main Authors: Wheeler, Patricia A., Kokkinakis, Steven A.
• Other Authors: College of Oceanography
• Format: Article in Journal/Newspaper
• Language: English; unknown
• Published: American Society of Limnology and Oceanography, Inc.
• Subjects: Pacific; Subarctic
• Online Access: https://ir.library.oregonstate.edu/concern/articles/6108vc82f

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.