Ammonium Recycling Limits Nitrate Use in the Oceanic Subarctic Pacific

• Published in: Limnology and oceanography Vol. 35; no. 6; pp. 1267 - 1278
• Main Authors: Wheeler, Patricia A., Kokkinakis, Steven A.
• Format: Journal Article
• Language: English
• Published: Waco, TX American Society of Limnology and Oceanography 01-09-1990
• Subjects: Animal, plant and microbial ecology; Biological and medical sciences; Biomass; Cruises; Euphotic zone; Fundamental and applied biological sciences. Psychology; Marine; Nitrates; Nitrogen; Physiological assimilation; Phytoplankton; Quaternary ammonium compounds; Recycling; Surface water; Pacific Ocean; IN, North Pacific; INE, Subarctic Pacific; Nutrient; Ammonium; Nitrates; Recycling; Pasture; Seasonal variation
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.1990.35.6.1267

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 $17 \mu M while NH_4^+$ concentrations ranged from 0 to $0.4 \mu M$. 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 $0.3 \mu M$ 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.