Vertical distribution of ammonium in stratified oligotrophic waters

Abstract

A method for measuring nanomolar concentrations of ammonium in seawater was used for the first time under field conditions in the Sargasso Sea and Gulf Stream. Ammonium concentrations ranged from below the limit of detection (3.5 nM) to 164 nM NH4+. Subsurface ammonium maxima (16–59 nM NH4+) occurred within the seasonal thermocline and near the deep chlorophyll a maximum in association with the primary nitrite maximum. The latter feature was observed in both the Sargasso Sea and Gulf Stream, suggesting that it may be common in stratified oligotrophic waters. Vertically integrated nitrite concentrations in the upper 140 m generally exceeded those for ammonium, suggesting more efficient use of reduced nitrogen by the microplankton.