Nitrification in the euphotic zone as a source for nitrite, nitrate, and nitrous oxide at Station ALOHA

Abstract

We measured chemoautotrophic bacterial nitrification rates in the lower euphotic zone at the North Pacific Time-series Station ALOHA using low-level chemical assays and inhibitor-sensitive radiocarbon uptake experiments. These measurements were compared with independent nitrification rate estimates based on nitrous oxide distributions, nitrate assimilation rates based on nitrate changes during an in situ incubation, and historical estimates of nitrification and nitrate assimilation from this region. Ammonium oxidation rates ranged from 1.0 to 137.4 μmol m-3 d-1, and nitrite oxidation rates varied from undetectable to 138.0 μmol m-3 d-1. Conservative estimates of depth-integrated euphotic zone nitrification rates from the complete three-cruise data set were 0.69 mmol m-2 d-1 for nitrite oxidation and 1.64 mmol m-2 d-1 for ammonium oxidation. The highest nitrification rates were found below the primary nitrite maximum, suggesting a significant contribution of euphotic zone ammonium oxidation to the nitrite pool below but not within this feature. A mass balance of nitrous oxide indicates that this gas is produced within the euphotic zone at a rate of 1.68-7.94 μmol m-2 d-1. The nitrous oxide production rate provides an independent estimate of the total euphotic zone nitrification rate in the range of 0.34-1.59 mmol m-2 d-1. We estimate that the total euphotic zone nitrification rate is 47-142% of the concurrent nitrate assimilation rate, indicating that nitrification in the euphotic zone is an important source of regenerated nitrate.