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

John E. Dore and David M. Karl

Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu 96822

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.