NITRIFICATION IN THE EUPHOTIC ZONE AS A SOURCE FOR NITRITE, NITRATE, AND NITROUS-OXIDE AT STATION ALOHA

We measured chemoautotrophic bacterial nitrification rates in the lowe r euphotic zone at the North Pacific Time-series Station ALOHA using l ow-level chemical assays and inhibitor-sensitive radiocarbon uptake ex periments. These measurements were compared with independent nitrifica tion rate estimates based on nitrous oxide distributions, nitrate assi milation rates based on nitrate changes during an in situ incubation, and historical estimates of nitrification and nitrate assimilation fro m this region. Ammonium oxidation rates ranged from 1.0 to 137.4 mu mo l m(-3) d(-1), and nitrite oxidation rates varied from undetectable to 138.0 mu mol 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 cont ribution of euphotic zone ammonium oxidation to the nitrite pool below but not within this feature. A mass balance of nitrous oxide indicate s that this gas is produced within the euphotic zone at a rate of 1.68 -7.94 mu mol m(-2) d(-1). The nitrous oxide production rate provides a n independent estimate of the total euphotic zone nitrification rate i n 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.