Articles | Volume 10, issue 11
Research article
19 Nov 2013
Research article |  | 19 Nov 2013

Measurements of nitrite production in and around the primary nitrite maximum in the central California Current

A. E. Santoro, C. M. Sakamoto, J. M. Smith, J. N. Plant, A. L. Gehman, A. Z. Worden, K. S. Johnson, C. A. Francis, and K. L. Casciotti

Abstract. Nitrite (NO2) is a substrate for both oxidative and reductive microbial metabolism. NO2 accumulates at the base of the euphotic zone in oxygenated, stratified open-ocean water columns, forming a feature known as the primary nitrite maximum (PNM). Potential pathways of NO2 production include the oxidation of ammonia (NH3) by ammonia-oxidizing bacteria and archaea as well as assimilatory nitrate (NO3) reduction by phytoplankton and heterotrophic bacteria. Measurements of NH3 oxidation and NO3 reduction to NO2 were conducted at two stations in the central California Current in the eastern North Pacific to determine the relative contributions of these processes to NO2 production in the PNM. Sensitive (< 10 nmol L−1), precise measurements of [NH4+] and [NO2] indicated a persistent NH4+ maximum overlying the PNM at every station, with concentrations as high as 1.5 μmol L−1. Within and just below the PNM, NH3 oxidation was the dominant NO2 producing process, with rates of NH3 oxidation to NO2 of up to 31 nmol L−1 d−1, coinciding with high abundances of ammonia-oxidizing archaea. Though little NO2 production from NO3 was detected, potentially nitrate-reducing phytoplankton (photosynthetic picoeukaryotes, Synechococcus, and Prochlorococcus) were present at the depth of the PNM. Rates of NO2 production from NO3 were highest within the upper mixed layer (4.6 nmol L−1 d−1) but were either below detection limits or 10 times lower than NH3 oxidation rates around the PNM. One-dimensional modeling of water column NO2 production agreed with production determined from 15N bottle incubations within the PNM, but a modeled net biological sink for NO2 just below the PNM was not captured in the incubations. Residence time estimates of NO2 within the PNM ranged from 18 to 470 days at the mesotrophic station and was 40 days at the oligotrophic station. Our results suggest the PNM is a dynamic, rather than relict, feature with a source term dominated by ammonia oxidation.

Final-revised paper