1Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland 21613, USA
2Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, USA
3Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA
*now at: Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA
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 or archaea and assimilatory nitrate (NO3–) reduction by phytoplankton or 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), high-resolution 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 of up to 50 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– profiles supported direct rate measurements of a net biological sink for NO2– just below the PNM. Residence time estimates of NO2– within the PNM were similar at the mesotrophic and oligotrophic stations and ranged from 150–205 d. Our results suggest the PNM is a dynamic, rather than relict, feature with a source term dominated by ammonia oxidation.