Nitrite (NO<sub>2</sub><sup>−</sup>) is a substrate for both oxidative and reductive microbial metabolism. NO<sub>2</sub><sup>−</sup> 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 NO<sub>2</sub><sup>−</sup> production include the oxidation of ammonia (NH<sub>3</sub>) by ammonia-oxidizing bacteria and archaea as well as assimilatory nitrate (NO<sub>3</sub><sup>−</sup>) reduction by phytoplankton and heterotrophic bacteria. Measurements of NH<sub>3</sub> oxidation and NO<sub>3</sub><sup>−</sup> reduction to NO<sub>2</sub><sup>−</sup> were conducted at two stations in the central California Current in the eastern North Pacific to determine the relative contributions of these processes to NO<sub>2</sub><sup>−</sup> production in the PNM. Sensitive (< 10 nmol L<sup>−1</sup>), precise measurements of [NH<sub>4</sub><sup>+</sup>] and [NO<sub>2</sub><sup>−</sup>] indicated a persistent NH<sub>4</sub><sup>+</sup> maximum overlying the PNM at every station, with concentrations as high as 1.5 μmol L<sup>−1</sup>. Within and just below the PNM, NH<sub>3</sub> oxidation was the dominant NO<sub>2</sub><sup>−</sup> producing process, with rates of NH<sub>3</sub> oxidation to NO<sub>2</sub><sup>−</sup> of up to 31 nmol L<sup>−1</sup> d<sup>−1</sup>, coinciding with high abundances of ammonia-oxidizing archaea. Though little NO<sub>2</sub><sup>−</sup> production from NO<sub>3</sub><sup>−</sup> was detected, potentially nitrate-reducing phytoplankton (photosynthetic picoeukaryotes, <i>Synechococcus</i>, and <i>Prochlorococcus</i>) were present at the depth of the PNM. Rates of NO<sub>2</sub><sup>−</sup> production from NO<sub>3</sub><sup>−</sup> were highest within the upper mixed layer (4.6 nmol L<sup>−1</sup> d<sup>−1</sup>) but were either below detection limits or 10 times lower than NH<sub>3</sub> oxidation rates around the PNM. One-dimensional modeling of water column NO<sub>2</sub><sup>−</sup> production agreed with production determined from <sup>15</sup>N bottle incubations within the PNM, but a modeled net biological sink for NO<sub>2</sub><sup>−</sup> just below the PNM was not captured in the incubations. Residence time estimates of NO<sub>2</sub><sup>−</sup> 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.