Measurements of nitrite production in and around the primary nitrite maximum in the central California Current
Nitrite (NO 2−) is a substrate for both oxidative and reductive microbial metabolism. NO 2− 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 2− production include the oxidation of ammonia (NH 3) by ammonia-oxidizing bacteria and archaea as well as assimilatory nitrate (NO 3−) reduction by phytoplankton and heterotrophic bacteria. Measurements of NH 3 oxidation and NO 3− reduction to NO 2− 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 2− production in the PNM. Sensitive (< 10 nmol L −1), precise measurements of [NH 4+] and [NO 2−] indicated a persistent NH 4+ maximum overlying the PNM at every station, with concentrations as high as 1.5 μmol L −1. Within and just below the PNM, NH 3 oxidation was the dominant NO 2− producing process, with rates of NH 3 oxidation to NO 2− of up to 31 nmol L −1 d −1, coinciding with high abundances of ammonia-oxidizing archaea. Though little NO 2− production from NO 3− was detected, potentially nitrate-reducing phytoplankton (photosynthetic picoeukaryotes, Synechococcus, and Prochlorococcus) were present at the depth of the PNM. Rates of NO 2− production from NO 3− 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 NH 3 oxidation rates around the PNM. One-dimensional modeling of water column NO 2− production agreed with production determined from 15N bottle incubations within the PNM, but a modeled net biological sink for NO 2− just below the PNM was not captured in the incubations. Residence time estimates of NO 2− 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.