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

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.