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

[摘要] Nitrite (NO₂⁻) is a substrate for both oxidative and reductivemicrobial metabolism. NO₂⁻ accumulates at the base of the euphoticzone in oxygenated, stratified open-ocean water columns, forming a featureknown as the primary nitrite maximum (PNM). Potential pathways ofNO₂⁻ production include the oxidation of ammonia (NH₃) byammonia-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 theeastern North Pacific to determine the relative contributions of theseprocesses to NO₂⁻ production in the PNM. Sensitive(< 10 nmol L⁻¹), precise measurements of [NH₄⁺] and[NO₂⁻] indicated a persistent NH₄⁺ maximum overlying the PNMat every station, with concentrations as high as1.5 μmol L⁻¹. Within and just below the PNM, NH₃oxidation was the dominant NO₂⁻ producing process, with rates ofNH₃ oxidation to NO₂⁻ of up to 31 nmol L⁻¹ d⁻¹,coinciding with high abundances of ammonia-oxidizing archaea. Though littleNO₂⁻ production from NO₃⁻ was detected, potentiallynitrate-reducing phytoplankton (photosynthetic picoeukaryotes,Synechococcus, and Prochlorococcus) were present at thedepth of the PNM. Rates of NO₂⁻ production from NO₃⁻ werehighest within the upper mixed layer (4.6 nmol L⁻¹ d⁻¹) but wereeither below detection limits or 10 times lower than NH₃ oxidation ratesaround the PNM. One-dimensional modeling of water column NO₂⁻production agreed with production determined from ¹⁵N bottle incubationswithin the PNM, but a modeled net biological sink for NO₂⁻ just belowthe PNM was not captured in the incubations. Residence time estimates ofNO₂⁻ within the PNM ranged from 18 to 470 days at the mesotrophic stationand was 40 days at the oligotrophic station. Our results suggest the PNM is adynamic, rather than relict, feature with a source term dominated by ammoniaoxidation.

[学科分类] 地球化学与岩石