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

Santoro, A. E.; Sakamoto, C. M.; Smith, J. M.; Plant, J. N.; Gehman, A. L.; Worden, A. Z.; Johnson, K. S.; Francis, C. A.; Casciotti, K. L.

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.



Santoro, A. E. / Sakamoto, C. M. / Smith, J. M. / et al: Measurements of nitrite production in and around the primary nitrite maximum in the central California Current. 2013. Copernicus Publications.


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