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

A. E. Santoro, C. M. Sakamoto, J. M. Smith, J. N. Plant, A. L. Gehman, A. Z. Worden, K. S. Johnson, C. A. Francis, K. L. Casciotti
2013 Biogeosciences  
<p><strong>Abstract.</strong> Nitrite (NO<sub>2</sub><sup>&amp;minus;</sup>) is a substrate for both oxidative and reductive microbial metabolism. NO<sub>2</sub><sup>&amp;minus;</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>&amp;minus;</sup> production include the oxidation of ammonia (NH<sub>3</sub>) by ammonia-oxidizing bacteria and
more » ... ing bacteria and archaea as well as assimilatory nitrate (NO<sub>3</sub><sup>&amp;minus;</sup>) reduction by phytoplankton and heterotrophic bacteria. Measurements of NH<sub>3</sub> oxidation and NO<sub>3</sub><sup>&amp;minus;</sup> reduction to NO<sub>2</sub><sup>&amp;minus;</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>&amp;minus;</sup> production in the PNM. Sensitive (< 10 nmol L<sup>&amp;minus;1</sup>), precise measurements of [NH<sub>4</sub><sup>+</sup>] and [NO<sub>2</sub><sup>&amp;minus;</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>&amp;minus;1</sup>. Within and just below the PNM, NH<sub>3</sub> oxidation was the dominant NO<sub>2</sub><sup>&amp;minus;</sup> producing process, with rates of NH<sub>3</sub> oxidation to NO<sub>2</sub><sup>&amp;minus;</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>&amp;minus;</sup> production from NO<sub>3</sub><sup>&amp;minus;</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>&amp;minus;</sup> production from NO<sub>3</sub><sup>&amp;minus;</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>&amp;minus;</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>&amp;minus;</sup> just below the PNM was not captured in the incubations. Residence time estimates of NO<sub>2</sub><sup>&amp;minus;</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.</p>
doi:10.5194/bg-10-7395-2013 fatcat:j547y6iznzcndllrouhlu24lca