Controls on nitrite oxidation in the upper Southern Ocean: insights from winter kinetics experiments in the Indian sector

Abstract. Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season, potentially weakening the spring/summer biological CO2 sink. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis-Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115±2.3 to 245±18 nM, suggesting that nitrite oxidizing bacteria (NOB) require a minimum (i.e., "threshold") nitrite concentration to produce nitrate. The half-saturation constant ranged from 134±8 to 403±24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150 nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two- to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. We hypothesize that the apparent threshold nitrite requirement of NOB indicates nitrite undersaturation of the heme-rich nitrite oxidoreductase enzyme, perhaps driven by the limited availability of iron in surface waters.

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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO3.4

Geoscientific Model Development ◽

10.5194/gmd-8-3119-2015 ◽

2015 ◽

Vol 8 (10) ◽

pp. 3119-3130 ◽

Cited By ~ 18

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Climate Model ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Antarctic Bottom Water

Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1600359113 ◽

2016 ◽

Vol 113 (38) ◽

pp. 10601-10606 ◽

Cited By ~ 99

Author(s):

Laura A. Bristow ◽

Tage Dalsgaard ◽

Laura Tiano ◽

Daniel B. Mills ◽

Anthony D. Bertagnolli ◽

...

Keyword(s):

Nitrogen Loss ◽

Strong Dependence ◽

Ammonium Oxidation ◽

Nitrite Oxidation ◽

Fixed Nitrogen ◽

Oxygen Minimum Zones ◽

Oxidation Rates ◽

Minimum Zone ◽

Nanomolar Range ◽

Oxygen Minimum

A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O2) sensitivity of these processes down to the O2concentrations present in the OMZ core (<10 nmol⋅L−1) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O2levels between 5 nmol⋅L−1and 20 μmol⋅L−1. Rates of both processes were detectable in the low nanomolar range (5–33 nmol⋅L−1O2), but demonstrated a strong dependence on O2concentrations with apparent half-saturation constants (Kms) of 333 ± 130 nmol⋅L−1O2for ammonium oxidation and 778 ± 168 nmol⋅L−1O2for nitrite oxidation assuming one-component Michaelis–Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis–Menten model, indicating a high-affinity component with aKmof just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O2affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O2is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.

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Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-2949-2015 ◽

2015 ◽

Vol 8 (3) ◽

pp. 2949-2972 ◽

Cited By ~ 2

Author(s):

C. Heuzé ◽

J. K. Ridley ◽

D. Calvert ◽

D. P. Stevens ◽

K. J. Heywood

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Surface Waters ◽

Bottom Water ◽

Deep Convection ◽

Mixed Layer Depth ◽

Vertical Mixing ◽

Weddell Sea ◽

Cmip5 Models ◽

Antarctic Bottom Water

Abstract. Most CMIP5 models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross Seas. To identify the triggering mechanisms leading to Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing are warmer, open larger polynyas and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

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