MEECE-nitrogen

Aims of this project and methods:

In this project, the nitrogen transfers within the Namibian upwelling (Northern part of the Benguela upwelling system) were estimated: advection by the mean circulation, submesoscale activity (eddies/filaments), and flux to the atmosphere (N₂O flux). For this purpose, we developed a Biogeochemical model for Eastern Boundary Upwelling System: BioEBUS, taking into account specific processes in Oxygen Minimum Zones (OMZs): nitrification, denitrification and anammox. 

Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking, and nitrification play a key role for the low-oxygen water content, N loss, and N₂O concentrations in the OMZ. Moreover, the explicit parameterization of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is necessary to improve the representation of microbial activity linked with the OMZ.

Fig.1: (Left): Vertical profiles of maximum N₂O concentrations between 22-24°S and 10-15°E (in 10^-3 mmol N₂O m^-3) for different parameterizations (Nevison et al., 2003; Suntharalingam et al., 2000, 2012), β coefficient (Suntharalingam et al., 2000, 2012) and sensitivity analyses on key parameters. Vertical profiles were 8 yr averaged. Red stars represent in situ measurements from the FRS Africana cruise in December 2009 during an active upwelling event.

In the mixed layer, the total N offshore export is estimated (see Fig. 3.a) at 10°E off the Walvis Bay area, with a positive mesoscale contribution of 20%.

Extrapolated to the whole BUS, the coastal N source for the subtropical gyre corresponds to 0.1±0.04 molNm⁻² yr⁻¹. This N flux represents a major source of N for the gyre compared with other N sources, and contributes 28% of the new primary production estimated for the South Atlantic subtropical gyre.

Export production (see Fig.3.a) helps to maintain an OMZ off Namibia in which coupled nitrification, denitrification and anammox processes lead to losses of fixed N and N₂O production. However, neither N losses nor N₂O emissions significantly impact the main N exports of the Walvis Bay area.

Fig.2 (Above):(a) Minimum oxygen concentrations (mmol O₂ m^-3) simulated in the OMZ, (b) depth (m) of these minimum oxygen concentrations (c) oxygen advection term and (d) oxygen mixing term (mmol O₂ m^-3 d^-1).

The studied area does not significantly contribute to N₂O emissions (0.5 to 2.7 %) compared to the global coastal upwelling emissions. Locally produced N₂O is mostly advected southward by the poleward undercurrent.

Over the slope, submesoscale dynamics represent a sink of nutrients due to a secondary circulation (Fig.3.b). (Gutknecht et al., 2013b)

Fig.3 (Right): (a) Full nitrogen budget in the mixed layer (10¹⁰ molNyr⁻¹). Fluxes of DIN are indicated in black, DON are in light grey and PON in dark grey. (b) Nitrate deficit (mmolNm⁻³) between the eddy-resolving simulation and a non eddy-resolving simulation, featuring the role of eddies on vertical distribution of nutrients in the Benguela upwelling system. Dashed lines represent nitrate isocontours for the eddy-resolving simulation. Fluxes: 8 yr means, averaged between 22 and 24° S.