Eastern boundary upwelling systems (EBUS) areregions of high primary production often associated withoxygen minimum zones (OMZs). They represent key regionsfor the oceanic nitrogen (N) cycle. By exporting organic mat-ter (OM) and nutrients produced in the coastal region to theopen ocean, EBUS can play an important role in sustainingprimary production in subtropical gyres. However, losses offixed inorganic N through denitrification and anammox pro-cesses take place in oxygen depleted environments such asEBUS, and can potentially mitigate the role of these regionsas a source of N to the open ocean. EBUS can also representa considerable source of nitrous oxide (N2O) to the atmo-sphere, affecting the atmospheric budget of N2O.In this paper a 3-D coupled physical/biogeochemicalmodel (ROMS/BioEBUS) is used to investigate the N bud-get in the Namibian upwelling system. The main processeslinked to EBUS and associated OMZs are taken into account.The study focuses on the northern part of the Benguela up-welling system (BUS), especially the Walvis Bay area (be-tween 22◦S and 24◦S) where the OMZ is well developed.Fluxes of N off the Walvis Bay area are estimated in orderto understand and quantify (1) the total N offshore exportfrom the upwelling area, representing a possible N sourcethat sustains primary production in the South Atlantic sub-tropical gyre; (2) export production and subsequent losses offixed N via denitrification and anammox under suboxic con-ditions (O2<25 mmol O2m−3); and (3) the N2O emission tothe atmosphere in the upwelling area.In the mixed layer, the total N offshore export is estimatedas 8.5±3.9×1010mol N yr−1at 10◦E off the Walvis Bayarea, with a mesoscale contribution of 20 %. Extrapolated tothe whole BUS, the coastal N source for the subtropical gyrecorresponds to 0.1±0.04 mol N m−2yr−1. This N flux rep-resents a major source of N for the gyre compared with otherN sources, and contributes 28 % of the new primary produc-tion estimated for the South Atlantic subtropical gyre. Export production (16.9±1.3×1010mol N yr−1) helpsto maintain an OMZ off Namibia in which coupled ni-trification, denitrification and anammox processes lead tolosses of fixed N and N2O production. However, neitherN losses (0.04±0.025×1010mol N yr−1) nor N2O emis-sions (0.03±0.002×1010mol N yr−1) significantly impactthe main N exports of the Walvis Bay area.The studied area does not significantly contribute to N2Oemissions (0.5 to 2.7 %) compared to the global coastal up-welling emissions. Locally produced N2O is mostly advectedsouthward by the poleward undercurrent.
Gutknecht E., Dadou I., Le Vu B., Cambon G., Sudre J., Garçon V., Machu E., Rixen T., Kock A., Flohr A., Paulmier A. and Lavik G. (2013) Nitrogen transfers and air-sea N2O fluxes in the upwelling off Namibia within the oxygen minimum zone: a 3-D model approach. Biogeosciences Discussions 8(2): 3537-3618. 10.5194/bgd-8-3537-2011
@article{Gutknecht2013,
Title = {Nitrogen transfers and air-sea N2O fluxes in the upwelling off Namibia within the oxygen minimum zone: a 3-D model approach},
Author = {Gutknecht, E. and Dadou, I. and Le Vu, B. and Cambon, G. and Sudre, J. and Garçon, V. and Machu, E. and Rixen, Tim and Kock, A. and Flohr, A. and Paulmier, A. and Lavik, G.},
Editor = {},
Journal = {Biogeosciences Discussions},
Year = {2013},
Pages = {3537-3618},
Volume = {8},
Doi = {10.5194/bgd-8-3537-2011},
Abstract = {Eastern boundary upwelling systems (EBUS) areregions of high primary production often associated withoxygen minimum zones (OMZs). They represent key regionsfor the oceanic nitrogen (N) cycle. By exporting organic mat-ter (OM) and nutrients produced in the coastal region to theopen ocean, EBUS can play an important role in sustainingprimary production in subtropical gyres. However, losses offixed inorganic N through denitrification and anammox pro-cesses take place in oxygen depleted environments such asEBUS, and can potentially mitigate the role of these regionsas a source of N to the open ocean. EBUS can also representa considerable source of nitrous oxide (N2O) to the atmo-sphere, affecting the atmospheric budget of N2O.In this paper a 3-D coupled physical/biogeochemicalmodel (ROMS/BioEBUS) is used to investigate the N bud-get in the Namibian upwelling system. The main processeslinked to EBUS and associated OMZs are taken into account.The study focuses on the northern part of the Benguela up-welling system (BUS), especially the Walvis Bay area (be-tween 22◦S and 24◦S) where the OMZ is well developed.Fluxes of N off the Walvis Bay area are estimated in orderto understand and quantify (1) the total N offshore exportfrom the upwelling area, representing a possible N sourcethat sustains primary production in the South Atlantic sub-tropical gyre; (2) export production and subsequent losses offixed N via denitrification and anammox under suboxic con-ditions (O2<25 mmol O2m−3); and (3) the N2O emission tothe atmosphere in the upwelling area.In the mixed layer, the total N offshore export is estimatedas 8.5±3.9×1010mol N yr−1at 10◦E off the Walvis Bayarea, with a mesoscale contribution of 20 %. Extrapolated tothe whole BUS, the coastal N source for the subtropical gyrecorresponds to 0.1±0.04 mol N m−2yr−1. This N flux rep-resents a major source of N for the gyre compared with otherN sources, and contributes 28 % of the new primary produc-tion estimated for the South Atlantic subtropical gyre. Export production (16.9±1.3×1010mol N yr−1) helpsto maintain an OMZ off Namibia in which coupled ni-trification, denitrification and anammox processes lead tolosses of fixed N and N2O production. However, neitherN losses (0.04±0.025×1010mol N yr−1) nor N2O emis-sions (0.03±0.002×1010mol N yr−1) significantly impactthe main N exports of the Walvis Bay area.The studied area does not significantly contribute to N2Oemissions (0.5 to 2.7 %) compared to the global coastal up-welling emissions. Locally produced N2O is mostly advectedsouthward by the poleward undercurrent.},
}
TY - JOUR
AU - Gutknecht, E.
AU - Dadou, I.
AU - Le Vu, B.
AU - Cambon, G.
AU - Sudre, J.
AU - Garçon, V.
AU - Machu, E.
AU - Rixen, Tim
AU - Kock, A.
AU - Flohr, A.
AU - Paulmier, A.
AU - Lavik, G.
TI - Nitrogen transfers and air-sea N<sub>2</sub>O fluxes in the upwelling off Namibia within the oxygen minimum zone: a 3-D model approach
T2 - Biogeosciences Discussions
PY - 2013
SP - 3537-3618
VL - 8
DO - 10.5194/bgd-8-3537-2011
AB - Eastern boundary upwelling systems (EBUS) areregions of high primary production often associated withoxygen minimum zones (OMZs). They represent key regionsfor the oceanic nitrogen (N) cycle. By exporting organic mat-ter (OM) and nutrients produced in the coastal region to theopen ocean, EBUS can play an important role in sustainingprimary production in subtropical gyres. However, losses offixed inorganic N through denitrification and anammox pro-cesses take place in oxygen depleted environments such asEBUS, and can potentially mitigate the role of these regionsas a source of N to the open ocean. EBUS can also representa considerable source of nitrous oxide (N2O) to the atmo-sphere, affecting the atmospheric budget of N2O.In this paper a 3-D coupled physical/biogeochemicalmodel (ROMS/BioEBUS) is used to investigate the N bud-get in the Namibian upwelling system. The main processeslinked to EBUS and associated OMZs are taken into account.The study focuses on the northern part of the Benguela up-welling system (BUS), especially the Walvis Bay area (be-tween 22◦S and 24◦S) where the OMZ is well developed.Fluxes of N off the Walvis Bay area are estimated in orderto understand and quantify (1) the total N offshore exportfrom the upwelling area, representing a possible N sourcethat sustains primary production in the South Atlantic sub-tropical gyre; (2) export production and subsequent losses offixed N via denitrification and anammox under suboxic con-ditions (O2<25 mmol O2m−3); and (3) the N2O emission tothe atmosphere in the upwelling area.In the mixed layer, the total N offshore export is estimatedas 8.5±3.9×1010mol N yr−1at 10◦E off the Walvis Bayarea, with a mesoscale contribution of 20 %. Extrapolated tothe whole BUS, the coastal N source for the subtropical gyrecorresponds to 0.1±0.04 mol N m−2yr−1. This N flux rep-resents a major source of N for the gyre compared with otherN sources, and contributes 28 % of the new primary produc-tion estimated for the South Atlantic subtropical gyre. Export production (16.9±1.3×1010mol N yr−1) helpsto maintain an OMZ off Namibia in which coupled ni-trification, denitrification and anammox processes lead tolosses of fixed N and N2O production. However, neitherN losses (0.04±0.025×1010mol N yr−1) nor N2O emis-sions (0.03±0.002×1010mol N yr−1) significantly impactthe main N exports of the Walvis Bay area.The studied area does not significantly contribute to N2Oemissions (0.5 to 2.7 %) compared to the global coastal up-welling emissions. Locally produced N2O is mostly advectedsouthward by the poleward undercurrent.
ER -
