Enhanced Nitrogen Loss by Eddy-Induced Vertical Transport in the Offshore Peruvian Oxygen Minimum Zone

Abstract. The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

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Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

Frontiers in Marine Science ◽

10.3389/fmars.2017.00152 ◽

2017 ◽

Vol 4 ◽

Cited By ~ 5

Author(s):

Peter Stief ◽

Ann Sofie B. Lundgaard ◽

Álvaro Morales-Ramírez ◽

Bo Thamdrup ◽

Ronnie N. Glud

Keyword(s):

Costa Rica ◽

Nitrogen Loss ◽

Oxygen Minimum Zone ◽

Fixed Nitrogen ◽

Minimum Zone ◽

Oxygen Minimum

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Distribution and relative quantification of key genes involved in fixed nitrogen loss from the Arabian Sea oxygen minimum zone

Indian Ocean Biogeochemical Processes and Ecological Variability - Geophysical Monograph Series ◽

10.1029/2008gm000730 ◽

2009 ◽

pp. 187-203 ◽

Cited By ~ 11

Author(s):

Amal Jayakumar ◽

S. Wajih A. Naqvi ◽

Bess B. Ward

Keyword(s):

Arabian Sea ◽

Nitrogen Loss ◽

Oxygen Minimum Zone ◽

Relative Quantification ◽

Fixed Nitrogen ◽

Minimum Zone ◽

Key Genes ◽

Oxygen Minimum

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Extreme N<sub>2</sub>O accumulation in the coastal oxygen minimum zone off Peru

Biogeosciences ◽

10.5194/bg-13-827-2016 ◽

2016 ◽

Vol 13 (3) ◽

pp. 827-840 ◽

Cited By ~ 33

Author(s):

A. Kock ◽

D. L. Arévalo-Martínez ◽

C. R. Löscher ◽

H. W. Bange

Keyword(s):

Linear Relationship ◽

Coastal Upwelling ◽

Nitrogen Loss ◽

Full Range ◽

Oxygen Minimum Zone ◽

Open Ocean ◽

N2o Production ◽

Minimum Zone ◽

Oxygen Minimum ◽

Oxygen Concentrations

Abstract. Depth profiles of nitrous oxide (N2O) were measured during six cruises to the upwelling area and oxygen minimum zone (OMZ) off Peru in 2009 and 2012/2013, covering both the coastal shelf region and the adjacent open ocean. N2O profiles displayed a strong sensitivity towards oxygen concentrations. Open ocean profiles with distances to the shelf break larger than the first baroclinic Rossby radius of deformation showed a transition from a broad maximum close to the Equator to a double-peak structure south of 5° S where the oxygen minimum was more pronounced. Maximum N2O concentrations in the open ocean were about 80 nM. A linear relationship between ΔN2O and apparent oxygen utilization (AOU) could be found for measurements within the upper oxycline, with a slope similar to studies in other oceanic regions. In contrast, N2O profiles close to the shelf revealed a much higher variability, and N2O concentrations higher than 100 nM were often observed. The highest N2O concentration measured at the shelf was ∼  850 nM. Due to the extremely sharp oxygen gradients at the shelf, N2O maxima occurred in very shallow water depths of less than 50 m. In the coastal area, a linear relationship between ΔN2O and AOU could not be observed as extremely high ΔN2O values were scattered over the full range of oxygen concentrations. The data points that showed the strongest deviation from a linear ΔN2O ∕ AOU relationship also showed signals of intense nitrogen loss. These results indicate that the coastal upwelling at the Peruvian coast and the subsequent strong remineralization in the water column causes conditions that lead to extreme N2O accumulation, most likely due to the interplay of intense mixing and high rates of remineralization which lead to a rapid switching of the OMZ waters between anoxic and oxic conditions. This, in turn, could trigger incomplete denitrification or pulses of increased nitrification with extreme N2O production.

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