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Biogeosciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/bg-2016-542
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
07 Mar 2017
Review status
This discussion paper is under review for the journal Biogeosciences (BG).
Vertical segregation among pathways mediating nitrogen-loss (N2 and N2O production) across the oxygen gradient in a coastal upwelling ecosystem
Alexander Galán1,a, Bo Thamdrup2, Gonzalo S. Saldías3,4, and Laura Farías5,6,7 1CREA – Centro Regional de Estudios Ambientales, Universidad Católica de la Santísima Concepción, Av. Colón 2766, Talcahuano 4270789, Chile.
2Department of Biology and Nordic Center for Earth Evolution (NordCEE), University of Southern Denmark, Odense M, Denmark
3College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, USA
4Centro FONDAP de Investigación en Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Valdivia, Chile
5Departamento de Oceanografía, Universidad de Concepción, Concepción, Chile
6Laboratorio de Procesos Oceanográficos y Clima (PROFC), Universidad de Concepción, Concepción, Chile
7Centro de Ciencia del Clima y la Resiliencia (CR2), Chile
aformerly at: Departamento de Oceanografía, Universidad de Concepción, Concepción, Chile
Abstract. The upwelling system off central Chile (36.5°ree: S) is seasonally subjected to oxygen (O2) deficient waters, with a strong vertical gradient in O2 varying from oxic to anoxic conditions on a scale of a few meters (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g., anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N-loss and its speciation, in the form of N2 and N2O, were studied using 15N-tracer incubations, inhibitor assays, the natural abundance of nitrate isotopes, and hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth), over the continental shelf off Concepción, Chile. Results of 15N-labeled incubations revealed a higher N removal activity during the austral summer, with denitrification as the dominant N2 producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline. Notwithstanding, a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 μmol L−1), relative to the hypoxic bottom waters (< 20 μmol O2 L−1). Different pathways were responsible for N2O produced in the oxycline and bottom waters, with ammonium oxidation and dissimilatory nitrite reduction, respectively, as the main subsidiary processes. Ammonium produced by DNRA could sustain both anammox and nitrification rates, including the ammonium utilized for N2O production. The temporal and vertical variability of δ15N-NO3 confirms that multiple N-cycling processes are modulating the isotopic nitrate composition over the shelf off central Chile during spring and summer. N removal processes in this coastal system appear to be related to the availability and distribution of oxygen and particles, which are a source of organic matter and the fuel for the production of other electron donors (i.e., ammonium) and acceptors (i.e., nitrate and nitrite), after its remineralization. These results highlight the links between several pathways involved in N-loss. They also establish that different mechanisms, supported by alternative N substrates, are responsible for a substantial accumulation of N2O, frequently observed as hotspots in the oxycline and bottom waters. Considering the extreme variation in oxygen observed in several coastal upwelling systems, these findings could help to understand the ecological and biogeochemical implications due global warming where intensification and/or expansion of the oceanic OMZs is projected.

Citation: Galán, A., Thamdrup, B., Saldías, G. S., and Farías, L.: Vertical segregation among pathways mediating nitrogen-loss (N2 and N2O production) across the oxygen gradient in a coastal upwelling ecosystem, Biogeosciences Discuss., doi:10.5194/bg-2016-542, in review, 2017.
Alexander Galán et al.
Alexander Galán et al.
Alexander Galán et al.

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Short summary
This investigation characterize the variability of pathways involved in N-loss in the water column over the continental shelf off central Chile during the development of the upwelling season. Our results highlight the links between several pathways involved in N-removal and considering the extreme variation in oxygen observed could help to understand the ecological and biogeochemical implications due global warming where intensification and/or expansion of the oceanic OMZs are projected.
This investigation characterize the variability of pathways involved in N-loss in the water...
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