MEPS 451:1-14 (2012)  -  DOI:

Assimilation and regeneration of inorganic nitrogen in a coastal upwelling system: ammonium and nitrate utilization

C. Fernandez1,2,*, L. Farías2,3

1UPMC Univ Paris 06 and CNRS, UMR 7621, LOMIC, Observatoire Océanologique, 66651, Banyuls/mer, France
2Departamento de Oceanografía, Centro de Investigación Oceanográfica en el Pacífico Sudoriental (COPAS), Universidad de Concepción, Concepción, Chile
3Laboratorio de Procesos Oceanográficos y Clima, Universidad de Concepción, Casilla 160-C, Concepción, Chile

ABSTRACT: The main processes involved in nitrogen cycling (as NO3 and NH4+ assimilation and their regeneration) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36°S). The study area shows seasonal surface fertilization and the development of subsurface hypoxia in austral spring and summer. The rates of NO3 uptake during active upwelling were 5 times higher than in non-upwelling seasons. Uptake of NH4+ was almost half that of NO3 uptake rates during upwelling periods, and similar to NO3 uptake rates in the absence of upwelling. Nitrification experiments showed higher rates during active, compared to non-active, upwelling seasons. NH4+ oxidation was coupled with NO2 oxidation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO3 regeneration via nitrification resulted from higher activity of NO2 oxidation compared to NH4+ oxidation. On the other hand, archaeal NH4+ oxidation had the potential for processing a large fraction of NH4+ and could therefore co-occur with bacterial NO2 oxidation. NH4+ utilization in this coastal upwelling is thus in the same range as NO3 assimilation. NH4+ oxidation is affected by oxygen concentration in the water column, leading to occasional decoupling of NH4+ and NO2 oxidation, particularly at the oxycline. This study gives preliminary evidence of the importance of archaeal–bacterial interactions in the nitrification process and highlights the role of ammonium in fueling annual primary production in coastal upwelling systems.

KEY WORDS: Coastal upwelling · Nitrogen uptake · Ammonium oxidation · Archaea

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Cite this article as: Fernandez C, Farías L (2012) Assimilation and regeneration of inorganic nitrogen in a coastal upwelling system: ammonium and nitrate utilization. Mar Ecol Prog Ser 451:1-14.

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