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Marine Ecology Progress Series

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MEPS 288:35-43 (2005)  -  doi:10.3354/meps288035

Ammonium cycling under a strong oxygen gradient associated with the Oxygen Minimum Zone off northern Chile (~23°S)

Verónica Molina1,2,3,*, Laura Farías2,3,4, Yoanna Eissler3, Luis Antonio Cuevas1,3,Carmen E. Morales3,4, Rubén Escribano3,4

1Programa de Postgrado, Departamento Oceanografía, 2Programa Regional de Oceanografía Física y Clima (PROFC),
3Centro de Investigación Oceanográfica en el Pacífico Sur-Oriental (FONDAP-COPAS), 4Departamento de Oceanografía, Universidad de Concepción, PO Box 160-C, Correo 3, Concepción, Chile

ABSTRACT: Ammonium (NH4+) cycling rates under different dissolved oxygen (DO) conditions in the water column, at a coastal upwelling area off northern Chile (~23°S), were estimated. Net NH4+ regeneration (eukaryotes and prokaryotes) and NH4+ oxidation (nitrifying bacteria) rates were examined by means of selective inhibitor assays (cycloheximide and allylthiourea) under dark conditions. Whole water samples for incubations were taken in the oxycline, low-DO zone (30 m; 69 µM DO), and in the upper boundary of the Oxygen Minimum Zone (OMZ), suboxic zone (50 m; <5 µM DO). Net NH4+ regeneration and consumption were also determined in size-fractionated (<200 µm) experiments with water samples obtained from the suboxic zone (50 m) and the base of the mixed layer, oxic zone (15 m; 104 µM DO). Results indicate that, in the oxycline, prokaryotes were responsible for most of the NH4+ cycling, with regeneration and oxidation rates of ~1.3 and 0.56 µM d–1, respectively. This, in turn, favoured NH4+ and NO3 accumulation in this layer, characterised by strong physical–chemical gradients (temperature, salinity and DO), and accompanied by lower abundances of cyanobacteria (0.09 × 105 cells ml–1) and heterotrophic nanoflagellates (HNF; 15.6 cells ml–1). In the oxic and suboxic layers, eukaryotes appear to be the main contributors to net NH4+ regeneration (4.6 to 17.7 µM d–1), supporting a high net NH4+ dark prokaryote consumption, including high potential NH4+ oxidation (0.95 to 1.34 µM d–1) in the suboxic zone. The abundances of bacteria, cyanobacteria and HNF were higher in these layers (>1.2 × 106, >0.9 × 105 and >37 cells ml–1, respectively), indicating a potential large impact on NH4+ cycling.

KEY WORDS: Ammonium regeneration · Nitrification potential · Bacterioplankton · Heterotrophic nanoflagellates · Oxygen Minimum Zone · Upwelling area · Northern Chile

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