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

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