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 ?M DO), and in the upper boundary of the Oxygen Minimum Zone (OMZ), suboxic zone (50 m; <5 ?M DO). Net NH4 + regeneration and consumption were also determined in size-fractionated (<200 ?m) experiments with water samples obtained from the suboxic zone (50 m) and the base of the mixed layer, oxic zone (15 m; 104 ?M DO). Results indicate that, in the oxycline, prokaryotes were responsible for most of the NH 4 + cycling, with regeneration and oxidation rates of ?1.3 and 0.56 ?M 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 ?M d-1), supporting a high net NH 4 + dark prokaryote consumption, including high potential NH4 + oxidation (0.95 to 1.34 ?M 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. © Inter-Research 2005.