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

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 exam- ined 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; 1.2 × 10 6 , >0.9 × 10 5 and >37 cells ml -1 , respectively), indicating a potential large impact on NH4 + cycling.