ed between 15 and 47 nmol N L21 d21. There was no obvious trend in nitrifying activity between coastal and open-ocean stations. Ammonia oxidation was generally confined to the upper OMZ, where O2 was still measurable. Distribution of nitrate reduction activity. Nitrate reduction was measured to nitrite N-cycling in the Namibian and Peruvian OMZs Distribution of anammox activity. Over the Namibian shelf a strong increase in the N-deficit was observed below the oxycline. Minimum values for N were O2 Sensitivity of N-Cycling in OMZs 6 O2 Sensitivity of N-Cycling in OMZs However, despite an apparent lack of O2 in situ shipboard experiments revealed NH3 oxidation activity also at St. 54 at 75 m as well as in the central OMZ at St. 62. Oxygen sensitivity of anammox and coupled N-cycle processes Oxygen sensitivity of anammox. Anammox activity, as indicated by 15N14N production from 15NH4+ and 15NO22, was measurable in all O2 manipulation experiments without lag phase at the 12695532 Namibian shelf stations. Oxygen concentration and N2 formation showed a significant negative correlation for the incubations with 15NH4+ as well as 15NO22 at St. 206 and the one with 15NH4+ at St. 252. Similar responses to increased O2 were observed for the incubations amended with 15NH4+ and 15NO22 at both stations. Activity decreased with increasing O2 and was, on average,,85%,,70% and,50% of the anoxic control at,3.7,,8.1 and,11.3 mmol L21 of oxygen, respectively. Over the course of the incubation O2 concentrations in the 15Nlabeling experiments did not vary significantly. No substantial difference in 15N14N production was observed between 15NH4+-labeled incubations with and without ATU. This indicates that anammox rather than coupled nitrification-denitrification was the process responsible for the production of 15N-labeled N2 at 1112 mmol L21 of dissolved O2. In the OMZ off Peru, 15N14N production rates in 15NH4+ and 15 NO22 incubations decreased with increasing O2 concentrations in all O2 manipulation experiments. However, substantial differences in the O2 sensitivity of anammox were 14642775 found between stations. Over the Peruvian shelf, adjusted O2 levels and N2 production were linearly and negatively correlated up to 14.4 mmol L21 O2 at St. 44 and 10.9 mmol L21 at St. 62. No rates were detectable beyond,20 
mmol L21 of O2. At the open-ocean stations in the Peruvian OMZ, anammox activity appeared to be more sensitive to the added O2. At St. 36,,30% activity of the anoxic control experiment remained detectable when O2 was increased from the in situ,1.2 mmol L21 to 5.5 mmol L21 of O2 in the 120 m sample. In comparison, anammox was fully inhibited at 2.8 mmol L21 of O2 already in the 180 m sample, where O2 was not detectable by the STOX sensor in situ. A similarly strong O2 response was seen at St. 54, where rates dropped to zero at 4.0 mmol L21 of O2 in the 75 m incubation experiment. Oxygen sensitivity of nitrate reduction to nitrite. Nitrate reduction rates in the O2 sensitivity assay carried out for the Namibian OMZ waters, decreased with increasing O2 concentrations. The incubation experiments at St. 206 revealed a stronger negative response to elevated O2 levels than those AZD1152 performed at St. 252. Activity at St. 206 was reduced to,30% of the anoxic control in the highest O2 treatment, whereas a doubling of the O2 concentration led to a decrease in NO32 reduction rates to,60% of the control experiment at St. 252. In the Peruvian OMZ, production of 15NO22 from 15NO32 was nev