Nitrification and nitric oxide in the oxygen minimum of the eastern tropical North Pacific

Ammonium oxidation rates (V_NH4 ), nitric oxide concentrations [NO], and NO formation rates (V_NO) were measured below the euphotic zone off Baja California. Vertically integrated nitrification rates were 1.1-2.7 mmol N m^-2 d^-1, compared with VERTEX estimates (in a less productive nearby region) of 0.56 mmol m^-2 d^-1 for the loss of sinking organic N between 100 and 2000 m. Integrated ammonium oxidation rates thus may provide valid independent estimates of organic N recycling. NO was detected at picomolar levels in the water column at oxygen concentrations between 0 and 100 μM. Many samples actively generated NO, which required an in situ source, and yielded flux estimates. A crude estimate of the vertically integrated NO flux for the region, 0.19 mmol N m^-2 d^-1, is 13% of the average integrated nitrification flux, suggesting the NO plays a significant role as an intermediate product in the nitrogen cycle. Most NO and NO production were found in the region of active nitrification outside the zone of active denitrification. However, V_NH4 and V_NO are interrelated in a complex way that suggests, but does not prove, a link between the two. NO may arise in part from an intermediate compound that is filterable through 0.2 μm filters, possibly a soluble byproduct of nitrification such as hydroxylamine.