We quantified the removal of fixed nitrogen as N2 production by anammox and N2 and N2O production by denitrification over a distance of 1900 km along the coast of Chile and Peru, using short-term incubations with 15N-labeled substrates. The eastern tropical South Pacific (ETSP) holds an oxygen minimum zone (OMZ) characterized by an anoxic, nitrate- and nitrite-rich layer of ~200 m thickness downwards from 30 – 90 m water depth. Oxygen minimum zones account for less than 0.1% of the ocean volume but play a disproportionally large role in biogeochemical cycling and account for 20 – 40% of the oceanic fixed nitrogen removal. We applied the novel STOX sensor for oxygen with detection limits around 10 nM and found that oxygen concentrations were below detection the core of the OMZ and that removal of fixed nitrogen almost only occurred when oxygen was below detection.
The removal of fixed nitrogen has traditionally been assumed to be due to denitrification but with the discovery of anammox and the recent implementation of 15N-techniques it appears that anammox is responsible for almost all of the N2 production. However, stoichiometric calculations indicate that this is not possible and that denitrification is needed for the mineralization of organic matter and production of NH4+ for anammox. Our data from frequent sampling along a 1900 km cruise track parallel to the coast of Chile and Peru show that denitrification does indeed occur, but less frequent and at higher rates than anammox. Denitrification was on average responsible for 70% of the N2 production and this high contribution was mainly driven by a few measurements of very high denitrification rates, and the patchy distribution likely explains that the process was missed in earlier studies, which covered fewer locations