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The upwelling system off central Chile (36.5° S) is seasonally\udsubjected to oxygen (O2)-deficient waters, with a strong vertical\udgradient in O2 (from oxic to anoxic conditions) that spans a few metres\ud(30–50 m interval) over the shelf. This condition inhibits and/or\udstimulates processes involved in nitrogen (N) removal (e.g. anammox,\uddenitrification, and nitrification). During austral spring (September 2013)\udand summer (January 2014), the main pathways involved in N loss and its\udspeciation, in the form of N2 and/or N2O, were studied using\ud15N-tracer incubations, inhibitor assays, and the natural abundance of\udnitrate isotopes along with hydrographic information. Incubations were\uddeveloped using water retrieved from the oxycline (25 m depth) and bottom\udwaters (85 m depth) over the continental shelf off Concepción, Chile.\udResults of 15N-labelled incubations revealed higher N removal activity\udduring the austral summer, with denitrification as the dominant N2-producing pathway, which occurred together with anammox at all times.\udInterestingly, in both spring and summer maximum potential N removal rates\udwere observed in the oxycline, where a greater availability of oxygen was\udobserved (maximum O2 fluctuation between 270 and\ud40 µmol L−1) relative to the hypoxic bottom waters\ud( <  20 µmol O2 L−1). Different pathways were\udresponsible for N2O produced in the oxycline and bottom waters, with\udammonium oxidation and dissimilatory nitrite reduction, respectively, as the\udmain source processes. Ammonium produced by dissimilatory nitrite reduction\udto ammonium (DNiRA) could sustain both anammox and nitrification rates,\udincluding the ammonium utilized for N2O production. The temporal and\udvertical variability of δ15N-NO3− confirms that multiple\udN-cycling processes are modulating the isotopic nitrate composition over the\udshelf off central Chile during spring and summer. N removal processes in this\udcoastal system appear to be related to the availability and distribution of\udoxygen and particles, which are a source of organic matter and the fuel for\udthe production of other electron donors (i.e. ammonium) and acceptors (i.e.\udnitrate and nitrite) after its remineralization. These results highlight the\udlinks between several pathways involved in N loss. They also establish that\uddifferent mechanisms supported by alternative N substrates are responsible\udfor substantial accumulation of N2O, which are frequently observed as hotspots\udin the oxycline and bottom waters. Considering the extreme variation in\udoxygen observed in several coastal upwelling systems, these findings could\udhelp to understand the ecological and biogeochemical implications due to global\udwarming where intensification and/or expansion of the oceanic OMZs is\udprojected

biogeosciences

Vertical segregation among pathways mediating nitrogen loss (N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O production) across the oxygen gradient in a coastal upwelling ecosystem