Effect of seasonal changes in bottom water oxygenation on sediment N oxides and N₂O cycling in the coastal upwelling regime off central Chile (36.5°S)

An intensive and seasonal coastal upwelling process, which attains maximal expression during late austral spring and summer, drives well-known changes in organic matter production and, therefore, in O₂ content in the water column. These variables have a concomitant effect on N sediment processes over the continental shelf off central Chile (36.5°S), which, in turn, can affect the NO₃^-, NO₂^-, and N₂O content in the bottom water. Hydrographic characteristics, benthic NO₃^- and NO₂^- fluxes, and denitrification rates were measured from 1998 to 2001 (with at least seasonal frequency). In order to elucidate how benthic N₂O recycling responds to different O₂ and nutrient levels and how it affects the bottom water N₂O content, net N₂O cycling was measured in December 2001 in sediment slurry incubations under different manipulated dissolved O₂ levels (anoxic: 0 μM; hypoxic: 22.3 μM; oxic: 44.6 μM) and without (natural) and with the addition of NO₃^- and NH₄⁺ (enriched experiments). Dissolved O₂ and NO₃^- contents (and also PO₄^{3 -}) showed clear seasonal patterns according to the oceanographic regime, i.e., from hypoxic waters rich in nutrients during the upwelling season to oxic waters with less nutrient contents during the non-upwelling season. The bottom water, on the other hand, was influenced by benthic organic mineralization, which consumes O₂ as well as other electron acceptor N-species such as NO₃^-. Benthic NO₃^- fluxes (2.62-5.08 mmol m^-2 d^-1) were always directed into the sediments, whereas denitrification rates varied from 0.6 to 2.9 mmol m^-2 d^-1. N₂O was also consumed at rates of 5.53 and 4.56 μmol m^-2 d^-1 under anoxia and hypoxia, but N₂O consumption rates were reduced to almost half under oxic conditions in both natural and a NH₄⁺-enriched experiments. With the NO₃^--enriched experiments, however, N₂O consumption was very high (up to 24.25 μmol m^-2 d^-1) under anoxic and hypoxic conditions, suggesting that high NO₃^- levels induce more N₂O reduction to N₂ by denitrification. N₂O production rates were only measured when oxic conditions were observed in the NO₃^--enriched experiment, suggesting some role of nitrification. Thus, N cycling in the sediments seems to affect the observed NO₃^-, NO^2-, and N₂O content in the bottom water and, therefore, in the entire water column due to vertical advection associated with coastal upwelling.