Assimilation and regeneration of inorganic nitrogen in a coastal upwelling system: ammonium and nitrate utilization

The main processes involved in nitrogen cycling (as NO3- and NH4+ assimilation and their regeneration) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36 degrees S). The study area shows seasonal surface fertilization and the development of subsurface hypoxia in austral spring and summer. The rates of NO3- uptake during active upwelling were 5 times higher than in non-upwelling seasons. Uptake of NH4+ was almost half that of NO3- uptake rates during upwelling periods, and similar to NO3- uptake rates in the absence of upwelling. Nitrification experiments showed higher rates during active, compared to non-active, upwelling seasons. NH4+ oxidation was coupled with NO2- oxidation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO3- regeneration via nitrification resulted from higher activity of NO2- oxidation compared to NH4+ oxidation. On the other hand, archaeal NH4+ oxidation had the potential for processing a large fraction of NH4+ and could therefore co-occur with bacterial NO2- oxidation. NH4+ utilization in this coastal upwelling is thus in the same range as NO3- assimilation. NH4+ oxidation is affected by oxygen concentration in the water column, leading to occasional decoupling of NH4+ and NO2- oxidation, particularly at the oxycline. This study gives preliminary evidence of the importance of archaeal-bacterial interactions in the nitrification process and highlights the role of ammonium in fueling annual primary production in coastal upwelling systems.