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

Abstract

The main processes involved in nitrogen cycling (as NO3 � and NH4 + assimilation and their re- generation) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36°S). The study area shows seasonal surface fertil- ization 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 peri- ods, 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 � oxi- dation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO3 � regener- ation 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 up welling is thus in the same range as NO3 � assimilation. NH4 + oxidation is af- fected 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.