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

Abstract

The main processes involved in nitrogen cycling (as NO 3- and NH 4+ assimilation and their regeneration) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36°S). The study area shows seasonal surface fertilization and the development of subsurface hypoxia in austral spring and summer. The rates of NO 3- uptake during active upwelling were 5 times higher than in non-upwelling seasons. Uptake of NH 4+ was almost half that of NO 3- uptake rates during upwelling periods, and similar to NO 3- uptake rates in the absence of upwelling. Nitrification experiments showed higher rates during active, compared to non-active, upwelling seasons. NH 4+ oxidation was coupled with NO 2- oxidation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO 3- regeneration via nitrification resulted from higher activity of NO 2- oxidation compared to NH 4+ oxidation. On the other hand, archaeal NH 4+ oxidation had the potential for processing a large fraction of NH 4+ and could therefore co-occur with bacterial NO 2- oxidation. NH 4+ utilization in this coastal up welling is thus in the same range as NO 3- assimilation. NH 4+ oxidation is affected by oxygen concentration in the water column, leading to occasional decoupling of NH 4+ and NO 2- 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. © Inter-Research 2012.