THE LEADING ROLE OF AMMONIUM IN THE NITROGEN UPTAKE REGIME OF SOUTHERN-OCEAN MARGINAL ICE ZONES

The nitrogen signature of marginal ice zones in the Southern Ocean oft en departs from the ''classical'' Antarctic nutrient profiles with hig h nitrate and low ammonium values in the surface layer. Weddell Sea ma rginal ice zones e.g. show enhanced nitrate depletions, amounting to s imilar to 500 mmol N m(-2). Additionally, ammonium stocks in the upper layer can be up to 7% of the inorganic nitrogen pool. The correspondi ng nitrogen uptake regime suggests elevated nitrate assimilation durin g the early phase of the productive season and significantly reduced n itrate assimilation at a later stage. Absolute as well as specific nit rate uptake rates decrease by an order of magnitude when ammonium stoc ks exceed 1.7% of the total inorganic nitrogen. The variability in nit rogen utilisation reflects physiological changes in the phytoplankton assemblage. In response to increased ammonium stocks phytoplankton sho w a reduction in their capacity to take up nitrate and following an in itial diatom bloom non-siliceous phytoplankton become predominant in a regenerated production regime.