AMMONIUM REGENERATION IN THE SCOTIA-WEDDELL CONFLUENCE AREA DURING SPRING 1988

The presence of large ammonium stocks and the related enhanced quantities of microheterotrophs in the Scotia-Weddell Confluence area indicate the importance of ammonium remineralization processes there. Maximal ammonium concentrations in the marginal ice zone amounted to more than 2-mu-mol N l-1 and remineralization rates in this zone were consistently higher than those measured in the adjacent open ocean and permanently ice-covered zones. At one station, characterized by a large kill swarm, the remineralization rate was 18 nmol N l-1 h-1; rates in the marginal ice zone generally ranged between 5 and 9 nmol N l-1 h-1. Protozoan activity was of predominant importance for the remineralization process; only after the krill event was a very high bacterial activity observed. The presence of krill does not contribute in a direct way to the ammonium pool, but probably initiates the bacterial breakdown activity in algal debris and faecal pellets. The increased remineralization leads to enhanced availability of ammonium in the water and to decreased f-ratios for phytoplanktonic nitrate assimilation. Nevertheless, nitrate uptake represented at least 30% of inorganic nitrogen assimilation, significantly higher than values measured for oligotrophic systems.