Inorganic nitrogen metabolism by Antarctic phytoplankton with special reference to ammonium cycling

Nitrogen metabolism of planktonic populations in coastal waters of the southern Scotia Sea was studied in February-March 1981 aboard R/V M e l d l e . In spite of the high concentration of nitrate present in these waters (21 to 29 FM), up to 93 % of phytoplankton nitrogen was assimilated in the form of ammonium, with the overall mean being 78 %. Time course experiments showed that uptake of nitrate and ammonium during nighttime amounted to approximately 15 and 50 %, respectively, of daytime values. This light dependency of nitrate, and to a lesser extent ammonium, is reflected in the depth profiles of nitrate and ammonium assimilation. Over the shallow waters of the Scotia Arc close to Elephant Island, most of the ammonium uptake was assoc~ated wlth nanoplankton (< 20 ,pm size). Estimating the flux of ammonium through the food web from the biomass and metabolic activities of bacteria, phytoplankton and zooplankton suggests that heterotrophic microbial organisms are responsible for most of the ammonium which is regenerated and assirmlated by phytoplankton. Data from isotope dilution experiments confirmed that micrograzers do account for most of the regenerated ammonium. The high concentrahons of ammonium in these coastal waters (average of 2.1 FM) are discussed relative to seasonal events and to zooplankton activity.