NITROGEN UPTAKE BY THE SUMMER SIZE-FRACTIONATED PHYTOPLANKTON ASSEMBLAGES IN THE WESTLAND, NEW-ZEALAND, UPWELLING SYSTEM

The assimilation of nitrate (NO3-), ammonium( NH4+), and urea by size- fractionated natural assemblages of phytoplankton, as a function of th e individual ambient nitrogen (N) nutrient concentrations, was investi gated off the west coast of South Island, New Zealand, in February 198 9. New production (rho NO3-) contributed by all three size classes was higher inshore than offshore, and accounted for 40-61% and 19-29% of the total N production (Sigma rho N), respectively. The reverse was tr ue for regenerated production [rho NH4+ rho urea], a clear indication of the importance of NH4+ and urea in offshore waters. Picoplankton (< 2 mu m fraction), both inshore and offshore, was the major user of th e regenerated N, and accounted for 43-62% of total regenerated product ion. The assimilation rates of all three size components achieved satu ration at the larger additions of NO3-, NH4+, and urea, and the half-s aturation constants (K-s) of all three N nutrients for the three size classes in summer were higher than the respective ambient N concentrat ions. Low ambient dissolved nitrogenous nutrients (DNN), low dissolved reactive phosphate (DRP), and low V-amb/V-max ratios recorded in summ er off Westland imply that the natural assemblages of phytoplankton, e specially the larger components in inshore waters, were under nutrient stress.