NITROGEN-METABOLISM IN LAKE KIZAKI, JAPAN .5. THE ROLE OF NITROGEN-FIXATION IN NITROGEN REQUIREMENT OF PHYTOPLANKTON

In situ nitrogen fixation and ammonium and nitrate uptake by phytoplankton in Lake Kizaki, in which the blue green algae, Anabaena spp. was the dominant species in summer, were measured using the nitrogen 15 tracer method in 1983. Nitrogen fixation rate was detectable from July to September and its maximum was reached in August. Nitrogen fixation rates were generally less than 10% of those of ammonium uptakes, though the fixation rates at the deepest part of the trophogenic zone was fairly similar to those of ammonium uptake in August. In this layer, the nitrogen fixation process was an important nitrogen source for phytoplankton. The effect of ammonium and nitrate on nitrogen fixation was investigated. In the presence of ammonium at 1-2 .mu.g atom N .cntdot. l-1, nitrogen fixation was completely suppressed, while nitrate did not affect nitrogen fixation where it was present even in relatively high concentration (above 10 .mu.g atom N .cntdot. l-1). Ammonium was preferentially used by phytoplankton, so that the in situ ammonium uptake rate where concentration was 1-2 .mu.g atom N .cntdot. l-1 was almost sufficient to meet the nitrogen requirement of phytoplankton. In such situation, nitrogen uptake by fixation should not be required. However, the nitrate uptake was often suppressed with ammonium at concentrations less than 0.5 .mu.g atom N .cntdot. l-1, and hence the nitrogen requirement of phytoplankton would be satisfied by nitrogen fixation, but not with nitrate even at high concentration.