NITROGENOUS NUTRIENT-UPTAKE BY PHYTOPLANKTON AND AMMONIUM REGENERATION BY MICROBIAL ASSEMBLAGE IN LAKE BIWA

In situ rates of nitrate, ammonium and urea uptake by the phytoplankto n assemblage, and the regeneration rate of ammonium by the microbial a ssemblage, in Lake Biwa were measured using the nitrogen 15 tracer met hod from 1985 to 1987. The rate of total nitrogen (sum of ammonium, ni trate and urea) uptake was in the range of 62-594 ng N l(-1) h(-1). Th e percentage contribution of ammonium uptake was 41-92%, that of urea 4-58% and that of nitrate <1-28% of total uptake. The annual mean new production which was supported by nitrate uptake was similar to 18% of the total production in 1986. The phytoplankton assemblage in Lake Bi wa preferentially utilized regenerated nitrogen, such as ammonium and urea, whose concentration was much lower than that of nitrate througho ut the observation period without in summer. The in situ nitrogen upta ke rate was almost sufficient to meet the nitrogen requirement of the phytoplankton assemblage, except in midsummer when the nitrate concent ration was below the detection limit of 0.3 mu g N l(-1). In the troph ogenic layer, the rate of ammonium regeneration was 66-272 ng N l(-1) h(-1). Although the ambient ammonium concentration in the trophogenic layer was maintained at around the half-saturation constant for ammoni um uptake kinetics, the ammonium uptake rates were always highly corre lated with ammonium regeneration rates. From the size fractionation ex periments and estimates from the literature, it was suggested that the microbial assemblage <1 mu m may have been the most important agent r esponsible for the ammonium regeneration processes in the trophogenic layer.