Nitrogenous nutrient uptake by phytoplankton and ammonium regeneration by microbial assemblage in Lake Biwa

In situ rates of nitrate, ammoniwn and urea uptake by the phytoplanktonassemblage, and the regeneration rate of ammonium by the microbialassemblage, in Lake Biwa were measured using the nitrogen 15tracer method from 1985 to 1987. The rate of total nitrogen(sum of ammonium, nitrate and urea) uptake was in the rangeof 62–594 ng N^–1 r^–1 h^–1. The percentagecontribution of ammonium uptake was 41–92%, that of urea4–58% and that of nitrate <1–28% of total uptake.The annual mean new production which was supported by nitrateuptake was 18% of the total production in 1986. The phytoplanktonassemblage in Lake Biwa preferentially utilized regeneratednitrogen, such as ammonium and urea, whose concentration wasmuch lower than that of nitrate throughout the observation penodwithout in summer. The in situ nitrogen uptake rate was almostsufficient to meet the nitrogen requirement of the phytoplanktonassemblage, except in midsummer when the nitrate concentrationwas below the detection limit of 0.3 µg N r^–1. Inthe trophogemc layer, the rate of ammonium regeneration was66–272 ng N 1^–1 h^–1 Although the ambient ammoniumconcentration in the trophogenic layer was maintained at aroundthe half-saturation constant for ammonium uptake kinetics, theammomum uptake rates were always highly correlated with ammoniumregeneration rates. From the size fractionation experimentsand estimates from the literature, it was suggested that themicrobial assemblage <1 µm may have been the most importantagent responsible for the ammonium regeneration processes inthe trophogenic layer.