Obvious differences between the uptake kinetics of the two principal nitrogen sources were observed in all seasons. For total assemblages, potential maximum uptake rates of ammonium were always higher than those of nitrate, and half uptake saturation constants of nitrate were not remarkably lower than those of ammonium, indicating the uptake preference of microplankton for ammonnium to nitrate. Furthermore, the uptake of nitrate was influenced much by environmental factors (such as temperature, light etc.). In winter, variations of nitrate uptake with increasing nitrate concentration could not fit the Michaelis-Menten equation.

Significant differences of nitrogen uptake among different size fractions were also found. First, nitrate was mainly uptaken by netplankton, and ammonium was mainly uptaken by nanoplankton and picoplankton. Secondly, the light dependences for both nitrate and ammonium uptake of netplankton and nanoplankton were much stronger than that of picoplankton. These findings suggested that new production is contributed mainly by netplankton and regenerated production isicontributed mainly by nanoplankton, especially picoplankton.