Abstract
Measurements of ammonium and nitrate uptakes by natural phytoplankton assemblages from Jiaozhou Bay at various combinations of ammonium and nitrate concentrations with15N trace techniques showed that uptake rate of either nitrogen was influenced by the presence of the other but that the influence of ammonium on nitrate uptake was much greater than that of nitrate on ammonium uptake. The influence mechanism of ammonium on nitrate uptake manifested as competition at lower concentrations and as inhibition at higher concentrations (ammonium concentration >0.6 μmol/L), but no total inhibition appeared within the concentration range of the experiments (0–10 μmol/L). The influence of nitrate on ammonium uptake seems to be a result of competition for uptake sites on the cell surface. In view of the in situ nutrient concentration in the given marine environment, it is believed that both nitrogen sources are utilized by phytoplankton. Nitrate uptake in the presence of ammonium and ammonium uptake in the presence of nitrate are expressed by a non-linearly and a linearly calibrated Michaelis-Menten equation respectively. In addition, a non-linearf-ratio model is given which may be useful in practical studies on new production in Jiaozhou Bay.
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