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Inhibition kinetics of nitrate uptake by ammonium in size-fractionated oceanic phytoplankton communities: implications for new production and f-ratio estimates

Ammonium inhibition of nitrate uptake by natural phytoplankton communities was investigated in two size fractions ( 2 μm) in the northeast basin of the Atlantic Ocean. Kinetics of the inhibition process were assessed in surface waters by measuring nitrate uptake rates over a range of ammonium concentrations with 15 N tracer techniques. Extremely low levels of ammonium were shown to significantly inhibit nitrate uptake in both small and large fractions at all seven sampling stations. The half-inhibition constant was significantly lower and the maximum inhibition was generally higher in the 2 μm size fractions (K i = 132 + 72 nmol L -1 , I max = 0.67 ± 0.12), demonstrating that the ammonium inhibition was more pronounced in the small size class. Based on the surface ammonium concentrations of the study area, inhibition kinetics parameters predict a mean reduction of nitrate uptake of 27.8% (±12.6) for 2 μm fraction which leads to a mean reduction of 20.8% (± 11.9) for the total community. The importance of the inhibition size specificity in regulating new production and the f-ratio is discussed.