Measurement of nitrate and ammonium uptake at ambient concentrations in oligotrophic waters of the North-East Atlantic Ocean

Nitrate and ammonium measurements at low (4 to 650 nmol N dm(-3)) nanomolar concentrations have been combined with N-15-nitrogen tracer experiments to determine uptake rates in oligotrophic regions of the NE Atlantic during the summer of 1996 between 50 degrees and 36 degrees N along the 20 degrees W meridian. The highest nutrient and chlorophyll concentrations were measured at the north of the transect, where the nitrate concentration was 650 nmol dm(-3); at the southern, oligotrophic stations, nitrate concentration was ca 4 nmol dm(-3) Nitrate uptake has been determined at concentrations of nitrate (ambient plus added N-15 tracer) as low as 7 nmol N dm(-3). Nitrate uptake rate versus concentration along the transect can be described by a hyperbolic curve with parameter values rho(max) = 2.77 nmol N dm(-3) h(-1) and K-NO3 = 20 nmol N dm(-3). Along the transect, there was a decrease in the f-ratio from 0.39 to 0.02 and in the relative preference index for nitrate from 0.7 to 0.1. There was no direct indication of ammonium inhibition of nitrate uptake. The observed range in nitrate concentration of 2 orders of magnitude had the greatest influence over the interaction between ammonium concentration and nitrate uptake. There was an exponential decrease in nitrate uptake rate with increases in the ratio of ammonium:nitrate concentration.