MEPS 187:295-300 (1999)  -  doi:10.3354/meps187295

ABSTRACT: Nitrate and ammonium measurements at low (4 to 650 nmol N dm^-3) nanomolar concentrations have been combined with ¹⁵N-nitrogen tracer experiments to determine uptake rates in oligotrophic regions of the NE Atlantic during the summer of 1996 between 50° and 36° N along the 20° 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 ¹⁵N 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 ρ_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 δ-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.

KEY WORDS: ¹⁵N uptake · Oligotrophic · Nanomolar nitrate/ammonium · δ-ratio