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Marine Ecology Progress Series

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

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

Andrew Rees*, Malcolm Woodward, Ian Joint

NERC Centre for Coastal and Marine Sciences, Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, United Kingdom

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 ρmax = 2.77 nmol N dm-3 h-1 and KNO3 = 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

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