Contribution of nitrate to the uptake of nitrogen by phytoplankton in an ocean margin environment

Rates of nitrate and ammonium uptake by phytoplankton were measured from July 1990 to March 1995 in the surface waters at several stations located along the continental margin of the NE Atlantic Ocean. Total inorganic nitrogen assimilation ranged from 2.3 to 95 nM h(-1) and exhibited two maxima during the spring bloom and in fall at the beginning of the vertical mixing of the water column. Seasonal and spatial changes in the nitrogen uptake regime (f-ratios) were estimated (1) by correcting ammonium uptake rates with an isotope dilution model, and (2) by evaluating the inhibition of nitrate uptake by ammonium, using a variation of the Michaelis-Menten equation. Overall, nitrate uptake rates paralleled carbon fixation rates, and f-ratios followed the well-known function of nitrate. During spring, new production, sensu Dugdale & Goering (1967), accounted for 46 to 85% of the total inorganic nitrogen production. It can gain in importance through vertical mixing in fall (0.29 < f < 0.82), after a period of predominant regenerated production in summer (0.07 < f < 0.41). Although new production appeared to be quantitatively important on average (mean f = 0.53), kinetic data suggested that ammonium was utilised preferentially throughout the full spectrum of nitrogen concentrations observed during this study. Moreover, the inhibition of nitrate uptake by ambient levels of ammonium was estimated to range from 8 to 50%. Therefore, it is suggested that the supply of regenerated nitrogenous nutrients, combined with feedback mechanisms (preference and inhibition), triggers a switch-over from predominantly new production towards regenerated production, even before the complete exhaustion of nitrate in the surface water. Overall, these results indicate a leading role for ammonium in regulating the removal of nitrate in this margin ecosystem.