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 fromJuly 1990 to March 1995 in the surface waters at several stations locatedalong the continental margin of the NE Atlantic Ocean. Total inorganicnitrogen assimilation ranged from 2.3 to 95 nM h−1 andexhibited two maxima during the spring bloom and in fall at the beginning ofthe vertical mixing of the water column. Seasonal and spatial changes in thenitrogen uptake regime (f-ratios) were estimated (1) by correcting ammoniumuptake rates with an isotope dilution model, and (2) by evaluating theinhibition of nitrate uptake by ammonium, using a variation of theMichaelis-Menten equation. Overall, nitrate uptake rates paralleled carbonfixation rates, and f-ratios followed the well-known function of nitrate.During spring, new production, sensu Dugdale & Goering (1967), accountedfor 46 to 85% of the total inorganic nitrogen production. It can gainin importance through vertical mixing in fall (0.29 < f < 0.82), aftera period of predominant regenerated production in summer (0.07 < f <0.41). Although new production appeared to be quantitatively important onaverage (mean f = 0.53), kinetic data suggested that ammonium wasutilised preferentially throughout the full spectrum of nitrogenconcentrations observed during this study. Moreover, the inhibition ofnitrate uptake by ambient level of ammonium was estimated to range from 8 to50%. Therefore, it is suggested that the supply of regeneratednitrogenous nutrients, combined with feedback mechanisms (preference andinhibition), triggers a switch-over from predominantly new productiontowards regenerated production, even before the complete exhaustion ofnitrate in the surface water. Overall, these results indicate a leading rolefor ammonium in regulating the removal of nitrate in this margin ecosystem.