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Hydrobiologia 353: 139–152, 1997.c 1997 Kluwer Academic Publishers. Printed in Belgium.139Contribution of nitrate to the uptake of nitrogen by phytoplankton in anocean margin environmentMarc Elskens1, Willy Baeyens & Leo GoeyensVrije Universiteit Brussel, Laboratory ofAnalytical Chemistry, Pleinlaan 2, B-1050 Brussels, Belgium1Correspondence to M. Elskens, Laboratorium voor Analytische Chemie (ANCH), Vrije Universiteit Brussel,Pleinlaan 2, B-1050 Brussels, Belgium(Tel: 32-2-6293264, Fax: 32-2-6293274, e-mail: melskens@vnet3.vub.ac.be).Received 5 November 1996; in revised form 15 April 1997; accepted 27 May 1997AbstractRates of nitrate and ammonium uptake by phytoplankton were measured from July 1990 to March 1995 in thesurface waters at several stations located along the continental margin of the NE Atlantic Ocean. Total inorganicnitrogen assimilation ranged from 2.3 to 95 nM hat the beginning of the vertical mixing of the water column. Seasonal and spatial changes in the nitrogen uptakeregime ( -ratios) were estimated (1) by correcting ammonium uptake rates with an isotope dilution model, and (2)by evaluating the inhibition ofnitrate uptake by ammonium, using a variation ofthe Michaelis-Menten equation.Overall, nitrate uptake rates paralleled carbon fixation rates, andnitrate. During spring, new production, sensu Dugdale & Goering (1967), accounted for 46 to 85% of the totalinorganic nitrogen production. It can gain in importance through vertical mixing in fall (0 29period of predominant regenerated production in summer (0 07to be quantitatively important on average (mean0 53), kinetic data suggested that ammonium was utilisedpreferentially throughout the full spectrum of nitrogen concentrations observed during this study. Moreover, theinhibition ofnitrate uptake by ambient levels ofammonium was estimated to range from 8 to 50%. Therefore, it issuggested that the supply of regenerated nitrogenous nutrients, combined with feedback mechanisms (preferenceand inhibition), triggers a switch-over from predominantly new production towards regenerated production, evenbefore the complete exhaustion of nitrate in the surface water. Overall, these results indicate a leading role forammonium in regulating the removal ofnitrate in this margin ecosystem.1and exhibited two maxima during the spring bloom and in fall-ratios followed the well-known function of0 82), after a0 41). Although new production appearedIntroductionDuring the past decades, the new production para-digm, introduced by Dugdale & Goering (1967), haslargely governed our understanding of the relationbetween inorganic nitrogen sources (mainly ammoni-umandnitrate), primaryproductionandparticle exporttowards the aphotic layer of marine systems. Basical-ly, a distinction is made according to the differencesin inorganic nitrogen source. New production is asso-ciated with allochtonous nutrients, mainly nitrate anddinitrogen, whereas regenerated production is fuelledwith in situ regenerated compounds such as ammoni-um. According to the definition of Eppley & Peter-son (1979), new production has two components: (i)accumulation ofbiomass in the euphotic layer and (ii)export oforganic matter to the deep. This definition ofnew production is termed net community productionalso (Minas et al., 1986).In addition to dissolved inorganic nitrogen, dis-solved organic nitrogen assumes an important role inthe nitrogen uptake regime ofphytoplankton (Antia etal., 1991; Bronk et al., 1994) and in nitrogen miner-alisation processes occurring in deeper layers as wellas in the upper mixed layer (Toggweiler, 1989; Bronket al., 1994). A better view of the nitrogen flow inthe ocean’s surface, based on a quantitative determi-nation of a maximal number of processes, can addconsiderably to the unravelling of the biogeochemi-cal nitrogen cycle. On the other hand, the distinction