Low contribution of N2 fixation to new production and excess nitrogen in the subtropical northeast Atlantic margin

We used N-15-labeled substrates to measure dinitrogen (N-2) fixation, nitrate (NO3-) and ammonium (NH4+) uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system (Cape Ghir, similar to 31 degrees N) and an open ocean grid (bounded between 25 degrees-42 degrees N and 20 degrees W) in the Canary Current region during the summer of 2009. New production (Pnew=NO3- uptake+N-2 fixation+DON released from NO3- uptake-NO3- regeneration) was higher in the upwelling than in the open ocean zone (0.126 and 0.014 mu mol N L-1 h(-1), respectively), while regenerated production (Preg=NH4+ uptake+DON released from NH4+ uptake+NH4+ regeneration) was similar in both zones (0.157 and 0.133 mu mol N L-1 h(-1), respectively). The resulting f-ratio (Pnew/Pnew+Preg) for the open ocean and upwelling zones was 0.08 and 0.48, respectively. The availability of nitrogen in excess of that expected from Redfield stoichiometry is generally attributed to N-2 fixation. A previous study indicated that our open ocean grid zone had an excess nitrogen production rate of 40 +/- 22 x 10(10) mol N yr(-1). We revisited this budget including new dissolved organic matter and NO3- fluxes through the Strait of Gibraltar and estimated a revised nitrogen excess rate of 22 +/- 19 x 10(10) mol N yr(-1). The average volumetric rate of N-2 fixation for this zone was only 1.3 x 10(-3) nmol N L-1 d(-1), indicating that its influence in Pnew and nitrogen excess production in this part of the Atlantic is negligible. (c) 2013 Elsevier Ltd. All rights reserved.