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

Abstract

We used 15N-labeled substrates to measure dinitrogen (N2) fixation, nitrate (NO3-) and ammonium (NH4+) uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system (Cape Ghir, ∼31°N) and an open ocean grid (bounded between 25°–42°N and 20°W) in the Canary Current region during the summer of 2009. New production (Pnew= NO3- uptake+N2 fixation+DON released from NO3- uptake NO3- regeneration) was higher in the upwelling than in the open ocean zone (0.126 and 0.014 µmol 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 µmol 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 N2 fixation. A previous study indicated that our open ocean grid zone had an excess nitrogen production rate of 40±22×1010 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±19×1010 mol N yr−1. The average volumetric rate of N2 fixation for this zone was only 1.3×10−3 nmol N L−1 d−1, indicating that its influence in Pnew and nitrogen excess production in this part of the Atlantic is negligible.