Abstract
Abstract. Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay in May 2014. Substantial N2 fixation activity was observed at 8 of the 10 stations sampled, ranging overall from 81 to 384 µmol N m−2 d−1 (0.7 to 8.2 nmol N L−1 d−1), with two sites close to the Iberian Margin situated between 38.8 and 40.7∘ N yielding rates reaching up to 1355 and 1533 µmol N m−2 d−1. Primary production was relatively lower along the Iberian Margin, with rates ranging from 33 to 59 mmol C m−2 d−1, while it increased towards the northwest away from the peninsula, reaching as high as 135 mmol C m−2 d−1. In agreement with the area-averaged Chl a satellite data contemporaneous with our study period, our results revealed that post-bloom conditions prevailed at most sites, while at the northwesternmost station the bloom was still ongoing. When converted to carbon uptake using Redfield stoichiometry, N2 fixation could support 1 % to 3 % of daily PP in the euphotic layer at most sites, except at the two most active sites where this contribution to daily PP could reach up to 25 %. At the two sites where N2 fixation activity was the highest, the prymnesiophyte–symbiont Candidatus Atelocyanobacterium thalassa (UCYN-A) dominated the nifH sequence pool, while the remaining recovered sequences belonged to non-cyanobacterial phylotypes. At all the other sites, however, the recovered nifH sequences were exclusively assigned phylogenetically to non-cyanobacterial phylotypes. The intense N2 fixation activities recorded at the time of our study were likely promoted by the availability of phytoplankton-derived organic matter produced during the spring bloom, as evidenced by the significant surface particulate organic carbon concentrations. Also, the presence of excess phosphorus signature in surface waters seemed to contribute to sustaining N2 fixation, particularly at the sites with extreme activities. These results provide a mechanistic understanding of the unexpectedly high N2 fixation in productive waters of the temperate North Atlantic and highlight the importance of N2 fixation for future assessment of the global N inventory.
Highlights
Dinitrogen (N2) fixation is the major pathway of nitrogen (N) input to the global ocean and thereby contributes to sustaining oceanic primary productivity (Falkowski, 1997)
9.3◦ W), which appeared to be located within an anticyclonic mesoscale eddy as evidenced by the downwelling structure detected in the chlorophyll a (Chl a) and NO−3 + NO−2 profiles (Fig. 2a, c) at this location
primary production (PP) was relatively homogenous in the Bay of Biscay and along the Iberian Margin (Bel9, Bel-11, Bel-13 and Geo-1) with average rates ranging from 33 to 43 mmol C m−2 d−1, except for station Bel-7, where it was slightly higher (52 mmol C m−2 d−1; Fig. 3a, b and Table S2), likely due to the presence of an anticyclonic mesoscale structure at this location
Summary
Dinitrogen (N2) fixation is the major pathway of nitrogen (N) input to the global ocean and thereby contributes to sustaining oceanic primary productivity (Falkowski, 1997). Trichodesmium, in particular, was long considered as the most active diazotroph in the global ocean. It has mostly been reported in tropical and subtropical oligotrophic oceanic waters which are thought to represent the optimal environment for its growth and N2fixing activity (Dore et al, 2002; Breitbarth et al, 2007; Montoya et al, 2007; Needoba et al, 2007; Moore et al, 2009; Fernández et al, 2010; Snow et al, 2015). Past estimates of global annual N2 fixation were mainly based on information gathered from tropical and subtropical regions, while higherlatitude areas have been poorly explored for diazotrophic activity (Luo et al, 2012)
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