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.
Evidence of high N<sub>2</sub> fixation rates in the temperate northeast Atlantic