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. Diazotrophic activity and primaryproduction (PP) were investigated along two transects (Belgica BG2014/14 andGEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay inMay 2014. Substantial N2 fixation activity was observed at 8 of the10 stations sampled, ranging overall from 81 to384  µ mol N m −2  d −1 (0.7 to8.2 nmol N L −1  d −1) , with two sites close to the Iberian Marginsituated between 38.8 and 40.7 ∘  N yielding rates reaching up to 1355and 1533  µ mol N m −2  d −1 . Primary production wasrelatively lower along the Iberian Margin, with rates ranging from 33 to59 mmol C m −2  d −1 , while it increased towards the northwestaway from the peninsula, reaching as high as135 mmol C m −2  d −1 . In agreement with the area-averaged Chl  a satellite data contemporaneous with our study period, our results revealedthat post-bloom conditions prevailed at most sites, while at thenorthwesternmost station the bloom was still ongoing. When converted tocarbon uptake using Redfield stoichiometry, N2 fixation couldsupport 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 couldreach up to 25 %. At the two sites where N2 fixation activitywas the highest, the prymnesiophyte–symbiont CandidatusAtelocyanobacterium thalassa (UCYN-A) dominated the nifH sequencepool, while the remaining recovered sequences belonged to non-cyanobacterialphylotypes. At all the other sites, however, the recovered nifHsequences were exclusively assigned phylogenetically to non-cyanobacterialphylotypes. The intense N2 fixation activities recorded at the timeof our study were likely promoted by the availability ofphytoplankton-derived organic matter produced during the spring bloom, asevidenced by the significant surface particulate organic carbonconcentrations. Also, the presence of excess phosphorus signature in surfacewaters seemed to contribute to sustaining N2 fixation, particularlyat the sites with extreme activities. These results provide a mechanisticunderstanding of the unexpectedly high N2 fixation in productivewaters of the temperate North Atlantic and highlight the importance of N2 fixation for future assessment of the global N inventory.

Evidence of high N&lt;sub&gt;2&lt;/sub&gt; fixation rates in the temperate northeast Atlantic

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