Evidence of high N 2 fixation rates in productive waters of the 1 temperate Northeast Atlantic 2 3

Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 23 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay (38.8–46.5° N; 8.0–19.7° W) in May 2014 24 close to the end of the spring bloom. We report substantial N2 fixation activities, reaching up to 65 nmol N L -1 d -1 and 25 1533 μmol N m -2 d -1 close to the Iberian Margin between 38.8° N and 40.7° N. Similar figures in the basin have only 26 been reported in the temperate and tropical western North Atlantic waters with coastal, shelf or mesohaline 27 characteristics, as opposed to the mostly open ocean conditions studied here. In agreement with previous studies, the 28 qualitative assessment of nifH gene diversity (encoding the nitrogenase enzyme that fixes N2) suggested a predominance 29 of heterotrophic diazotrophs, and the absence of filamentous cyanobacteria. At the sites where N2 fixation activity was 30 highest sequences affiliated to UCYN-A1, obligate symbiont of eukaryotic prymnesiophyte algae, were recovered. The 31 remaining phylotypes were non-cyanobacterial diazotrophs, known to live in association with suspended particles and 32 zooplankton (i.e., Bacteroidetes, Firmicutes and Proteobacteria). Outside the area of exceptional activity, N2 fixation in 33 the open ocean and at shelf-influenced sites was also relatively high, ranging from 81 to 384 μmol N m -2 d -1 , but was 34 undetectable in the central Bay of Biscay. We propose that the unexpectedly high heterotrophic N2 fixation activity 35 recorded at the time of our study was sustained by the availability of phytoplankton derived organic matter (dissolved 36 and/or particulate) resulting from the ongoing to post spring bloom. We pose that this organic material not only sustained 37 bacterial production, but also provided sufficient nutrients essential for the nitrogenase activity (e.g., phosphorus). 38 Dissolved Fe was supplied through atmospheric dust deposition during the month preceding our study and through 39 advection of surface waters from the subtropical region and the shelf area. Our findings stress the need for a more 40 1 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-220 Manuscript under review for journal Biogeosciences Discussion started: 23 May 2018 c © Author(s) 2018. CC BY 4.0 License.