Nitrogen fixation in the Southern Ocean: a case of study of the Fe-fertilized Kerguelen region (KEOPS II cruise)

Abstract. N₂ fixation rates were measured during the KEOPS2 cruise in the HNLC area of Southern Ocean and in naturally iron-fertilized waters (Kerguelen Island 49.25° S, 69.58° E) using the ¹⁵N isotopic technique. We detected N₂ fixation within the mixed layer at all stations, from the surface to 140 m depth. The data shows high variability with rates ranging between 0.42 and 20.11 nmol N L⁻¹ d⁻¹. The highest rates were concentrated in the euphotic layer and maximum values were obtained north of polar front (station F-L), which coincide with a positive N^* ([NO₃]–16[PO₄]), high chlorophyll concentration and dissolved iron. N₂ fixation rates were also obtained in stations with moderate (A3-2; E-4W) and also low (R-2) iron levels as well as Chl a, suggesting that beside the microbial biomass, its composition/structure is a driving factor controlling N₂ fixation activities. Molecular analysis showed a diazotrophic community dominated by heterotrophic bacterioplankton. Size fractioned experiments indicated that most of N₂ fixating activities came from <5 μm community and it was on line with molecular analysis revealing a low diversity diazotrophic community dominated by heterotrophic bacterioplankton. This study shows for first time N₂ fixation is occurring in the Southern Ocean, at rates exceeding previous reports for high latitudes. Our findings suggest an indirect role of dFe in the regulation of N₂ fixation through the enhancement of regenerated primary production and the availability of phytoplankton-derived dissolved organic matter, which in turn may stimulate heterotrophic bacterioplankton.