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

Abstract

N 2 fixation rates were measured during the KEOPS2 cruise in the HNLC area of South-ern Ocean and in naturally iron-fertilized waters (Kerguelen Island 49.25 • S, 69.58 • E) using the 15 N isotopic technique. We detected N 2 fixation within the mixed layer at all stations, from the surface to 140 m depth. The data shows high variability with rates 5 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 * ([NO 3 ]–16[PO 4 ]), high chlorophyll concentration and dissolved iron. N 2 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 micro-10 bial biomass, its composition/structure is a driving factor controlling N 2 fixation activi-ties. Molecular analysis showed a diazotrophic community dominated by heterotrophic bacterioplankton. Size fractioned experiments indicated that most of N 2 fixating activi-ties came from < 5 µm community and it was on line with molecular analysis revealing a low diversity diazotrophic community dominated by heterotrophic bacterioplankton. 15 This study shows for first time N 2 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 N 2 fixation through the enhancement of regenerated primary production and the availability of phytoplankton-derived dissolved organic matter, which in turn may stimulate heterotrophic bacterioplankton.