In the naturally iron-fertilized surface waters of the northern Kerguelen Plateau region, the early spring diatom community composition and contribution to plankton carbon biomass were investigated and compared with the High Nutrient Low Chlorophyll (HNLC) surrounding waters (October-November 2011, KEOPS 2). The large iron-induced blooms were dominated by small diatom species belonging to the genera Chaetoceros (Hyalochaete) and Thalassiosira, which rapidly responded to the onset of favorable light-conditions in the meander of the Polar Front. In comparison, the iron-limited HNLC area was typically characterized by autotrophic nanoeukaryote-dominated communities and by larger and more heavily silicified diatom species (e.g. Fragilariopsis spp.). Our results support the hypothesis that diatoms are valuable vectors of carbon export to depth in naturally iron-fertilized systems of the Southern Ocean. Comparison with the diatom assemblage composition of a sediment trap deployed in the iron-fertilized area suggests that the dominant Chaetoceros (Hyalochaete) cells were less efficiently exported than the less abundant yet heavily silicified cells of Thalassionema nitzschioides and Fragilariopsis kerguelensis. Our observations emphasize the strong influence of species-specific diatom cell properties combined with trophic interactions on matter export efficiency, and illustrate the tight link between the specific composition of phytoplankton communities and the biogeochemical properties characterizing the study area.
Project: KErguelen Ocean and Plateau compared Study 2 (KEOPS2) (http://www.com.univ-mrs.fr/~queguiner/KEOPS_2.html)Seawater samples were collected from 12 L bottle CTD-rosette equipped with a Seabird SBE 911-plus CTD unit. Samples were taken at four to six depths relative to varying Photosynthetically Active Radiation. Diatom counts (live and empty cells) were made following the Utermöhl method (Hasle 1978). Specific diatom C biomass was assessed following the methodology of Cornet-Barthaux et al. (2007). The species-specific contribution to live cell abundance and to C biomass are then calculated from these results.
Supplement to: Lasbleiz, Marine; Leblanc, Karine; Armand, Leanne K; Christaki, Urania; Georges, Clément; Obernosterer, Ingrid; Quéguiner, Bernard; King, Gary M (2016): Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean. FEMS Microbiology Ecology, 92(11), fiw171
