Description: Little is known about life cycle details in open ocean diatoms, such as the preparation for overwintering or timing of sexual reproduction. We applied SHERPA, a diatom image analysis software, to the valves of Fragilariopsis kerguelensis (O’Meara) Hust. captured in a Polar Frontal Zone sediment trap (54°S, 141.45°E, 800m), to investigate these events. The time-series analysis revealed four significant phases: 1) Prolific vegetative reproduction phase: The fraction of smaller valves increased significantly during late spring and early summer, representative of ongoing and potentially rapid seasonal vegetative reproduction. 2) Ceasing vegetative reproduction phase: The bias for a smaller sized population notably reversed from mid-summer through to early autumn, and an increase in the minimum valve size occurred in conjunction with the end of the vegetative productive phase observed from sediment trap fluxes. 3) Sexual reproduction phase: Valves in the initial cell size range (≥ 76µm), from which sexual reproduction can be inferred, occurred principally in autumn. 4) Overwintering vegetative phase: During late autumn and through winter, valve size distributions remained nearly symmetrical with low percentages of smaller valves, and a very low vegetative reproduction rate is hypothesized. The distribution shift towards smaller valves from Phase 1 reflects the spring bloom event. We hypothesize that initially in Phase 2 the very strong distribution shift may be resultant of two concurrent factors: a) a cessation of the productive phase due to a change in environmental factors (e.g. light, nutrient availability), and b) grazing selection pressure was enhanced on the population due to the rapid increase in smaller valves. We speculate, from our observations during Phases 3 and 4, that an overwintering strategy is in place for the species. In this phase only large cells maintain sufficient storage capacity to survive a Southern Ocean winter, and could even sustain a source of ready supplies for inoculating the population in the next spring season. Such a “tactic” relieves the limitation of minimum size restrictions impacting on enhanced generation cycles. The results of this time-series size analysis from sediment trap fluxes, provides the first indication of the life cycle and survival strategy for Fragilariopsis kerguelensis.

Description: 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. 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. Furthermore, our results corroborate observations of the exported diatom assemblage from a sediment trap deployed in the iron-fertilized area, whereby 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.