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Some aspects of the life cycle of the Southern Ocean diatom Fragilariopsis kerguelensishave been investigated previously, but many of its details have not been surveyed in nature. We investigated material from a two-year sediment trap time series by high-throughput imaging and image analysis, looking for morphometric signals of life cycle stages. Valve length distributions appeared close to unimodal but positively (right-) skewed. Size cohorts resulting from synchronized sexual reproduction events were not clearly distinguishable. Nevertheless, based on changes in valve length distributions, we found three general seasonal phases. These corresponded to periods of proliferation (with higher proportions of smaller cells during late spring/early summer), cessation of growth (relative loss of smaller cells during late summer/early autumn), and overwintering (little change in size distributions, with an increased proportion of large cells). We discuss possible causes of these signals, and their relevance to growth, sexual activity and adaption to environmental conditions, such as grazing pressures and the need for an overwintering strategy.

Diatoms are siliceous phytoplankton that play a major role in global carbon fixation, as well as forming the basis of most marine food webs. In the Southern Ocean, diatoms are responsible for significant volumes of carbon export and sequestration in the deep sea. Diatoms are unicellular and microscopic, thus diatom taxonomy has naturally progressed as imaging and molecular technologies have developed. Despite recent advancements, some aspects of diatom taxonomy remain unclear. The genus Shionodiscuswas separated from Thalassiosirain 2006, and possesses, as a group, internal extensions of the marginal strutted processes as well as a labiate process on the valve face, usually distant from the margin. The features distinguishing Shionodiscusfrom Thalassiosira, and Shionodiscusspp. from each other, are difficult to examine under light microscopy, and most taxonomists will group similar Shionodiscusspecies together when encountered, rather than discriminating species. As a result, the full extent of Shionodiscusdiversity is rarely examined in individual studies. In this study, sediment trap material captured in the Australian sector of the Subantarctic Zone was examined using both light and scanning electron microscopy, and compared to known measurements on Shionodiscusspecies and varieties. This study had two aims; to catalogue for the first time the diversity of Shionodiscusspp. within the Australian sector, and to create a formalized set of criteria for grouping Shionodiscusspecies, when only light microscopy analysis is possible. By documenting Shionodiscusdiversity in the Australian sector and establishing a standard identification protocol, researchers will be better able to determine the ecological significance of the genus in diatom assemblages.