Diatom Species of the Genus Rhizosoleniafrom Southern Ocean Sediments: Distribution and Taxonomic Notes

The classification of diatom taxa in the genus Rhizosolenia Brightwell in Southern Ocean waters and sediments remains unclear. Our review demonstrates that considerable intertaxa confusion has occurred and continues to occur as a result of misapplied taxonomy. In general, valves of Rhizosolenia taxa within total diatom sediment assemblages are a minor, though common component, with only a few taxa sufficiently abundant for statistical analysis. We have endeavoured to clarify through literature and data set assessment the currently accepted taxon concepts in terms of otaria morphology, this being one of the most useful features available for the discrimination of fossil rhizosolenioids in Southern Ocean sediments. Otaria morphology allows separation of incomplete valves of preserved Rhizosolenia specimens to be determined from descriptions related to the complete cell. The identification of R. styliformis Brightwell, R. antennata f. semispina Sundström, and R. polydactyla Castracane f. polydactyla are addressed here in detail. The presence of Rhizosolenia species A, a taxon with similarities to the otaria-lacking, original descriptions of R. hebetata f. semispina (Hensen) Gran, is described and illustrated. Taxonomic notes, light microscopic representation and distributional data for several other Antarctic Rhizosolenia taxa are provided. The documentation of down-core occurrences of fossil rhizosolenioids in published material is poor. Our core material indicates relative abundance increases for Rhizosolenia species A and R. antennata f. semispina in glacial time sequences, which may constitute a valuable biostratigraphic tool for palaeoenvironmental reconstructions. Comments on winter resting stages are provided.