Comparative seasonal biogeography of mineralising nannoplankton in the Scotia Sea: Emiliania huxleyi, Fragilariopsis spp. and Tetraparma pelagica

The Southern Ocean is an important biogeochemical region on a global scale, in which mineralising phytoplankton play a role in cycling energy, carbon and nutrients. Mineralising phytoplankton with cells 2–20 μm in diameter (nannoplankton) are poorly enumerated by traditional preservation and microscopy techniques, yet may fulfil an important role in the Southern Ocean. Here we define the spatial and temporal biogeography for these mineralising nannoplankton assessed by scanning electron microscopy in conjunction with an array of biological, physical, and chemical variables during two cruises to the Scotia Sea region of the Southern Ocean. The cruises encompassed two seasons, austral summer (January–February 2008) and austral autumn (March–April 2009). The biogeography of the three most numerous mineralising nannoplankton groups, the coccolithophore Emiliania huxleyi, the smaller (<10 μm) species of the diatom genus Fragilariopsis, and chrysophytes of the genus Tetraparma (mostly Tetraparma pelagica) were found to be related to the boundaries of the major circumpolar fronts. E. huxleyi abundances were relatively high in the northern water masses (maximum of 650 cells ml−1), while T. pelagica abundances were high in the southern water masses (maximum of 1910 cells ml−1). Small Fragilariopsis spp. abundances were also highest in the southern water masses (maximum of 1820 cells ml−1), but this group was present throughout the Scotia Sea. Multivariate statistical analysis found that the most influential environmental variables controlling mineralising nannoplankton biogeography were sea surface temperature and silicate concentration. Estimates of biomass indicated that the Scotia Sea mineralising nannoplankton community formed a substantial part of the total phytoplankton community, particularly south of the Southern Antarctic Circumpolar Current Front (SACCF) during the austral autumn, where mineralising nannoplankton biomass reached 36% of the total phytoplankton biomass. The results that are obtained suggest that traditional microscopic surveys of large Southern Ocean phytoplankton may underestimate total biomass by excluding key mineralising nannoplankton groups. Greater appreciation of the ecological significance of mineralising nannoplankton in the Southern Ocean will improve our understanding of the relationships between environmental parameters, primary production, and the biological carbon pump in this ecosystem.

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