Temporal and spatial variability of phytoplankton fluxes in the Australian and New Zealand Sectors of the Southern Ocean

Phytoplankton are key to global carbon cycling, and critical to understanding a changing climate. Phytoplankton such as diatoms remove CO2 from the atmosphere via photosynthesis, of which one fifth is exported to the deep ocean in a process termed the “Biological Pump”. In opposition to the Biological Pump, the “Carbonate Counter-Pump” releases CO2, driven by calcifying phytoplankton such as coccolithophores. Thus carbon export depends upon phytoplankton community composition, quantified with sediment traps, which preserve a time series of sinking particles. Sediment trap deployments are patchy in the Subantarctic and Subtropics and little work has been done quantifying the phytoplankton. This thesis quantified assemblages and flux of diatoms and coccolithophores from Australian and New Zealand deployments, were not previously well characterized, and discusses their role in export. Subantarctic Australian traps captured among the highest coccolith fluxes of the southern hemisphere, while diatoms were the main silica-exporters. Species-level phytoplankton seasonal ecological succession was also reported for the first time in the Australian region. Scanning Electron Microscopy culminated in a taxonomic study describing the poorly-known diatom genus Shionodiscus, improving our understanding of key Australian taxa. In Subantarctic New Zealand, a 48-day “pulse” bloom of Pseudo-nitzschia diatoms comprised 98%of annual diatom flux. New Zealand Subtropical traps exhibited strong coastal and benthic phytoplankton input, providing evidence for significant particle advection as a result of local oceanography, the Wairarapa Eddy system. Finally, diatom and coccolith fluxes from 46 sediment trap deployments were mapped from the Subtropics to Antarctica, revealing a broad trend of increasing diatom flux from 30° S to the coast of Antarctica, which will potentially inform future trapping efforts. Records of phytoplankton seasonality and abundance are key to understanding the physical and chemical drivers of regional differences in the Biological Pump, and how carbon cycling may experience regional change in the future, under future climate change. The work undertaken for this thesis provides a valuable record against which future studies may compare, and makes a compelling argument for the continuation of sediment trap studies.