Constraining iron sources and cycling during seasonal bloom development in the Australian sector of the Southern Ocean

The East Australian Current (EAC) is a major western boundary current that entrains iron from a variety of potential sources including riverine, sediment resuspension, lateral exchange of shelf waters, and atmospheric dust inputs as it flows south along the eastern margin of Australia. In waters southwest of Tasmania, large spring phytoplankton blooms are associated with the mixing of iron-enriched EAC waters and nutrient-rich, iron-depleted (HNLC) Southern Ocean waters. These blooms play an important role in the drawdown of CO₂ in this region. During austral spring 2018, trace element and isotope sampling was undertaken along a transect from subtropical waters into subantarctic waters in the Australian sector of the Southern Ocean to quantify sources of new iron to the EAC and iron fluxes to seasonal blooms in HNLC waters. We will present trace metal concentrations and Fe stable isotope signatures of dissolved and particulate phases to compare Fe biogeochemistry in subtropical EAC waters and in HNLC waters in the subantarctic zone to finger-print various inputs of iron to subtropical waters and examine how they are transformed upon utilization by Fe-limited phytoplankton communities.