The bioavailability of iron from different sources to phytoplankton, driving substantial carbon dioxide uptake of the large blooms downstream of South Georgia Island, remains unknown. Although geochemical characterization suggests that iron from glacial meltwater and groundwater is bioavailable, phytoplankton iron uptake measurements are lacking. In this study, additional to assessing iron chemical speciation and weathering processes, iron-55 uptake by a natural phytoplankton community was quantified in seawater sampled from low and high chlorophyll waters around South Georgia, to which iron from nearshore sources (glacial meltwater and groundwater) was added. Iron bioavailability depended on the chemistry of the fertilized seawater and the chemical composition of the source itself. Aggregation of dissolved organic matter in high chlorophyll water scavenged dissolved iron, making it unavailable to phytoplankton. In low chlorophyll water, as opposed to iron from groundwater, iron from glacial meltwater was bioavailable to phytoplankton and would increase carbon dioxide fixation by 80-100%.