Contrasting bioavailability of iron in glacial vs. groundwater to Antarctic phytoplankton: Linking iron uptake to geochemistry

Abstract Iron (Fe) sources and their bioavailability to phytoplankton, driving substantial CO₂ uptake of the large blooms downstream of South Georgia Island, remain largely unknown. Although geochemical characterization suggests that Fe from glacial meltwater and groundwater is bioavailable, Fe uptake measurements by phytoplankton are missing. In this study, additional to geochemical processes, ⁵⁵Fe uptake by a natural phytoplankton community was quantified in seawater sampled from low and high chlorophyll waters around South Georgia, to which nearshore sources (glacial meltwater and groundwater) were added. Fe bioavailability, depended on the in-situ chemistry of the fertilized seawater and the chemical characteristics of the Fe source itself. Aggregation of dissolved organic matter in high chlorophyll water scavenged dissolved Fe, making it unavailable to phytoplankton. In low chlorophyll water, Fe from glacial meltwater was bioavailable to phytoplankton, as opposed to Fe from groundwater. As global warming accelerates glacial melt, CO₂ uptake in this region may increase.