Iron uptake rates of nearshore sources around South Georgia by a natural phytoplankton community (RV Polarstern, PS133.2)

A shipboard experiment was conducted during the RV Polarstern expedition PS133.2 to test the bioavailability of Fe in different nearshore sources. To this end 0.2 µm-filtered natural Antarctic seawater was collected from a low and high chlorophyll region (LCW and HCW, respectively) around South Georgia Island (Fig. 1). To the LCW and HCW, Fe from the following natural sources was added: groundwater (GW) from a subaerial seepage and proglacial meltwater (MW) from King Haakon Bay and proglacial meltwater (MW) and estuarine water (EW) from Cumberland Bay. The addition of Fe (unfiltered water) from these natural sources was compared to the addition of iron chloride (FeCl3). A set of LCW and HCW bottles was left unamended (no addition). After allowing each treatment to equilibrate for 24 h, samples for dFe concentrations and Fe speciation were taken. To quantify the Fe bioavailability of each treatment, radioactive Fe (55FeCl3) was added and left to equilibrate for another 24 h before aliquots of the pre-concentrated Antarctic phytoplankton community (taxonomic composition, Supplementary Fig. 1) were added to the treatments. The LCW and HCW without Fe addition were further split into biotic and abiotic bottles, with only the biotic bottles receiving the phytoplankton concentrate. 55Fe uptake over 24 h by the respective phytoplankton community was then determined in each treatment.