Retention of dissolved iron and Fe-II in an iron induced Southern Ocean phytoplankton bloom

During the 13 day Southern Ocean Iron RE-lease Experiment (SOIREE), dissolv ed iron concentrations decreased rapidly following each of three iron-enric hments, but remained high (> 1 nM, up to 80% as Fe-II) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (sp reading of iron-fertilized waters) and particle scavenging, The latter may only be explained by a joint production-maintenance mechanism; photoreducti on is the only candidate process able to produce sufficiently high Fe-II, b ut as such levels persisted overnight (8 hr dark period) - ten times the ha lf-life for this species - a maintenance mechanism (complexation of Fe-II) is required, and is supported by evidence of increased ligand concentration s on day 12. The source of these ligands and their affinity for Fe-II is no t known. This retention of iron probably permitted the longevity of this bl oom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.