Retention of dissolved iron and Fe^II in an iron induced Soutern Ocean phytoplankton bloom

During the 13 day Southern Ocean Iron RElease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron-enrichments, 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 (spreading of iron-fertilized waters) and particle scavenging. The latter may only be explained by a joint production-maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high Fe^II, but as such levels persisted overnight (8 hr dark period) - ten times the half-life for this species - a maintenance mechanism (complexation of Fe^II) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for Fe^II is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.