Abstract

Seasonal progression of dissolved iron (DFe) concentrations in the upper water column were examined during four occupations in the Atlantic sector of the Southern Ocean. DFe inventories from euphotic and aphotic reservoirs decreased progressively from July to February, while dissolved inorganic nitrogen decreased from July to January with no significant change between January and February. Results suggest that between July and January, DFe loss from both euphotic and aphotic reservoirs was predominantly in support of phytoplankton growth (iron‐to‐carbon uptake ratio of 16 ± 3 μmol/mol), highlighting the importance of the “winter DFe reservoir” for biological uptake. During January to February, excess loss of DFe relative to dissolved inorganic nitrogen (iron‐to‐carbon uptake ratio of 44 ± 8 μmol/mol and aphotic DFe loss rate of 0.34 ± 0.06 μmol·m⁻²·day⁻¹) suggests that scavenging is the dominant removal mechanism of DFe from the aphotic, while continued production is likely supported by recycled nutrients.