Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub‐Antarctic Zone of the Southern Atlantic Ocean

Seasonal progression of dissolved iron (DFe) concentrations in the upper water column was 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 mu mol/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 mu mol/mol and aphotic DFe loss rate of 0.34 +/- 0.06 mu mol.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. Plain Language Summary Trace metal iron is one of the limiting nutrients for primary productivity in the Southern Ocean; however, the relative importance of seasonal iron supply and sinks remains poorly understood, due to sparse data coverage across the seasonal cycle and lack of high-resolution dissolved iron (DFe) measurements. Here we present four "snapshots" of DFe measurements at a single station in the southeast Southern Atlantic Ocean (one in winter and three in late spring-summer), to address the seasonal evolution of DFe and dissolved inorganic nitrogen (DIN) concentrations within the biologically active sunlit and subsurface reservoirs. We observed a seasonal depletion of DFe inventories from July to February, while DIN inventories decrease from July to January with no concomitant changes between January and February. This suggests that in addition to biological uptake in the sunlit layer, the observed decrease in DFe inventories below this (relative to DIN) is driven by aggregation and incorporation of iron particles into larger "marine snow" sinking particles, while nutrient recycling is driving the observed continuation of primary productivity during late summer. Our results provide insight into seasonal change of DFe availability in different reservoirs where interplay between removal and supply processes are controlling its distributions and bioavailability to support upper surface primary production.