Lagrangian reconstruction of a glaciogenic iron delivery to the Kerguelen blooms, Southern Ocean: comparison of SWOT-merged products with conventional altimetry

There is general agreement that iron is the main nutrient limiting primary productivity in the Southern Ocean. In contrast to the average low biological productivity elsewhere in the Southern Ocean, the Kerguelen region is home to massive blooms that extend hundreds of kilometers offshore, and serve as a backbone to rich ecosystems. The blooms have been shown to be sustained by continental iron inputs, in particular by the resuspension of iron-enriched sediments over the plateau, transported eastward by the Antarctic Circumpolar Current. However, iron inputs from glacial erosion and ice melt may be another iron source. In particular, two of the outlet glaciers of Kerguelen's Cook Ice Cap transport iron-enriched lithogenic material downstream to the coastal waters of the Golfe des Baleiniers. Whether the circulation is able to connect the glacier outlets to the pelagic area, and how much of the pelagic bloom can be influenced by glaciogenic iron, are two open questions that we address here. Using in situ and satellite data, we show the persistence, on an interannual basis, of a chlorophyll-enriched plume connected to the Golfe des Baleiniers and driven by a horizontal advection of iron. Using a Lagrangian methodology,  we reconstruct the horizontal advection of iron and show that glaciogenic iron supply influences up to a third of the spatial extent of the open ocean bloom onset. We find that the new high resolution SWOT observations allow a significant reduction in altimetry biases attributable compared to previous products, allowing a better representation of fine scale biogeochemical structures. Our results are particularly relevant in the context of the negative mass balance of ice caps and glacial retreat evidenced both on Kerguelen and other Southern Ocean islands in the context of climate change.