Glaciogenic iron transport pathways to the Kerguelen offshore phytoplankton bloom

Abstract. In contrast to the average low biological productivity across most of the Southern Ocean, the Kerguelen region is one of the few subantarctic regions to host massive phytoplankton blooms, extending hundreds of kilometers offshore. These blooms play a crucial role in the Southern Ocean carbon cycle and support a diverse ecosystem of patrimonial and commercial significance. The Kerguelen blooms are associated with a subsurface iron source that supplies surface waters both on the Plateau and offshore. The mechanisms of iron enrichment have only been partially elucidated. The resuspension of iron-enriched sediments over the Plateau, transported offshore by the Antarctic Circumpolar Current, is one mechanism that has been studied in the past years. However, the Kerguelen Islands host a glacier system, and two of the outlet glaciers of Kerguelen’s Cook Ice Cap are likely to provide 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 open ocean, and how much of the offshore bloom extension can be reached by glaciogenic iron is not known. Using in situ and satellite data, including observations from the recent SWOT satellite mission, we reconstruct the horizontal advection of iron and show that glaciogenic iron supply reaches up to one third of the spatial extent of the offshore bloom onset. These findings have significant implications in the context of ongoing ice cap mass loss and glacier retreat observed on Kerguelen and other Southern Ocean islands under climate change.