Iron availability influences nutrient drawdown in the Heard and McDonald Islands region, Southern Ocean

At the southern part of the northern Kerguelen Plateau (Southern Ocean) is an active volcanic hotspot, hosting volcanically active Heard Island and McDonald Islands (HIMI), the former of which is largely covered by glaciers. While offshore waters are persistently Fe limited, typical of the broader Southern Ocean, near shore waters over the Kerguelen plateau show variability in Fe distributions and support a high biomass of phytoplankton during austral spring-summer. This study investigates dissolved iron (DFe) and macronutrient distributions in waters surrounding HIMI during the Heard Earth-Ocean-Biosphere Interactions (HEOBI) voyage in January-February 2016. Comparison of surface DFe with macronutrient concentrations shows that the majority of the plateau is Fe limited in late summer and, based on comparison with previous voyages, also Fe limited in different years and earlier in the bloom season. The distribution of DFe drawdown from estimated winter inventories to observed late summer inventories shows that DFe availability drives macronutrient uptake on the plateau. The drawdown of silicic acid decreases relative to nitrate drawdown in proximity to HIMI, in agreement with classical diatom nutrient uptake behaviour under iron replete conditions. Comparison of Fe: nitrate and Fe: phosphate drawdown ratios with expected uptake stoichiometry suggest that recycling of Fe increases with distance from Fe sources on the plateau. Lastly, comparison with data from previous voyages shows that DFe distribution varies inter-annually due to complex oceanographic conditions on the plateau, with greatest variability observed over the rough bathymetry and strongly tidally influenced region closest to HIMI. Together these data highlight the central role of Fe in driving nutrient uptake and stoichiometry in the HIMI region of the Kerguelen Plateau.