| Summary: | Iron plays an important role in the regulation of biological productivity and the carbon cycle of the Southern Ocean. Recently, synchrotron X-ray spectromicroscopy revealed that molar iron to silicon (Fe:Si) ratios in living diatom samples collected from surface waters and ice in the coastal Antarctic are significantly higher than reported dissolved Fe:Si ratios of Circumpolar Deep Water. Upwelling of Circumpolar Deep Water is a dominant source of iron and silicon to coastal Southern Ocean surface waters. Thus with higher Fe:Si ratios, diatom production preferentially depletes dissolved iron relative to silicon, potentially contributing to perennial iron limitation in this region. Combining diatom and water column dissolved iron and silicon datasets with a simple inverse box model we estimate the regional coupled iron and silicon budget. Upwelling of subsurface waters cannot supply enough iron to balance the loss due to diatom production, which indicates that the closed budget requires additional iron sources or additional methods of silicon removal. To evaluate the ecological and biogeochemical impacts of the high Fe:Si ratio, a three-dimensional ocean biogeochemistry and ecosystem model is used to simulate the sensitivity of ocean productivity and nutrient cycling to a wide range of Fe:Si ratios in modeled diatoms. The Fe:Si ratio of diatoms regulates the surface iron and macronutrient distribution in vast regions beyond the Southern Ocean. A globally higher Fe:Si ratio strongly decreases subpolar productivity and is partially compensated by the moderate increase in subtropical productivity. Our results indicate that the Fe:Si ratio of diatoms has a global impact controlling the distribution of both micro- and macro-nutrients and associated biological production. M.S.
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