Analysis of Iron Sources in Antarctic Continental Shelf Waters

Previous studies showed that satellite-derived estimates of chlorophyll a in coastal polynyas over the Antarctic continental shelf are correlated with the basal melt rate of adjacent ice shelves. A 5-km resolution ocean/sea ice/ice shelf model of the Southern Ocean is used to examine mechanisms that supply the limiting micronutrient iron to Antarctic continental shelf surface waters. Four sources of dissolved iron are simulated with independent tracers, assumptions about the source iron concentration for each tracer, and an idealized summer biological uptake. Iron from ice shelf melt provides about 6% of the total dissolved iron in surface waters. The contribution from deep sources of iron on the shelf (sediments and Circumpolar Deep Water) is much larger at 71%. The relative contribution of dissolved iron supply from basal melt driven overturning circulation within ice shelf cavities is heterogeneous around Antarctica, but at some locations, such as the Amundsen Sea, it is the primary mechanism for transporting deep dissolved iron to the surface. Correlations between satellite chlorophyll a in coastal polynyas around Antarctica and simulated dissolved iron confirm the previous suggestion that productivity of the polynyas is linked to the basal melt of adjacent ice shelves. This correlation is the result of upward advection or mixing of iron-rich deep waters due to circulation changes driven by ice shelf melt, rather than a direct influence of iron released from melting ice shelves. This dependence highlights the potential vulnerability of coastal Antarctic ecosystems to changes in ice shelf basal melt rates. Plain Language Summary Phytoplankton in Antarctic coastal waters grow more rapidly relative to waters farther offshore. This growth is limited by the availability of light for photosynthesis and the supply of the micronutrient dissolved iron. Earlier studies suggest that satellite-based estimates of phytoplankton growth are related to the melting of nearby floating portions (called ice shelves) of the Antarctic ice sheet. In this study, a computer model, which includes melting of ice shelves, is used to examine the different sources of dissolved iron that supply the well-lit summer surface waters around Antarctica. Dissolved iron is available in the floating ice shelves, and the direct supply of this iron to coastal waters by melting of the bottom of the ice shelf is important for enhancing biological production. However, melting creates less dense water at the ice shelf base that rises and brings deep waters that contain dissolved iron towards the surface in front of the ice shelf. The model shows that this input provides a larger source of dissolved iron to the open surface waters in many coastal regions than does direct supply from the ice shelf meltwater. This implies that phytoplankton growth may be vulnerable to changes in ice shelf basal melt.