Modeling the Seasonal Cycle of Iron and Carbon Fluxes in the Amundsen Sea Polynya, Antarctica

The Amundsen Sea Polynya (ASP) is distinguished by having the highest net primary production per unit area in the coastal Antarctic. Recent studies have related this high productivity to the presence of fast-melting ice shelves, but the mechanisms involved are not well understood. In this study we describe the first numerical model of the ASP to represent explicitly the ocean-ice interactions, nitrogen and iron cycles, and the coastal circulation at high resolution. The study focuses on the seasonal cycle of iron and carbon, and the results are broadly consistent with field observations collected during the summer of 2010-2011. The simulated biogeochemical cycle is strongly controlled by light availability(dictated by sea ice, phytoplankton self-shading, and variable sunlight). The micronutrient iron exhibits strong seasonality, where scavenging by biogenic particles and remineralization play large compensating roles. Lateral fluxes of iron are also important to the iron budget, and our results confirm the key role played by inputs of dissolved iron from the buoyancy-driven circulation of melting ice shelf cavities (the "meltwater pump"). The model suggests that westward flowing coastal circulation plays two important roles: it provides additional iron to the ASP and it collects particulate organic matter generated by the bloom and transports it to the west of the ASP. As a result, maps of vertical particulate organic matter fluxes show highest fluxes in shelf regions located west of the productive central ASP. Overall, these model results improve our mechanistic understanding of the ASP bloom, while suggesting testable hypotheses for future field efforts. Plain Language Summary Winds in the coastal Antarctic regions tend to create openings in the sea ice cover called coastal polynyas. Such polynyas are found in multiple locations around the Antarctic continent including in the Amundsen Sea. This particular embayment features the fastest-melting ice shelves of the continent, but it is also distinguished by a large production of algae during the austral summer season. Recent studies have reported a statistical link between these two features but the mechanisms involved are not well understood. We use advanced computer simulations to gain insight into these mechanisms and use measurements collected in the Amundsen Sea to evaluate how realistic the simulations are. The results emphasize the two roles of the coastal circulation: it provides a necessary micronutrient (iron) to the algae and it also transports the algae away from where it grows during the summer. The computer simulations improve our understanding of why the Amundsen Sea Polynya is so productive, and they suggest new hypotheses that can be tested in future field efforts.