Self‐Shading and Meltwater Spreading Control the Transition From Light to Iron Limitation in an Antarctic Coastal Polynya

Dotson Ice Shelf (DIS) in West Antarctica is undergoing rapid basal melting driven by intrusions of warm, saline Circumpolar Deep Water (CDW) onto the continental shelf. Meltwater from DIS is thought to influence biology in the adjacent Amundsen Sea Polynya (ASP), which exhibits the highest net primary productivity (NPP) per unit area of any coastal polynya in the Southern Ocean. However, the relative importance of iron and light in colimiting the spring phytoplankton bloom in the ASP remains poorly understood. In this modeling study we first investigate the mechanisms by which ice shelves impact NPP, then map spatio‐temporal patterns in iron‐light colimitation, and finally examine the environmental drivers of iron and light supply. We find that ice shelf melting leads to greater upper ocean iron concentrations, both directly due to release of iron from sediments entrained at the glacier bed, and indirectly via a buoyancy‐driven overturning circulation which pulls iron from CDW to the surface. Both of these mechanisms increase NPP compared to experiments where ice shelf melt is suppressed. We then show that the phytoplankton self‐shading feedback delays the bloom and reduces peak NPP by 80% compared to experiments where light penetration is independent of chlorophyll. Compared to light limitation, iron limitation due to phytoplankton uptake is more important a) later in the season, b) higher in the water column, and c) further from the ice shelf. Finally, sensitivity experiments show that variability in CDW intrusion influences NPP by controlling the horizontal spreading of iron‐rich meltwater. Plain Language Summary The seas around Antarctica are covered for much of the year with sea ice. When gaps (known as polynyas) develop, the exposed ocean surface can exchange carbon dioxide with the atmosphere. These polynyas often host large seasonal blooms of floating algae—phytoplankton—which take up carbon as they photosynthesize. The most intense blooms tend to form close to the floating ice shelves which form the margins of the Antarctic Ice Sheet. In this study we construct a computer model of a polynya off West Antarctica. We show that the ice shelf helps to supply phytoplankton with the iron that they need to grow, allowing the polynya to take up more carbon. On the other hand, the early growth of the bloom results in a progressive “greening” of the polynya, which prevents sufficient light from reaching deeper phytoplankton. This slows the development of th