Key role of organic complexation of iron in sustaining phytoplankton blooms in the Pine Island and Amundsen Polynyas (Southern Ocean)

Primary productivity in the Amundsen Sea (Southern Ocean) is among the highest in Antarctica. The summer phytoplankton bloom in 2009 lasted for > 70 days in both the Pine Island and Amundsen Polynyas. Such productive blooms require a large supply of nutrients, including the trace metal iron (Fe). The organic complexation of dissolved Fe was investigated in the Amundsen Sea during the spring of 2009 to better understand the potential role of ligands in enhancing the local stock of dissolved Fe. The main sources of Fe and ligands to the Amundsen Sea are thought to be melting sea-ice and the Circumpolar Deep Water (CDW), which is modified (MCDW) on the continental shelf and upwells beneath the coastal glaciers and ice-shelves. Upwelling of relatively warm MCDW is also responsible for the rapid melting of the Pine Island Glacier (PIG) and surrounding ice-shelves, resulting in additional release of Fe into surface waters. At upwelling stations near ice shelves, organic ligands were nearly saturated with Fe, thus enhancing the stock of Fe and its availability to the phytoplankton community. However, ligands had little capacity to buffer additional Fe input from glacial melt. In these coastal upwelling regions, much of the glacial Fe supply is lost due to vertical export of Fe via scavenging and precipitation. Conversely, within the phytoplankton bloom in the nearby coastal polynyas, the uptake of Fe combined with the production of organic matter enhanced the abundance of relatively unsaturated organic ligands capable of stabilizing additional Fe supplied from glacial melt. These unsaturated dissolved organic ligands, combined with the continuous input of Fe (dissolved and particulate) from glacial melt, appear to favor the solubilization of Fe, thus increasing the stock of bioavailable Fe and fueling the phytoplankton bloom. (C) 2012 Elsevier Ltd. All rights reserved