Limiting iron concentrations early in the season off the West Antarctic Peninsula

Iron (Fe) has been shown to be a limiting nutrient for phytoplankton growth in Antarctic waters, even over the productive continental shelves surrounding the Antarctic continent. The abundance of dissolved Fe (dFe) in seawater is controlled by a balance between Fe input (via sediment resuspension, sea-ice and glacial melt, upwelling, atmospheric deposition, hydrothermal inputs and lateral and vertical diffusion from sources), stabilization processes via organic complexation that keep Fe in the dissolved phase, and by removal processes like (oxidative) precipitation, adsorptive scavenging, and phytoplankton uptake. Concentrations of dFe measured on a recent cruise in the Bellingshausen Sea ranged from 12 pM to 8.8 nM, with a median of 0.36 nM for the entire dataset. The dFe concentrations in the surface waters of the open ocean were extremely low with values well below 0.1 nM. Concentrations increased with depth to about 0.15 nM at 300 m and 0.2 -0.3 nM at 750 m. These very low concentrations were surprising, as it is currently assumed that dFe concentrations are replenished over the winter and available for uptake in the spring/summer. Samples were taken weeks after the ice had retreated and primary productivity was low. Bioassays conducted on board indicated primary productivity was limited by both light and dFe availability. No effects were seen for the addition of manganese or Vitamin B-12. Towards the continent over the continental shelf the highest concentrations (up to 8.8 nM) were observed. Over the shelf, high concentrations were observed near the surface as well as near the bottom, indicating both a surface source as well as a sedimentary source. Highest concentrations were generally observed closest to the continent along three transects from the open ocean to the coast, all showing a similar trend of low concentrations offshore with increasing concentrations towards the continent.