Size fractionation and bioavailability of iron released from melting sea ice in a subpolar marginal sea

We incubated Fe-limited seawater with sea-ice sections to evaluate which forms of iron (Fe) released from melting sea ice can favor phytoplankton growth. Biological availability (bioavailability) was approximated by fractionating Fe into soluble (<1000 kDa), colloidal (1000 kDa–0.2 μm), and labile particulate (>0.2 μm) sizes. Results show that phytoplankton thrived after the addition of sea ice. While the labile particulate fraction dominated the total Fe pool in sea ice, the concentration of dissolved Fe (<0.2 μm) was likely not enough to support phytoplankton growth in seawater over time. The concentrations and molar ratios of Fe, Mn and Al in acid-digested particles indicate that particulate Fe in sea ice were derived from multiple origins. Specifically, the Fe to Al ratio in sea ice was higher than in lithogenic material, suggesting that the sea ice were enriched with biogenic material. Our study suggests that particulate Fe from sea ice should be considered an important source of biologically available Fe in ice-covered marginal seas.