Physicochemical speciation of iron in the Baltic sea

Size fractionated classes of Fe in surface water at two stations, one in central Baltic Sea (the Landsort Deep) and one in Bothnian Sea, were measured from spring until autumn to evaluate temporal variations in the physicochemical speciation. Membrane filtration, cross-flow ultrafiltration and DGT (diffusive gradients in thin films) were among the applied techniques. Average concentrations for total, <0.22 and <1kDa Fe were 32, 7.0 and 5.9nM respectively at the Landsort Deep. For the Bothnian Sea, these figures were 96, 21 and 6.2nM. DGT-labile concentrations of Fe were significantly lower than <1kDa at both stations. This probably indicates that a large portion of the DGT-labile Fe was organic bound, which would slow down the diffusion rate. The DGTlabile Fe concentration was on average 0.6 nM at Landsort Deep, and 1.0 nM in the Bothnian Sea. If assuming that all DGT-labile Fe was bound to fulvic acids the concentrations becomes about 5 times higher, but still lower than <1kDa. No clear trends were observed for the DGT measured concentrations. In both areas the concentration of Fe in all but the DGT-labile fraction decreased 60-80% towards end of the sampling period. Average ratios between total Fe and <0.22μm were about 4-5 at both stations, but in the Bothnian Sea also a clear difference between <0.22μm and <1kDa was observed, indicating a significant colloidal fraction. At the Landsort Deep, a peak of <1kDa Fe was evident in end of July, despite stable total concentrations during the summer. A small peak of <1kDa Fe was also observed in the Bothnian Sea, but not as clear as in Landsort. These peaks coincide with drops in biomass, indicating that this is a mineralization event. We hypothesize that the difference in concentration between DGT-labile and