Dissolved iron(III) speciation in the high latitude North Atlantic Ocean

On voyages in the Iceland Basin in 2007 and 2009, we observed low (ca. 0.1 nM) total dissolved iron concentrations dFe] in surface waters (<150 m), which increased with depth to ca. 0.2–0.9 nM. The surface water dFe] was low due to low atmospheric Fe inputs combined with biological uptake, with Fe regeneration from microbial degradation of settling biogenic particles supplying dFe at depth. The organic ligand concentrations L_T] in the surface waters ranged between 0.4 and 0.5 nM, with conditional stability constants (log K′_FeL) between 22.6 and 22.7. Furthermore, L_T] was in excess of dFe] throughout the water column, and dFe was therefore largely complexed by organic ligands (>99%). The ratio of L_T]/dFe] was used to analyse trends in Fe speciation. Enhanced and variable L_T]/dFe] ratios ranging between 1.6 and 5.8 were observed in surface waters; the ratio decreased with depth to a more constant L_T]/dFe] ratio in deep waters. In the Iceland Basin and Rockall Trough, enhanced L_T]/dFe] ratios in surface waters resulted from decreases in dFe], likely reflecting the conditions of Fe limitation of the phytoplankton community in the surface waters of the Iceland Basin and the high productivity in the Rockall Trough.Below the surface mixed layer, the observed increase in dFe] resulted in a decrease of the L_T]/dFe] ratios (1.2–2.6) with depth. This indicated that the Fe binding ligand sites became occupied and even almost saturated at enhanced dFe] in the deeper waters. Furthermore, our results showed a quasi-steady state in deep waters between dissolved organic Fe ligands and dFe, reflecting a balance between Fe removal by scavenging and Fe supply by remineralisation of biogenic particles with stabilisation through ligands.