Iron (Fe) is an essential trace metal for phytoplankton growth, and its behavior in seawater is largely controlled by complexation with organic ligands. To better understand the role of ligands in Fe cycling, we investigated their vertical distribution in the Bering Sea and surrounding regions, which are a source area for North Pacific Intermediate Water (NPIW). Filtered seawater samples (<0.2 μm) were collected at seven stations in the western Bering Sea and the Sea of Okhotsk during cruises in 2018 and 2021. The concentration of Fe-binding organic ligands ([L]), their conditional stability constants (log K′), and inorganic Fe not complexed with ligands ([Fe′]) were determined using CLE-ACSV. Ligand concentrations ([L] = 1.7–8.1 nM) exceeded dissolved Fe in all samples, and more than 99% of Fe was estimated to be organically complexed. Compared to previous studies in the eastern equatorial Pacific, relatively high ligand concentrations were observed in intermediate and deep waters, suggesting that ligand complexation facilitates Fe transport. However, no clear relationship was found between ligand and dissolved Fe concentrations, indicating that ligand abundance alone cannot explain Fe distributions. In some deep and near-bottom samples, lower log K′ values and elevated [Fe′] were observed, likely reflecting the influence of reductive sediments. Considering that high [Fe′] is often associated with elevated colloidal Fe, our results suggest that Fe dynamics in this region are controlled not only by ligand complexation but also by colloidal processes.

Iron,the Bering Sea,Fe speciation