LAUSR

The chemical speciation of Cu is dominated by complexation with organic ligands. However, the size fractionation of dissolved Cu and its organic ligands are not well understood. In this study, we revealed new insights into the biogeochemical cycling of Cu in the subarctic Pacific based on both chemical speciation and size fractionation analyses. Our data show that the surface waters of the subarctic Pacific contain a pool of colloidal Cu that was not observed in the subtropical South Pacific Ocean. Both soluble and colloidal Cu in the surface waters were strongly bound to organic ligands. In the intermediate waters, strong soluble Cu‐binding organic ligands were present, which could be related to the Cu uptake mechanism of methanotrophs under low oxygen environments. In deep waters, dissolved Cu concentrations increased but soluble Cu concentrations remained mostly constant, indicating that there are sources of colloidal non‐labile Cu in the benthic layer. Dissolved Cu exhibited a positive linear relationship with silicate in the upper 1500 m while soluble Cu displayed a linear relationship with silicate throughout the water column. The results of this study show that dissolved Cu is bound by organic ligands of various sizes and binding strengths throughout the water column of the subarctic Pacific, and that the characteristics of these ligands change with depth.