Cu complexation by organic ligands in the sub-arctic NW Pacific and Bering Sea

Cu speciation was characterized at three stations in the sub arctic NW Pacific and Bering Sea using cathodic stripping voltammetry with the competing ligands benzoylacetone and salicylaldoxime. A single ligand model was fit to the titration data, yielding concentrations throughout the water column of ∼3–4 nM, and conditional stability constants ranging from 10^12.7 to 10^14.1, this range being partly due to the choice of competing ligand. Free Cu²⁺ in surface waters was 2–4×10⁻¹⁴ M, in close agreement with values reported by previous workers in the NE Pacific using anodic stripping voltammetry (ASV). However, those results showed that complexation by strong organic ligands becomes unimportant below 200–300 m, while our data indicated Cu is strongly complexed to depths as great as 3000 m. Free Cu²⁺ concentrations in surface waters reported here and in previous work are close to the threshold value where Cu can limit the acquisition of Fe by phytoplankton.