Copper complexation in the Northeast Pacific

Abstract

Copper titrations were conducted at sea with differential pulse anodic stripping voltammetry to examine the degree to which copper was associated with organic ligands. Greater than 99.7% of the total dissolved copper in surface waters of the central Northeast Pacific shallower than 200 m was estimated to be associated with strong organic complexes. Below 200 m, increasing proportions of inorganic or labile copper species were observed. At middepths (1,000 m), about 50–70% of the total dissolved copper was in the organically complexed form. Whereas total copper varies by a factor of only three from the surface to middepths (0.6–1.8 nM), copper complexation gives rise to extremely low cupric ion activities in surface waters ({Cu2+} = 1.4 × 10−14 M) and higher values at middepth ({Cu2+} = 10−11 M)—a variation of three orders of magnitude. Two classes of copper‐binding ligands were found to be responsible for this complexation: an extremely strong ligand class [log K′cond (Cu′) = 11.5] at low concentrations (∼ 1.8 nM) which dominated copper complexation in the surface waters and decreased with depth, and a weaker class of ligands [log K cond(Cu′) = 8.5] at higher concentrations (8–10 nM) which was observed throughout the water column and showed no apparent structure in its vertical distribution. These findings have significant implications concerning the toxicity and bioavailability of copper in open ocean systems.