The complexation of trace metals by organic ligands has long been proposed to play a significant role in determining trace metal speciation in seawater. However, analytical methodologies with sufficient specificity and sensitivity only recently have been developed to enable us to address questions of trace metal/organic complexation. In this study copper titrations were conducted at sea using differential pulse anodic stripping voltammetry on North Pacific samples to determine the extent of copper complexation with organic ligands. This study includes data from a transect from 33°N, 139°W to 55°N, 148°W and seasonal occupations of the VERTEX T-4 station (33°N, 139°W). The data indicate the presence of at least two copper binding ligands: L1, the stronger ligand, or ligand class, averages 2 nM with log K′l cond(Cu′) = 11.6; L2, the weaker ligand class varies between 5 and 10 nM witgh log K′2 cond(Cu′) = 8.6. The presence of these ligands strongly buffers the activity of copper(II) in surface waters at all stations. Seasonal variations in the distribution of strong copper complexing ligands follow seasonal variations in the depth of the mixed layer at T-4 and indicate a ligand source in the lower mixed layer. Ligand concentrations do not covary with the rates of primary productivity in a latitudinal transect from the central North Pacific to the subarctic. Ligand concentrations at all stations vary only by a factor of two and hold surface water copper(II) ion activities relatively constant (about 10-14 M), suggestive of a regulated ligand production mechanism. These results comprise a unique data set for copper complexation in the Pacific and may have significantly implications for the distribution of plankton between oligotrophic and subarctic systems. © 1990.
