Temporal and spatial variability of copper complexation by strong chelators in the Sargasso Sea

Copper(II) complexation in the upper water column was studied at the Bermuda Atlantic Time Series Station (BATS) from January 1992 to March 1993, and in the southern Sargasso Sea in April 1992, using adsorptive cathodic stripping voltammetry (ACSV). Copper titration data, analysed using a one ligand model, indicated that speciation was dominated by a strong ligand or ligand class, with a conditional stability constant of 1013.2. Total concentrations of copper and ligand were very similar, ranging from 0.9 nM to 2 nM. Concentrations of free cupric ion (Cuf2+) varied widely in the upper water column depending on whether Cu exceeded the ligand concentration or vice versa. Temporal and spatial variability in these parameters showed trends with hydrographic and biological parameters consistent with biological production and near-surface photochemical decomposition of the strong ligand. Under well stratified oligotrophic conditions, such as those prevailing year round at the southern Sargasso station and in the summer and autumn at BATS, the ligand showed a subsurface maxima coinciding roughly with the chlorophyll maximum. Ligand concentrations decreased below total Cu concentrations in the mixed layer, leading to pronounced increases in Cuf2+ concentrations. However, samples collected at BATS during or following periods of intense vertical mixing and biological activity showed excess ligand concentration throughout the upper water column and extremely low cupric ion concentrations. The spatial and temporal variability of the strong ligand at BATS is similar to that observed by Coale and Bruland (1990) in the NE Pacific, suggesting that Cu speciation in both regions is controlled by common processes. However, in that study, copper concentrations were always below the ligand concentration, so cupric ion concentrations did not display the great variability observed in the Sargasso Sea in this study.