Free cupric ion concentration and Cu(II) speciation in a eutrophic lake

The free cupric ion concentrations, [Cu2+], in the water column of eutrophic Lake Greifen were evaluated by means of ligand exchange with catechol and cathodic stripping voltametry of the Cu-catechol complexes. Total dissolved Cu, [Cu]T, was in the range 0.5–2.8 × 10−8 M, while [Cu2+] ranged from 10−16 to 10−14 M at different times and depths. These values of [Cu2+] are much lower than literature values for the open sea. Equilibrium models of titration data indicate the presence of very strong ligands with conditional stability constants of 1013.9–1014.9 and corresponding concentrations of 40–90 nM. Measured [Cu2+] was especially low during the algal bloom in spring and was higher in winter. p[Cu] and ratios of [Cu]T to [Cu2+] thus exhibit a seasonal pattern similar to that of algal productivity, suggesting that the Cu-complexing ligands are produced by algae.