Environmental Context. The bioavailability of dissolved metals in natural waters is directly affected by metal-sequestering agents. These agents include soil-derived matter and compounds released by microorganisms, since copper can support or inhibit aquatic microorganisms depending on concentration. During summer the levels of copper increase in surface waters, an effect intuitively attributable to increased ultraviolet light degrading the sequestering agents more effectively, leading to a concurrent release of the metal. This paper shows that the amount of degradation attributable to light is too low to explain the metal release and that a biological influence may instead be responsible.

Abstract. The influence of UVB irradiation on the Cu²⁺ binding by natural organic ligands in six alpine lakes on the South Island, New Zealand, has been investigated using competitive ligand equilibration with salicylaldoxime and detection by cathodic stripping voltammetry (CLE-CSV). During austral summer 2002–2003 the total dissolved Cu ([Cu]_T), the concentration of strong Cu²⁺-binding ligands ([L]_T), and their conditional stability constant K´´ were determined in surface samples of all six lakes. All lakes exhibited appreciable concentrations of a strong Cu²⁺ binding ligand with similar K´´ values and concentrations always exceeding [Cu_T], thus dominating Cu²⁺ speciation. Four lakes (Hayes, Manapouri, Wanaka, Te Anau) showed no appreciable trend in [L_T] throughout the summer, whereas in Lakes Wakatipu and Hawea [L_T] increased steadily throughout this period. Laboratory UVB irradiation of lake water samples using a 400 W mercury lamp with a Pyrex glass filter (λ > 280 nm) showed that Cu²⁺-binding ligands are destroyed by UVB radiation, causing [L]_T to decrease with a rate of –0.588 nmol L^–1 h^–1 (r² 0.88). From this we calculate that the in situ ligand destruction rate by UVB in summer for surface waters of these lakes is too small to significantly affect [L_T], and conclude that variations in ligand concentrations must result from seasonally variable biological factors.