Photochemical control of copper complexation by dissolved organic matter in Rocky Mountain streams, Colorado

We investigated photochemical, seasonal, and source effects on copper (Cu) complexation by dissolved organic matter (DOM). Cu-DOM complexation regulates Cu toxicity by decreasing the activity of the cupric ion ({Cu2+}), the most bioavailable Cu species. However, DOM is photochemically unstable under solar insolation. We analyzed Cu-DOM complexation before and after photooxidation of DOM collected from six rivers during spring runoff and late summer (n = 12 DOM samples). After irradiation of DOM for 24 h in a solar simulator (similar to 4 d of ambient insolation), we analyzed Cu-DOM complexation during potentiometric titrations of Cu into dissolved organic carbon concentrations of 5 mg L-1. In 10 DOM samples across the range of titrations (Cu, 7.8 x 10(-8) to 8.7 x 10(-6) mol L-1), photooxidation of DOM decreased Cu complexation, increasing {Cu2+} by an average of 156% +/- 28% ((x) over bar +/- SE). In one DOM sample, irradiation had no net effect on {Cu2+} (6% +/- 12%), whereas in another Cu complexation was enhanced (30% +/- 4%). Cu complexation that was indistinguishable before irradiation decreased significantly more during photooxidation of DOM in spring (185% +/- 25%) than in summer (74% +/- 14%). The specific ultraviolet absorption coefficient at 254 mu explained similar to 60% of the variation in conditional stability constants of Cu-DOM complexes regardless of DOM source, season, or extent of photooxidation. During a simulated contaminant event where 1.5 x 10-6 mol L-1 Cu was added to site waters, water chemistry reduced bioavailability in 6 of 12 cases to below the {Cu2+} expected to cause 50% mortality ({Cu2+}(ILC50)) in larval fish. However, after 6 d of photooxidation, none of the site waters remained below {CU2+}(LC50).