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

Brooks, Marjorie L., Diane M. McKnight, William H. Clements

Limnol. Oceanogr., 52(2), 2007, 766-779 | DOI: 10.4319/lo.2007.52.2.0766

ABSTRACT: 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 (~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% (mean ± 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 nm explained ~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+}LC50) in larval fish. However, after 6 d of photooxidation, none of the site waters remained below {Cu2+}LC50.

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