Toxicity of copper to larvalPimephales promelasin the presence of photodegraded natural dissolved organic matter

Abstract

Copper (Cu) complexation with natural dissolved organic matter (DOM) mitigates Cu bioavailability to aquatic biota by decreasing the activity of the cupric ion ({Cu2+}). However, DOM is photochemically unstable. In a previous study, we demonstrated that irradiation (~13 days sunlight) of DOM collected from a river decreased its binding-site density for Cu by 45%, but increased binding-site density in wetland DOM by 147%. Binding-site densities correlated positively with ketones and aldehydes (C-II groups). Herein, we determined the mortality of larval fathead minnows (FHM; Pimephales promelas) as a function of {Cu2+} in 96 h static-renewal toxicity tests without DOM. Next, we calculated {Cu2+} in control and photooxidized DOM and then predicted mortality of larval FHM in toxicity tests. Observed mortalities agreed with predictions (r2≈ 0.96) in treatments with lowest binding-site densities and proportions of C-II groups (highest {Cu2+}). However, treatments with ~50% lower {Cu2+}, containing higher proportions of C-II groups, had equally high mortalities and poorer fits with predictions (r2≈ 0.75), possibly indicating that Cu bound to C-II groups are bioavailable. To our knowledge, this study is the first to predict and then directly test the effects of DOM photooxidation on Cu toxicity.